Trying to find info on the affect of barrel length on velocity and pressure. Guess I've been looking in the wrong place, so any help would be greatly appreciated. JIM

Length has no effect on pressure. Barrel length will affect velocity up to a point.
Fairly mild cartridges like the 7x57, the .308 etc won't gain much be going above 22-23", the 7mm Rem Mag, 300 WinMag, etc. go below 24" and you will notice velocity loss.
The "over-bore" cartridges like the 30/378 etc you really need 26" and more. A lot has been written about this but a reasonable rule of thumb is for the 1st class mentioned about 20-25 fps for each inch of barrel, 30-24 fps per inch for the 7mm Mag,etc and over 50 fps for the biggest and fastest.

But then the longest barrel is not necessarily the best option. Shorter, stiffer barrels vibrate less and produce better accuracy in most rifles -- within reasonable limits.

In muzzle loaders, once you get out to about 26" you're topping out on velocity gains. Of course the longer sight radius has some effect on accuracy for iron sights.

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