I’ve been a LEO for 20 yrs and have shot in M17 to various MSA masks. We use the standard M4 stocks and before that we used MP5’s. Our unit has been taught to cant the rifle in. With new masks most agecies use, they are made pliable enough to accomodate this and end up working well. Practice and getting comfortable with your kit is as important.
Forgot to add… We use EOTechs on flat top receiver. Is there a better set up, probably, but that’s what we have issued to us.
Good luck…
Just tried it the other day. Haven’t done that since I was in, a long time ago. Anyway, with MSA Millennium and a Avon SF10:
MSA - was difficult to use. Had to cant at 45*, could use use with TA-33 but hard as hell.
Avon - was easier, cant at about 30*, easier to pick up reticle in TA-33.
Both were much easier to use with a T-1 in a high mount.
Magpul ACS was hard to get a good cheekweld because of storage compartments. The CTR was much easier for good placement. Less bulky.
Bottom line: Unless you’re kicking a door for something unfriendly, one of the last things you want to have to do is run a rifle wearing an APR. YMMV.
Hell, I figured he was just shooting a Bushmaster Suppressed 
I run a T1 on a KAC nvg tall mount and it helps while wearing a gas mask.
But when we do gas ops I run my pistol if at all possible much easier than using the long gun with the gas mask.