Ground attack aircraft questions

[Zeus]

What made an aircraft good at ground attack? Could any aircraft strafe ground targets, with their effectiveness mainly being dependent on their armament? Or were specific measures taken to make attacking ground targets easier/more effective (and did this influence their effectiveness against other aircraft)?



I am of course not talking about bombs or air-to-surface rockets .

[ChrisMAg2]

IMHO, speed is not a must for a good ground attack. Other then that, I think survivability at least against rifle ammo or better against a heavy MG (upto .50 Cal. or 13mm) should be useful, as well as air superiority. The latter is a must.
For instance...

[model299]

What made an aircraft good at ground attack? Could any aircraft strafe ground targets, with their effectiveness mainly being dependent on their armament? Or were specific measures taken to make attacking ground targets easier/more effective (and did this influence their effectiveness against other aircraft)?

I am of course not talking about bombs or air-to-surface rockets .

Then you need to go to this thread here (Best strafing aircraft in WWII???) for an at-length discussion of this very topic!

[Zeus]

Thanks for your replies, and the link. Definitely an interesting topic, unfortunately it focuses on only the best ground attack plane (like many topics here), with a few serious candidates... but this says nothing about other planes' performance in this area.

I'd still like to know what exactly makes a plane good at ground attack. Are the guns aimed differently (downwards), or can any plane with forward firing guns strafe ground targets? Is pure firepower (ie the number of guns, or the number of bullets a second) the most important aspect (the topic from the link seems to focus on this), or is it speed?

BTW my main concern is the effectiveness against ground targets, so survivability is (for my current purpose) not so important (I'm trying to stat WW2 aircraft for a game, and toughness/armnor is separate from attack effectiveness).

[HoHun]

Hi Zeus,

>BTW my main concern is the effectiveness against ground targets, so survivability is (for my current purpose) not so important

Hm. Ignoring defences, the best ground attack aircraft carried a good payload of bombs and rockets while being quite manoeuvrable to be able to hit targets accurately. Guns for strafing were generally found to be inferior to other ordnance, though they were undoubtly useful, too.

However, the increased effectiveness of defences later in the war forced a change away from slower attack aircraft to faster fighter bombers (while making strafing still less effective since repeated attack runs became highly dangerous).

In conclusion, I'd say you might not want to ignore survivability completely, though the key might not be armour, but speed instead.

Regards,

Henning (HoHun)

[Njaco]

HoHun, speed wasn't nessecarily needed for ground attack and late in the war Germany and Russia used outdated Biplanes as ground attack. Slow speed was more effective as far as hitting your target but also made yourself a target as well. I think the major reason why these old planes were used mostly at night.

[filnorm]

Please keep in mind the fact that 0.50 cal. bullets can easily penetrate a steam locomotive boiler, in other words in case of strafing rolling stock standard armamanet of fighters would be good enough

[Tony Williams]

Survivability was a key issue, not just in terms of add-on armour but in such matters as control-line redundancy. Air-cooled engines were also better (although the Ju-87 and Il-2 managed without).

Which armament was best depended on the targets. The Hurricane MK IIB's battery of 12 x .303 was great against troops in the open, but at the other extreme a high-velocity 30+mm gun was needed to plink tanks. The 20mm cannon was probably the best overall compromise (as it was for air fighting).

[Demetrious]

What characteristics make for a good strafing aircraft, hmm? Well, there are a few... I'm going to use the P-38J Lightning as an example, because it exemplifies many characteristics of a good strafing platform.

Stability as a shooting platform: The aerodynamic stability of an aircraft helps control recoil of the guns and aids greatly in accuracy when putting rounds on target. The P-38 had twin engines, a tail boom and the engines were counter-rotating, which made it an exceptionally stable shooting platform. The P-47, while not as stable, was more stable than many because the great weight of the aircraft made it more sturdy in the air. Such stability makes it much easier to make pinpoint attacks, rather then just drag your piper over the general area of the enemy vehicles on the ground and spray the entire landscape. It's worth noting that the Stuka, which was a sitting duck for enemy fighters, what with it's fixed landing gear and all, was also a very stable shooting platform.

Don't be so swift to discount the effects of recoil- remember that the recoil of multiple heavy machine guns and cannon would usually shave off 50MPH of forward airspeed from a fighter in the first second of firing, and it was common for pilots to keep on shooting until they stalled themselves out.

Gun Orientation: Guns mounted in the middle of the wing, like in most allied aircraft, require the guns to be zeroed in on a fixed convergence point, generally 300 feet or so in front of the aircraft. This puts a severe limitation on the maximum range of the guns. By contrast, the P-38 had guns mounted in the central nacelle, right in front of the pilot. This means they were zeroed from 0 to infinity. This allows for an aircraft to open fire on targets from as far out as the pilot dares, and also lets the pilot choose just how close to the ground they want to get. Pilots flying aircraft like the Spit or Mustang had no choice but to get down to within 300 feet of the target, and hose their fire across the ground in a true "strafing" run, weather they wanted to or not, whilst pilots with centrally mounted guns could fire a stream of bullets at one target from a distance, and break off while still a good distance away. Note that the modern A-10 has it's gun mounted on the centerline for a similar reason.

Imagine it from the perspective of the cockpit. With centrally mounted guns, you could spot your target from altitude, put your gun piper right over it, and open fire from a distance. While you are doing this, remember, you're pointing your nose directly at the target- and at the ground. With a fighter with wing-mounted guns, however, your maximum range is far less, and so you have to wait till you are much closer to the target before you can open fire. And then, you are so close to the ground, and flying right at it, that your firing window is much shorter. That's why those fighters tended to use the time-honored strafing tactic where they used flap settings and flying skills to simply keep their airplane in a slightly nose-down attitude (as well as their guns, naturally,) while still flying horizontally with the ground. That's how they achieve the effect of streams of bullets walking their way across the ground, like you always see in the movies. Such shooting is great for blowing away infantry and truck in the open, but not very useful for trying to pop tanks.

The Germans, being clever, made a habit of mounting their guns either in the engine cowling or the wing root, so their fighter aircraft rarely suffered from a convergence distance problem. Other notable ground attack aircraft that had their guns mounted centrally were the Bristol Beaufighter and the Mosquito.

Speed: When it comes to ground attack, slower is better. Consider the modern A-10A Thunderbolt II: in an age of Mach 3 fighter jets, the A-10 can barely top 450 MPH on average- which is about the speed of the P-51! When attacking ground targets, speed is not a necessity. Note that the Stuka, which was a sitting duck for enemy fighters largely because of it's slow, wallowing speed, was also an outstanding ground attack aircraft. Of course, any aircraft can slow down to a relatively low speed (varying slightly, depending on it's stall speed, of course,) but different aircraft handle very differently at low speeds. This, then, really goes back to the stability factor.

So, in essence, to answer your question, it wasn't just about the guns mounted on it. The stability and quality of the aircraft as a gunnery platform and the orientation of the guns made a big difference in how easily and how accurately the fighter could engage ground targets- not all fighters are equal when it comes to shooting at ground targets. Note also that while the things that make those aircraft good at shooting at the ground also made them good at shooting at enemy aircraft- like the P-38- those advantages were often less important against enemy airplanes, since those tended to get a hell of a lot closer (thus why other fighters designs didn't sweat targets past 300 feet very much.) Generally, characteristics that make a plane a good shooter make it a worse dogfighter.

Just my two cents.

[Tony Williams]

Don't be so swift to discount the effects of recoil- remember that the recoil of multiple heavy machine guns and cannon would usually shave off 50MPH of forward airspeed from a fighter in the first second of firing, and it was common for pilots to keep on shooting until they stalled themselves out.

Can you quote a source for that?

Guns mounted in the middle of the wing, like in most allied aircraft, require the guns to be zeroed in on a fixed convergence point, generally 300 feet or so in front of the aircraft. This puts a severe limitation on the maximum range of the guns. By contrast, the P-38 had guns mounted in the central nacelle, right in front of the pilot. This means they were zeroed from 0 to infinity.

Not quite. They didn't have to worry about convergence, but they still had to take account of range, because of the trajectory of the bullets. If the guns were adjusted to be exactly on the sight mark at 500 yards, for instance, they would shoot high at shorter distances and low at longer distances.

When it comes to ground attack, slower is better. Consider the modern A-10A Thunderbolt II: in an age of Mach 3 fighter jets, the A-10 can barely top 450 MPH on average- which is about the speed of the P-51! When attacking ground targets, speed is not a necessity. .

Again, not quite. You might not need very high speed in the attack, but it's useful afterwards to climb away from the attack as quickly as possible in order to avoid ground fire.

And even in the attack, the slower you go, the more exposed you are to ground fire and the longer the flak gunners have to get your range. Fighter-bomber pilots used to launch their attacks with the throttles wide open, I believe; I don't think they would ever deliberately slow down in the presence of ground fire.

And of course a high transit speed means that you can reach the targets more quickly, and are at less risk of interception by enemy fighters.

[Demetrious]

Can you quote a source for that?

Not a direct quote off the top of my head, but I've read that in more than one pilots account. Chuck Yeagers book, and also God Is My Copilot (account of a Flying Tiger ace, on the off chance you haven't heard of it.) Sadly, Google is terribly unhelpful in finding more solid information for you- my first post is actually in the first page of results. In any case, it shouldn't surprise you terribly that the combined recoil of 6 .50 caliber guns firing on full automatic can be drastic, to say the least. I think the reason that little tidbit of fighter lore isn't more widely recognized is because it was so dependent on fighter armament- lots of fighters carried rifle-caliber armaments (Spitfire and Zero, for example,) or their primary armament was rather lacking (2 13mm guns in the BF-109, meh.) Cannons were common and pack a punch, but if I recall correctly, many aircraft cannons were rather low-velocity, and mounted in much fewer numbers than the .50s that were packed into American fighters. So the effects of recoil vary widely from fighter to fighter.

Not quite. They didn't have to worry about convergence, but they still had to take account of range, because of the trajectory of the bullets. If the guns were adjusted to be exactly on the sight mark at 500 yards, for instance, they would shoot high at shorter distances and low at longer distances.

A nitpick, but a very accurate one. Bullets are not laser beams.

Again, not quite. You might not need very high speed in the attack, but it's useful afterwards to climb away from the attack as quickly as possible in order to avoid ground fire.

And even in the attack, the slower you go, the more exposed you are to ground fire and the longer the flak gunners have to get your range. Fighter-bomber pilots used to launch their attacks with the throttles wide open, I believe; I don't think they would ever deliberately slow down in the presence of ground fire.

And of course a high transit speed means that you can reach the targets more quickly, and are at less risk of interception by enemy fighters.

Well, as with all things, that depends on circumstances. What I said in my first post about stability as a shooting platform at lower speeds rather than low vs. high speed still stands, I think. The threats facing you as you make your attack will determine your airspeed, of course- if you're facing nothing more than light machine gun fire and small arms, and you have center-mounted guns that allow you to fire from 500 feet out or so, you'll take your sweet time to line up your shots. However, if you're flying your standard fighter bomber (P-47, Typhoon, etc) with wing-mounted guns set to converge at 100 or 150 feet, then you have no choice but to get uncomfortably close to the ground, where even MG-42's and small arms fire are going to be murderous. If I was flying most fighter-bombers, I'd floor the throttle too, and hit like the proverbial hurricane (as the Hurricane itself was wont to do...) Which is another point I should have mentioned for the OP's benefit- any aircraft doing ground attack is guaranteed to attract more than the usual amount of incoming fire, so even attacking the softest targets, a good strafing aircraft is well served by heavy armor.

Generally, I'd posit that speed is not essential for ground attack, but only strictly in the context of ground attack. In an overall analysis, it's pretty damn important, as you say.

Consider the A-10, the pinnacle of aircraft design thinking limited solely to the context of ground attack. In an age where the surface to air threats have evolved significantly, what with SAM's and even AAA guns guided by radar and aimed by computer, the A-10 has a top speed barely equivalent to that of most WWII fighter-bombers. Of course, the military still uses fast fighter-bombers like the F-16 for SEAD/Wild Weasel anti-air suppression missions, the A-10 needs air superiority before it can be fielded and standoff missiles equate with the SAM capabilities of the modern age- but the A-10 is still very slow, and yet a devastatingly effective ground attack aircraft.

In this analysis, I think looking at the Stuka would be instructive, as the WWII equivalent of the A-10. Yes, the Stuka was legendary at ground attack, and also very slow- but it was a dive-bomber. In a dive, gravity works for you, so you have a long firing window because gravity is working with your cannon shells and not arcing their trajectory, greatly increasing your effective range. Not only do you have a long firing window, but you're moving fast and hard to hit- of course, short of multiple coordinated batteries of light and medium flak cannons, it is often maddeningly difficult for ground based guns to hit aircraft in the first place. Regardless, the low airspeed of the Stuka didn't hinder it in the attack much.

However.

Even though the fact that one needs air superiority before one can easily field dedicated ground attack aircraft, (rather then more nimble but less effective fighter-bombers,) is pretty much a universal maxim, the Stuka was a legendary brick. From Wikipedia:

Perhaps the prime example of its vulnerability to fighters was the shooting down of five Stukas in the space of a few minutes, by the Australian ace Clive Caldwell in a P-40 Tomahawk on December 5, 1941, over Libya.

Now that is incredibly pathetic. This is also a good time to note that the Stuka developed it's fearsome reputation during the early engagements of the war, when the Germans were steamrolling over nations with little or no fighter support. Later on, when the Luftwaffe couldn't guarantee control of the skies, the Stuka was a target.

Now compare to our contemporary example of the A-10. The Warthog is designed from the ground up for one mission- destroying tanks. If it's on the ground, the Hog can kill it with brutal efficiency. However, it takes a shameful amount of time to reach the target, when compared with an F-16, and if enemy fighters show up it is lunchmeat. The tight turn radius means it could easily hide in the ground clutter and steal a snap shot at the tailpipe of an attacking bandit, but the plane doesn't even have any air intercept radar, and it's pathetic speed means that any air-to-air missile launched within it's envelope has a better than even chance of plugging it. The A-10's very design makes it the ultimate ground attack aircraft, but by the same token, the less effective F-16 has much higher survivability in an environment lacking total air superiority.

The lesson? Speed is not essential for ground attack aircraft, but makes it a hell of a lot more versatile and greatly increases survivability. One problem with ground attack is that since it requires getting close to the ground (obviously) it guarantees exposure to an obscene volume of fire, especially from small-caliber weaponry. An F-16 or a P-51 at speed is a harder target than an A-10 or Stuka for flak guns, but because of the high volume of fire, it's still quite likely that they will at least catch a few rounds- and fast fighters are generally far less forgiving of damage. The fact that the designers of the A-10 opted to turn it into a flying tank rather then give it more evasive speed is reflective of the assumption that ground attack aircraft are going to catch fire no matter what (I think.) I think armor is (slightly) more important than flat airspeed when tackling ground targets, because that seems to be the trade off decided on by the designers of many ground-attack aircraft, and one that worked for many good ground attack planes (the P-47's ability to withstand damage comes to mind.)

But, as you have pointed out and I have further ruminated on, low top airspeed creates a lot of other problems for ground attack aircraft, primarily a loss of versatility and capacity in all other aspects, which often destroy survivability anyway. Those lines were not so distinct for WWII aircraft, of course, because mission roles weren't as specialized in those days as they would later become. One thing that cannot be denied is that speed always has tremendous inherent benefits, both those you observed and more- the question of weather their value outweighs the benefit of armor or heaver guns or more ordinance, etc, is another matter.

Perhaps the answer is that the best ground attack aircraft are obviously those specialized to the role, and the sacrifice of manuverability is equally well spent either gaining an advantage in speed OR armor. I think of the de Haviland Mosquito, a tremendously effective aircraft on the complete opposite end of the armor/speed spectrum- made of wood of all things, and incredibly fast. That aircraft was more of a light bomber than a ground strafer, but aircraft serving in any role are well suited by the ability to get out of Dodge before interceptors can even get their wheels up.

So, to wrap up, I contend that how much airspeed you want is a simple trade-off with how versatile you want your aircraft to be as an effective combat platform. In any case, thank you for expanding the discussion to include the entire sortie, rather then just the the five seconds you're shooting at the little people on the ground, which I had limited myself to.

[Tony Williams]

In any case, it shouldn't surprise you terribly that the combined recoil of 6 .50 caliber guns firing on full automatic can be drastic, to say the least.

I'm not so sure. It's a difficult one to pin down, and fighters would certainly have varied a lot, as you say. Recoil tends to have been written about rather more (often erroneously) in the case of large-calibre cannon. This is what I put in Flying Guns – World War 2: Development of Aircraft Guns, Ammunition and Installations 1933-45:

This might be an appropriate moment to dispel one of the favoured myths of big-gun aircraft; that the recoil had a drastic effect on their speed. To take the example of the USAAF's B-25 fitted with a 75 mm M4 gun; the aircraft weighed around 12,000 kg and attacked at perhaps 400 km/h, the gun fired a 6.8 kg projectile at around 2,200 km/h. A simple rule of thumb is to multiply the weight by the speed to achieve a rough "momentum index" (it is actually a bit more complicated than this, as the expanding propellant gasses contribute to the recoil). It will be apparent that the aircraft has at least 200 times the momentum of the projectile, and a single shot will therefore not greatly slow it. In fact, at the end of an attack run in which several shots were fired, the plane would typically be slowed by 10-15 mph. The effect on fighter speed of long bursts of heavy gunfire (especially from automatic cannon) could be noticeable, particularly in a turning battle when the aircraft might be manoeuvring at the extremes of the flight envelope, close to stalling.

The threats facing you as you make your attack will determine your airspeed, of course- if you're facing nothing more than light machine gun fire and small arms, and you have center-mounted guns that allow you to fire from 500 feet out or so, you'll take your sweet time to line up your shots. However, if you're flying your standard fighter bomber (P-47, Typhoon, etc) with wing-mounted guns set to converge at 100 or 150 feet, then you have no choice but to get uncomfortably close to the ground, where even MG-42's and small arms fire are going to be murderous.

I think that you are underestimating common convergence distances.

In the Battle of Britain, the RAF fighters started off with their guns harmonized at 1,200 feet, but this was later reduced to 750 feet as a result of battle experience. This remained a fairly typical distance for the RAF for the rest of the war. The later USAAF fighters, with multiple wing-mounted .50s, often had each pair zeroed at a different distance in order to provide a reasonable concentration of fire at all practical ranges: so you might get the inner pair zeroed at around 500 feet, the next pair at 750 feet and the outer pair at 1,000 feet.

There is no doubt that centre-mounted guns permitted more precise shooting, not just because of the convergence issue but also because fuselages were more rigid than wings. This was particularly useful when attacking point targets such as tanks, which required heavy cannon to penetrate. The Hs 129 / MK 103 combination was highly accurate and effective.

[JoeB]

Can you quote a source for that?

Adapting your 75mm/B-25 example to a fighter: say it weighs 8,000lb. A .50 cal bullet weighs typically approx. 0.12lb plus 0.034lbs propellant also being thrown forward. Say the bullet goes 3000fps and the escaping propellant 4000fps. The momentum of one 'salvo' of 6*.50 cal is 6*(.034*4000+.12*3000)=2976ft-lb/sec. If we set the plane on a frictionless surface and fire the guns once, it will move off at (2976ft-lb/sec)/8000lbs=.37 ft/sec~.25mph in the opposite direction: conservation of momentum. So a 1 sec burst (~12-14 rpg at 750-850rpm cyclic rate) is worth only a few mph speed reduction.

I assume this effect was often quoted even by people with first hand knowledge as greater than seems possible for a few reasons: because the peak recoil force perceived was jarring, though of too short duration to actually affect speed that much. As you said the situation might be high angle of attack manuever: a pilot might just not have realized he would have stalled in the turn he attempted even without firing the guns. And, pilots sometimes fired pretty long (too long, gun damaging) bursts, during which the effect could pile up to a more significant number. The simple calc will overstate the effect for a long burst, in which case the aerodynamic drag and prop thrust will decrease and increase respectively, noticeably, producing excess thrust which offsets the recoil effect, relative to the example of firing the guns at rest on a frictionless surface. So hard to nail down exactly for a very long burst, but for a short one simple conservation of momentum seems to rule out a large effect.

Joe

[Graeme]

In the Battle of Britain, the RAF fighters started off with their guns harmonized at 1,200 feet, but this was later reduced to 750 feet as a result of battle experience. This remained a fairly typical distance for the RAF for the rest of the war.

Different nations, different standards? Gunnery notes for the RAAF from 1943 constantly refer to '400 yards'. They also refer to 'cones of fire' being around four feet in diameter at this range-a result of gun vibration. Did harmonization distances differ with air to air fighting compared to ground attack?

[Demetrious]

EDITED: Doh, ninja posters!

I'm not so sure. It's a difficult one to pin down, and fighters would certainly have varied a lot, as you say. Recoil tends to have been written about rather more (often erroneously) in the case of large-calibre cannon. This is what I put in Flying Guns – World War 2: Development of Aircraft Guns, Ammunition and Installations 1933-45:

I'll be damned. You wrote a book? Excellent.

[quote= It will be apparent that the aircraft has at least 200 times the momentum of the projectile, and a single shot will therefore not greatly slow it. In fact, at the end of an attack run in which several shots were fired, the plane would typically be slowed by 10-15 mph. The effect on fighter speed of long bursts of heavy gunfire (especially from automatic cannon) could be noticeable, particularly in a turning battle when the aircraft might be manoeuvring at the extremes of the flight envelope, close to stalling.[/quote]

Exactly. I've heard that the recoil of such massive guns as the 75mm howitzer mounted in some B-25's and the 57mm gun in the anti-shipping variant of the Mosquito would make the aircraft feel like it paused in the air for a second, but the rate of fire is so slow that it doesn't make for much of a problem. Multiple .50 caliber machine guns provide a sustained fire, not to mention the fact that fighters weigh far less than a bomber with a massive cannon. Odd that more tends to be written about the big gunships then recoil effect in fighter craft- after all, it would seem to be more of a concern for a fighter pilot (especially if they risk firing near their stall speed, as you wrote,) whereas even if the recoil was bad for the gunships, it'd be of little consequence to them.

EDIT:

So a 1 sec burst (~12-14 rpg at 750-850rpm cyclic rate) is worth only a few mph speed reduction.

I assume this effect was often quoted even by people with first hand knowledge as greater than seems possible for a few reasons: because the peak recoil force perceived was jarring, though of too short duration to actually affect speed that much.

Well. o_o Can't argue with math, that's for sure. And I find your explanation for varying pilots accounts quite plausible, because they're so subjective. As I've read on this forum and others, the truth of weather the Spitfire or the Mustang was the better turn fighter depends entirely on which WWII pilot you ask. ~_^

In the Battle of Britain, the RAF fighters started off with their guns harmonized at 1,200 feet, but this was later reduced to 750 feet as a result of battle experience. This remained a fairly typical distance for the RAF for the rest of the war. The later USAAF fighters, with multiple wing-mounted .50s, often had each pair zeroed at a different distance in order to provide a reasonable concentration of fire at all practical ranges: so you might get the inner pair zeroed at around 500 feet, the next pair at 750 feet and the outer pair at 1,000 feet.

I'll be damned twice. Most of the things I've read has alternately given ranges of 150-300 feet (or 100 yards, etc) which seemed to match with what I've read about pilots being trained to wait till their opponent filled their windscreen before they fired. And I have never heard of the USAAF zeroing pairs of guns at various ranges. Incredibly interesting. Thank you for correcting me.

That certainly changes some of my conclusions. For one, it means that fighter-bombers (especially the USAAF aircraft) had guns that produced a cone of fire out to their max convergence distance, which would be saturated with a hail of lead. That'd make strafing soft targets perfect childs play, as your fire would be scattered over a small area, rather than a pinpoint- and precise fire isn't important there, because you're not going to be potting tanks with .50s or .303's anyway.

Now you've got me all curious about the convergence distance of cannon vs. guns on aircraft. I wonder what the 20mm's on the Spitfire were set for, vs the .303s.

There is no doubt that centre-mounted guns permitted more precise shooting, not just because of the convergence issue but also because fuselages were more rigid than wings. This was particularly useful when attacking point targets such as tanks, which required heavy cannon to penetrate. The Hs 129 / MK 103 combination was highly accurate and effective.

Indeed. And with a low rate of fire and limited ammunition, you needed accuracy.
EDIT:

Different nations, different standards? Gunnery notes for the RAAF from 1943 constantly refer to '400 yards'. They also refer to 'cones of fire' being around four feet in diameter at this range-a result of gun vibration. Did harmonization distances differ with air to air fighting compared to ground attack?

Only four feet? Seems reasonably tight, if you're shooting at something the size of a fighter... of course, then distance comes into play, as the sight diagram in that picture shows. That'd be a reasonable explanation for zeroing the guns in at a fair distance- for a target up close, it fills your entire gunsight anyway, but if you're shooting at something at 400 yards, you'd need pinpoint accuracy to hit- as the 400 yard sight diagram shows.

It would seem I've mis-characterized and over-simplified the entire convergence distance issue...

EDIT THE SECOND:

Well, well, look what I found. It would appear that we were both right- they boresighted for the "lethal density pattern," sighting pairs of guns at varying ranges, but in practice they often re-sighted the guns to shorter and shorter ranges anyways based on combat experience and pilots average skill levels.

Throw in the fact that individual pilots would sometimes have their guns adjusted to meet their personal preferences/skill levels (depending on the theater and their Services tolerance of personal adjustments like that, and just how much money passed under the table to the crew chief and/or armorers) you have a real crap shoot on your hands.