They are both great guns. The .45ACP (with my G21SF) is just like a mellow pop, while the 10mm (in my G29, and my friend's G29SF and G20SF) is more of a mellow kick, if that makes sense. They are both pretty smooth recoiling, but the 10mm has a good bit more push (with full-power loads). I think snap or flip is about the same. With the G29/G29SF, you get a good bit more recoil and flip, but it's still VERY controllable. The G20SF, on the other hand, is such a big *****cat. You definitely know you're shooting something with lots of power, but the full grip and the added mass make it much smoother. Recoil with the G20SF is not much more than .45ACP+P in the G21SF, even with the hot stuff. A lot depends on the powder, too. Some powders tend to be a bit snappier, but I would never really call 10mm recoil snappy.
It's amazing how much the G20SF tames the recoil. After buying a G29SF to complement his G20SF, my friend said he wanted to put up the G20SF for a while to shoot the G29SF. He said it was because the G20SF really tames the recoil so much. With the G29s, you really know you're shooting something with some power. Another aspect that doesn't get mentioned is the noise. The 10mm guns are so much louder. And they have a deep, cannon-like report. It's a really unique experience with an automatic, and that's what makes them so fun. You will never leave the range with sore hands after shooting 10mm. I usually shoot at least 300 rounds, and almost all are always full-power. It won't numb your hand or anything.
The accuracy of 10mm is exceptional, especially at long range. We were at the range the other day, and we started shooting at a 12oz. water bottle out on the berm at 125 yards, just for fun. We never dreamed we would hit it. It was amazing. At that range, we were aiming right at it. We didn't even have to hold high with the sights. All of our shots were within about 12" of the bottle, and my friend hit it once. I never got lucky (we were low on ammo, and wasted the last of it on this). But I would be confident that I could hit a man-sized target at that range 8 out of 10 times, and I don't think I'm that great of a shot. There was no appreciable bullet drop, and this was all with a 3.78" barrel and factory polymer sights. While I didn't shoot the G21SF on that trip, I've shot at a 1"x2" railroad sign at 130 yards before, and had to hold the front post about 70% above the rear just to get close. Bullet drop was noticeably greater. Shots out to 75 yards on a milk-jug sized target are not very hard with 10mm.
The last thing I'll say about 10mm is that its versatility is amazing. You can buy standard 10mm range ammo that will have less felt recoil than .40S&W, but still more power, due to the slide mass. With my reloads, I shoot anything from 180gr bullets at 850fps to 1300fps. A range of 450fps with that bullet weight is awesome. Sorry for the long response.
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