.25g or .28g bb ?

[ThunderCactus]

I, too, many years ago found GBBRs somehow performed much better with heavier rounds, but there is a valid reason for that:

The only reason heavier weights perform better is because they decelerate less slowly in flight. Provided you can put enough backspin on them, they'll always get better range than light rounds until gravity starts overcoming the magnus effect.
That is, there's a point where a BB is so heavy and traveling forward so slowly that the magnus effect can't successfully counteract gravity very far. It's basically the same effect as not putting enough backspin on a heavy round.

That being said, the method of propelling the BB differs little *within a certain area*.
A ported aeg, GBBR, and P*, can all have very similar initial pressures when the BB first passes through the hop rubber.
More initial pressure, means you can apply more hop pressure before jams occur, which means more hop forced onto the BB.
The reason GBBR's seemingly do better on heavier ammo has little to do with the fact it's using pressurized gas (otherwise the exact same phenomenon would be apparent in HPA rigs, and some ported AEGs can generate well over 80psi of initial pressure), but because it's using a gas with a serious cooldown effect. THEREFORE they need to use a softer silicone based rubber in order to maintain hopup effectiveness, the byproduct of which is a softer rubber has more GRIP, and therefore is better suited to applying a proper amount of backspin on a heavier round.

If you put a nice soft 50 degree guarder rubber in a 1.6j AEG, you'll notice the range increases quite a bit on heavier ammo. Downside is softer rubbers wear out very quickly.
Rhop offers a soft, grippy silicone surface over a long distance. It requires less pressure to apply the same backspin which also significantly reduces the wear.

Problem with AEP pistols and long barreled guns that need full cylinders is that the initial pressure where the BB is being pushed past the hopup is very LOW. Meaning it's very easy to jam it by applying too much hop. This is why AEP's seem to have such crap range in comparison to gas pistols of the same muzzle energy.
There's also the matter of joule creep; pistols tend to maintain their muzzle energy very well, whereas AEP's and guns with under-volumed cylinders tend to lose muzzle energy as the BB weight increases.

As far as the difference between .25s and .28s....
They used to differ in manufacturing, where the .25s tended to have bubbles in them. Manufacturers used air to lighten the mixture to produce .25s.
The side effect of the large bubbles was a poorly balanced round that tended to wobble in the air, giving complete crap accuracy.
Recently they've been doing a better job of dispersing air pockets and having lots and lots of micro bubbles instead, resulting in a much more accurate .25 BB
Now in the 150ft range, I agree there's not a whole lot of difference between a .28 and .25, but where the weight really comes in handy is beyond 150ft.
Because heavier objects decelerate more slowly than lighter ones, the .28 carries it's momentum much further. Provided you have the backspin necessary to keep it afloat, the range will increase accordingly.
If you have, or know someone with, a gun that shoots in the 260-300ft range I encourage you to test the difference in range and accuracy between .25s and .28s because it's actually quite a substantial difference out at range.
.30s are becoming far more common in 1.5-1.6j range guns. And despite the added cost, it's definitely worth it for me. But I sometimes get asked "why not use .28s? It's only .02g lighter, how much difference could it make?" well a .28 is only .03g heavier than a .25, and it makes quite a big difference....

The last point I want to make is a quote from another thread on how to know when to use heavier ammo

For tuning, if your BB flies straight, then ends it's path by flaying off in a random direction, you need to use heavier ammo. Technically you do want it to fall short. At the end of a straight path of every BB, the forward velocity will become so slow that the magnus effect will take over completely, and depending how much the backspin has degraded, the BB is now very likely wobbling due to changing air conditions applying friction at differing amounts on the BB, and so combing the wobble and magnus effect that increases as forward velocity decreases: it'll veer off in a random direction. You want to balance between gravity and the magnus effect; Ideally, your BB should fly as far as possible, but succumb to the force of gravity before the BB flays out randomly. Using a BB that's too heavy will simply shorten your range.

Wow this post is a lot longer than I thought it would be....
Also, if you go back 4 or 5 years, I bet we'd have the exact same train of thought on how guns work, and we'd probably have the same progression of understanding. You're making all the right observations =)