muzzle veloc 1st gun with short barrel is more than muzzle vel of2nd gun with long barrel-acceleration affect?

Answer 1

It depends. Mostly on the speed of which the powder burns.

Specifically, can all of the powder used be burned (energy completely utilized) in the short barrel?

If the powder can burn completely in the short barrel, the velocity would be the same. OR...

The velocity from the longer barrel could be slightly less - if the longer barrel adds any parasitic drag (friction) past the point where the potential of the powder (energy) has been completely utilized.

If the potential of the powder cannot be fully utilized (or as fully utilized) in the shorter barrel as in the long one, then, ignoring any friction loss the velocity from the long barrel would be greater.

In guns that use the same ammunition, this can be a factor.
"Can", because so many different powder variants with different properties are available, you can generally find something that does pretty much what you want...

If you're using a Hi-Point 9mm carbine and a Hi-Point C9 - you'll do a little better with the carbine and it's 18" barrel - but not a lot more than the 3.5" barrel of a C9, because 9mm ammo is designed for use in shorter barreled guns (pistols).

Long and short were not defined.
Thus my original and general statement.

In practicality, the difference is mainly shown in very short barrel pistols produced for maximum concealment.

A .38 cal "Bodyguard" compact pistol with a 2" barrel will be measureably slower (I'd guess 10-15%) than at typical 4-6" barrel.

What was the question again?

Answer 2

short barrell gun would need lighter bullet or bigger powder load.

or both guns have identical bullets and very light powder load that allows bullet to slow down due to friction in longer muzzle