I know it's perfectly legal to do in both divisions (as long as you're not using those mag sleeves that try to blend the "too long" mag body into the frame's grip)...but I wonder how many make a habit of shooting subcompact or majorsub with mags from a larger Glock? The reason I ask is, I bought a G30 specifically for shooting majorsub, but I haven't been able to find deals on G30 mags. CDNN recently had (may still have) used G21 mags for about $13...and I couldn't pass up that sort of deal. I took the G30 to the range today with my new used G21 mags, and I think I actually shot the G30 better with the G21 mags than with the 10 round G30 mags. I did have one round fail to chamber. It was the first round in the mag. I'm not sure how it happened, but it was the opposite of a feed ramp nose dive. Right after it happened, I took the gun apart, dropped the offending round into the chamber, and dropped it back out, put everything back together, and it chambered no problem the second time. I guess I should measure the distance between the feed lips on all the G21 mags. Maybe this one was a little too wide?