40 is a snappy round by nature, the recoil actually surprised me the first time i shot my bros FN 40 because it seemed more energetic than my glock 30 in .45 ACP.
what brand of 10mm were you running? the reason i ask is because some 10mm ammo is actually not loaded very hot. when i picked up my glock 20 i told my FFL dealer i wanted some hot 10mm ammo and he sold me a few boxes of PMC, recoil felt weak. but other types of ammo have alot more power behind them.
the AR15 is one of, if not the very best rifle ever made. the more i shoot mine the more i love that platform of rifle.
put an ACOG TA31 on it and you have literal perfection