Ok once again:
1. The loss of energy from tumble effect on 150 meters is negligible if you consider that the cartridge was designed for 400 meter engagements. Your shooting at aluminium skin, how much energy do you need to puncture it anyway? If the tumble effect was so great, wouldn’t it cause and even greater entry hole?
2. Even with ball, the bullet would make a 7 mil entry hole. Ball designed in 1920 is still the same that you get today. If you want to add explosive ammo to the mix (which were available back then as well... just like modern ammo now) that would make an even greater entry hole.
3. This isn't an assault rifle you sling over your shoulder. Even a “modern” assault rifle fires at only 600 rounds per minute. I’m talking about a combined rate of fire of 9600 rounds per minute.
It’s a serious bit of kit which is mounted, meaning stable platform.
4. it’s pretty hard to find any figures on the dispersion for the gun, but I found a reference saying 1 meter per 100 meters. So that is 8 guns filling an area of 1.5 meters with 160 rounds per second on 150 meters.
The point I’m making is that the damage model focus too much on the components that make up the airframe and too little attention is paid to the structural damage incurred. If you lose 2 meters of fuselage area on a 9 meter plane, surely that would be a bad thing?