Commercial 7.62x39 loads are designed around 20" barreled guns, as this is the barrel length of 99.9% of all guns chambered in this caliber.  You won't see any difference worth noting by shortening your barrel to 22".  Unlike .223/5.56mm, the 7.62x39 cartridge doesn't have a "velocity floor", below which the cartridge loses a major amount of effectiveness.

-Troy

Surplus loads are definitely optimized for 20" barrels, and for the same reason: virtually every gun in this caliber has a 20" barrel.  I mentioned commercial loads because:

1. I assumed Gunplay would be using them for hunting
2. I assumed that HE assumed that they might be different.

As I'm sure you know, different powders have different pressure curves, and the powders selected for this caliber will be set to have peaked in pressure before the 12" mark, with pretty low pressures by the time the bullet exits at 20".  For that reason, you won't see much of an increase in velocity from barrels longer than 20".  Some, certainly, but not much.

Now, if you HANDLOAD, you could certainly select powders that take better advantage of your longer barrel by using slightly slower burning powders.  And with a bolt gun, you don't have a gas system to worry about.  But few people bother to handload for this caliber, and given the price of ammo, I don't blame them.

-Troy