The B20 was basically a necked up version of the 12.7mm Berezin machine gun. It fired the same ammo as the ShVAK (the "OF" HE-Frag round carried 6.7 grams of HE).Originally Posted by schwarzpanzer
This gun was used on the La7-FNV (approximately 386 deployed durring WWII), commonly refered to as the "3 cannon La-7".
The critical factor of this gun is its very low weight and very high reliability. Jam rates were in the 1:4000 rounds fired catagory and could often be cleared with a recharge (many guns like the Hispano have less than a 50% chance of successfully clearing a jam via gun recharging). The B-20 weighed only 25 kg including mounting hardware. This compares to:
.50 BMG = 30 kg
Hispano II = 50 kg
MG151/20 = 42 kg
MK108 = 44 kg
And none of those weights except the .50 include the mounting hardware, so add about another 20%. And most .50's were mounted in Edgewater recoil mounts and the weights of those are not included (as it is more than just a mount really). The Hispano in particular required a seriously heavy mount as it is not structurally sound on its own.
The B-20 rof was about 800 rpm in the La7 - this was a synchronized installation! Unsynchronized I believe the gun fired at 1150 rpms but I've not found any WWII data to confirm this (post-war it was used in the UBT format in bomber turrets). Muzzel velocity varied from about 770 m/s to about 825 m/s depending on the ammo type, perhaps more in the fixed mounted unsync'd installations. Ammo fit in the same belt links as the 12.7 mm ammo, so belt length was the same, therefore approximately the same number of rounds could be fit in a given gun tray as would fit for a .50 BMG. Eight B-20's could probably be mounted for just a little more cost in weight than six .50's.
So to my thinking, all things considered, the B-20 was the best aircraft gun of WWII. It had enough hitting power to down any target considering its velocity characteristics, RoF, and the number of guns that could be mounted.
As an example, consider an FW190A-8 could have carried 6 of these guns and would have been toting only 75% as much weight as the German armament of 4 x MG151/20's + 2 x MG131's, and it would have had only a little over half as much weight out in the outer guns creating less of an unsprung weight issue (which is bad for roll characteristics and overall manuverability). And the guns would have jammed only about 1/3rd as often as the MG151/20's!
For the weight of 4 Hispano's you could carry at least 10 B-20's! (including the required mounting weight).