Yep, what GUNNER 50 said,
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.
Clear as mud?
