Well since you feel you need a heavy load you shoyld have picked a 45 maybe !! Reason why You have plenty ole 45acp loads then there the +P loads and finally nothing more than a heavier recoil spring you could run 45super . .
Now I did give some options you might have over looked . If you feel you need 180gr then why not a federal , remmy , winny and hornday or something more basic made like pmc . S&B has had more than its share of problem ammo and heavy flat nose loads have been a problems in other handguns .
Since you came to air your issues here you have to expect reply's you may not like . But hey sorry to offer some unwanted advise about how YOU could check your mag issues with the slide off the frame but that's your problem to deal with .
The M&P 10mm is amongst the newest production 10mm offerings. The 10mm 220gr by underwood is a massively popular loading, and within SAAMI specs per underwood. This is not the pistol failing to feed, failing to eject, or any other similar issue, it's a bizarre mag drop problem that doesn't seem to impact the other M&P 10mms, nor the M&P 10's main competitors, the Glock 20 and 29.
So the solution to his factory pistol not handling a very common factory (if boutique) loading in a very bizarre way is to...go to the .45 ACP so he can deal with modifying it (which is often more than just a recoil spring...) to shoot .45 Super, a less popular round with less factory support that is pretty equivalent ballistically to the 10mm that he has? Why?
No, his pistol should work the first time around, or S&W should make it right. Ideally I do think the suggestions to have another person shoot it (or even shoot it weak hand) to make doubly sure it's not accidental magazine release contact, but the recoil really is nothing to write home about and I'm willing to take his word that he can properly shoot a handgun without dropping the magazine inadvertently like he does any other handgun.