It is possible for the Summer Special to work with both your G21 and G30 . Not the greatest but perfectly do able. Example 1(. Glock 30 Holster with a Glock 21) Your Glock 21s slide will pert rude from the bottom of the holster approx. 3/4" of an inch. This could possibly create issues if you have a high profile front sight. The sight could possibly hang up on the bottom lip of the holster while the pistol is being drawn. This would not be as much as of an issue with a stock front sight. Example 2. (Glock 21 Holster with a Glock 30) Your Holster will be approx ¾ of an inch longer then your Glock 30. If you frequently carry your Glock 30 in this holster, eventually the bottom of the holster may start of cave in because of lack of slide support. As you can see, neither choice is optimal, but will work.