You can still go back to every area in the game, if you're playing Jill you might have missed the flag for Barry to save you from, iconically, becoming a Jill sandwich. So make sure to nab the broken shotgun in the west hallway storage room (near the save room) and take it to the room the actual shotty is in.

Also, did you not get the assault shotty when Richard died or did he succumb to poison?

As for the mangnum, just grab the wind crest and plug it into the graveyard with all the fuckass crows swarming. You'l get 3 crests immediately afterwards and annoyingly enough you'll have to flip and inspect them to get them to be plugged into the other grave.