Since I've never heard of "peck" velocity I dunno exactly how to respond. Even if I assume "peck" means "peak" I'm still not sure how one would define that. But if you mean will a 20" barrel give you the velocity a longer barrel will then clearly not. Will it give you enough to do anything the .308 is capable of sure it will.
NO! I have read that .22 long rifle cartridges attain their peak velocity in about a 16-inch barrel. That is, with longer barrels, barrel friction overcomes the pressure behind the bullet and velocity decreases. Somewhere recently I saw a writeup about an experiment with a large centerfire cartridge which showed it to gain velocity even with a very long barrel, maybe 6-feet or so. Wish I could remember. I did an experiment cutting a 26-inch 7mm Weatherby barrel back to 24-inch then 22-inch before having the rifle rebarreled. It lost 40 fps for each inch the barrel was shortened. I did some physics calculations based on the energy of the bullet with the different barrel lengths and if I remember correctly, the pressure behing the bullet was about 8,000 or 10,000 psi. That is near the end of the barrel. Anyway, I bet a .308 Winch needs at least a 30-inch barrel to attain peak velocity.
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