Sand
I used to do it against my kneepad or thigh.
I was really more concerned about sand in the mag than seating. I generally repacked my mags every day or two, and was pretty particular about the way I loaded the rounds.
Surgical gloves covered mags for daily duty, and came off for missions/guard/convoys.
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"I won't be wronged, I won't be insulted, and I won't be laid a hand on. I don't do these things to others and I expect the same from them." - John Wayne
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