According to what I’ve heard, a longer barrel does increase damage, but not linearly.
Imagine a cannon, with its relatively long barrel. Remove the entire barrel and just put the cannonball on a wall as gunpowder is exploded. Or even just shorten the barrel so it all BARELY fits.
To a certain point, longer barrels are better (if unwieldy).
It really really depends on the round and the rifle. Sometimes a longer barrel allows more complete combustion of the propellant which means higher muzzle velocity which can mean more damage and a flatter trajectory (which isn’t exactly the same as “accuracy” but it would probably be easier to hit something.) On some rounds this won’t matter. Whether or not this would apply to the 7.62x54r I have no idea. I know it could to a .308 Winchester which is a comparable round.