Hi Paul, surely you must be joking, this is a recurring theme and given time this thread will most likely be highly 'popular!' Yeah, maybe anyway.
In my opinion (*groan*) it rather depends whether you're talking about PCP or spring guns. Most (excluding FAC rated) pneumatics do not have any recoil. So barrel length 'seems' to make little difference to accuracy, providing you can hold the thing still until the pellet has exited the barrel.
Guns with a significant recoil, though, are easier to shoot accurately in carbine form, because the lock time (i.e. the time interval from pulling the trigger until the projectile leaves the barrel) is shorter. Therefore, there is less likelihood of the gun moving or being pulled off target, while the pellet is still in the barrel and accuracy suffering as a result.
That's it, in a nutshell, or in a mouthfull of nutshells more like. Regards ... Geek