CDR, there is the occasional firearms platform that defies the conventional wisdom and the Glock 30 is one of those.
I've seen the G21 do 1.5" or a bit less at 25 yards for 5 shots, off the bench, but I've seen the G30 do UNDER 1.0".
If you ask in one of the general forums in GT, you'll see the same with the 9mm Glock 26 versus the 9mm full size Glock 17, and I for one have certainly seen the same with the snubby Glock 27 in .40 versus the Glock 22 full size in the same .40 S&W chambering.
The reason seems to be twofold. First, the shorter barrel gun has a relatively more rigid barrel, when you measure width vis-a-vis length. Second, and perhaps more important I think, is that the G30 in .45 ACP and the Baby Glocks in standard frame size have the double captive spring design in their recoil springs. This seems to better guarantee that the bullet is past the muzzle before the mechanism begins to unlock. That gives a more consistent barrel alignment shot to shot, in relation to the plane of the sights that are aiming the gun.
There is another gunmaker (which will remain nameless here) who made a 9mm double action auto that generally was doing well if it shot a 2.5" group at 25 yards out of the box. However, the company offered a variation for twice or more the price that did an inch at 25 yards. The guys at the "custom" section of the factory told me a big part of that was a different recoil spring assembly that guaranteed the bullet would be out of the muzzle before it began to unlock. Those guns used to go about $1500 retail. You get the same effect in the Glock 30 at no extra charge, with every standard G30 that leaves the factory.
As they say on the Internet, "Just sayin' ..."
The G36 has a narrower, lighter slide than the G30, and this may be why I for one have never seen a G36 shoot as accurately as a G30, on the average.
Just one guy's opinion,