This is a screen shot from a video. .223 barrel on my Thompson Contender.
For those who don't know gun powder doesn't explode it burns. When you pull the trigger and the gun fires the barrel becomes part of the chamber as the bullet travels out the end. This is why shorter barrels loose velocity when compared to longer barrels. I think a .308 looses around 200 feet per second for every inch that you shorten it.
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