It's because of the pressure differences in a 30-06/8mm cartridge compared to the 308.
(remember, the 1919 is a gas/recoil operated gun)
The booster orifice dia. needs to be a certain size-to-cartridge-relation to correctly operate the action.
Using a 308 booster on a 30-06/8mm gun is a NO-NO. The 308 booster has a smaller dia. hole. Using it with a 30-06/8mm cartridge causes higher gas pressure inside the bearing/booster. This higher pressure of gas can wreak havoc by producing harder recoil on the internals. In turn, causing the bolt to pound violently against the back plate, which in some cases (some mentioned here on the boards) can have disastrous consequences....from blown top covers, to brass fragments coming out of the back plate into people's thighs, hips and hands, buttocks (yes, buttocks)...etc
Not to mention (and god forbid) a possibility of the back plate failing and going through your face!
30-06/8mm boosters on a semi 308 gun works, kinda.. though, you'll probably get a lot of stoppages due to lower pressure 308 gas being expelled through the larger dia. 30-06/8mm booster hole.
Think of the 1919 booster like how a blank adapter works.
The pressure expelled out of a blank round is very low.. thus the hole in the adapter needs to be the right size in order to build up enough (correct) gas pressure to properly cycle the action.
that might not necessarily be the case......I put a 30-06 flash hider on my RF 1919, with a crankfire, and it works great....the only way to see if a 30-06 booster will work, is to actually try it on your gun.....worse case scenario if it doesn't work, you can either re-sell it, or keep it on the gun if you have it set up in your den as a show peice, just for looks
The difference between the muzzle pressure of military .30-06 and military 7.62mm NATO ball is negligible. Likewise for most military cartridges in that class; Turkish 7.92mmx57 being a notable exception as it appears to often be loaded with a relatively slow-burning powder that results in a higher muzzle pressure.
The smaller diameter orifice of the Israeli boosters is due to the Israeli belief that the "proper" rate of fire for an MG is in the 1000-1100 rpm range, not to any meaningful difference in muzzle pressure. What size orifices were fitted to Mk21 Mod0 and C5-series MGs?
For a semiautomatic gun the largest orifice that will reliably cycle the gun is the best choice, as the bolt velocity will be lower and less wear and tear on the gun will occur. If you're running a crank-fire device you may need to use a smaller orifice to prevent outrunning the gun.
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