You cant use a 45 can with a 556…
My mistake, … Bo?
Some 9mm and 45 cans are rated for 300blk but not all, and they may have restrictions on subs vs supers. You need to do your research.
Yeah, don’t wanna go in ½cocked 
… I only go in ,full cocked! 
Good input you are getting. I found out about this upper first hand, not mine but a friend. He was wanting to do the same as you but first troubleshoot feeding issues. Easy fix…l grabbed one of my Lancer dedicated 300 mags and that stopped FTF compared to the Magpul 556. When we removed the handguard he realized that what he wanted to do (like you) isn’t going to happen.
He ordered a PSA railed upper, got a adapter for a can and Lancer 300 mags. Happy shooter
But the Marauder sure does put out a crazy flame in low light! (and broad daylight too - just not so spectacular)
Well , I just took it to a gunsmith this morning and he made it seem like no problem… So I guess that remains to be seen, he said up front that he had never worked on one before but I suppose I’m giving him the benefit of the doubt. We will just have to see what he comes back with, hopefully it’s not just a pile of spare parts.