[ThunderCactus]

The difference is more likely due to the changed weight of the pistol affecting your handling of it.
Honestly at 25ft, groupings really don't mean anything at all, and if you're not using a gun vise, ESPECIALLY on a pistol, then the groupings are highly subject to your own personal skill.

AEG's, GBB's, GBBR's, HPA, spring, etc, they all have the exact same shot potential.
What matters to the BB is the pressure of the substance being used to center it in the barrel, and the consistency of that.
A .30g BB being shot at 1.6j from any of the above type of gun has the exact same potential for shot performance.
The BB doesn't care WHAT medium is being used to propel it; air, propane, CO2, nitrogen, compressed farts, they all do the same thing.

Barrel length doesn't matter because a) the hopup is applied at the very start of the BB's travel, and b) the rate at which the BB becomes centered in the barrel is proportional to the initial gas pressure generated.
What that means, is a gas pistol with a 5" barrel shooting 1j is going to perform the same as say an MP5K with a 5" barrel shooting 1j on a ported cylinder.

Okay so now range and accuracy have nothing to do with barrel length, since once the BB is centered in the barrel, it is centered, and no extra amount of barrel will ever make it any MORE centered than it already is. Depending on your initial pressure it could be centered in the first half inch, or the first 4 inches.

Barrel bore has nothing to do with range or accuracy either. Since the BB never actually touches the barrel wall, it just rides a cushion of compressed air down the barrel. So the size of the bore doesn't matter as long as the BB is still riding the air cushion.

When we talk about accuracy at range.....
Your gun is dead accurate to 160ft, like we're talking groupings the size of a quarter, but it has trouble hitting a volkswagon beetle at 240ft. Now think about this really thoroughly; do you think the accuracy loss between 160 and 240ft has to do with your barrel? Or do you think it has something to do with the BB and the hopup?
Factually, your barrel only makes your gun accurate within the first 50ft or so. Beyond that, hopup and BB weight take over.
Because the BB quite literally flies itself to target (the magnus effect turns it into a little wing), it's weight and the stability of it's backspin are what make up the bulk of your accuracy at long range. The rest of it is how rigid your barrel group is, fps consistency, air bubbles in the BB, various outside factors. But the majority of the performance is dictated by BB weight and backspin.

Simply put:
-If your BBs fly straight, then flay out wildly in random directions, you need to use heavier ammo
-If your BBs fly straight, drop gently, but the flight path varies slightly left to right, you need better BBs or more likely a better hop rubber. Sometimes this can be caused by the slack in all your parts (like the inner barrel being loose in the outer barrel, etc)
-If you get better range with a .25 than you do with a .28, you need a better hop rubber that puts more backspin on the BB
-as a rule of thumb, to select BB weight, keep increasing the weight of the BB until you start losing range. So if you run .30s, then gain range on .32s, then lose range on .36s, then you should use .32s, or upgrade your hopup to use .36s