[.357SIG]

You want to hear about technicality, so don't get mad. You are slamming the mags in WAY too hard. If you are pushing with so much force as to possibly pinch off a piece of skin, you should tone it down. Remember, the magazine isn't the strongest part of the gun, and beating on them can damage both the magazine catch and the polymer stop built on the magazine, as well as the counterparts on the gun itself. Training I've received from various sources has never required more than a good, firm push with the heel of your hand. Additionally, this method is more natural when doing a reload.

Having said that, you're just jarring the slide loose with the force of impact. To the slide catch, it is no different than thumbing it down and will not cause any abnormal wear.

The other part is that you're relying on it in your practice most of the time. It isn't the best thing to use as a training aid, as it isn't supposed to occur under normal conditions.