Will a MR556 lower fit a 6.5 upper?

Depending on the fit and the trigger group, it should be okay. I've shot my MR556 upper on the same lower that I use for my 6.5 Grendel without any issues...

Thank you for the reply. I don't know all the hr556 trigger groups so which one will work with a good quality upper, for instance an Alexander Arms? A lowers caliber is defined by the magazine I think but I may be wrong here. I an looking to buy an hk ar in the hope that it will be as versatile as a any generic custom build but with the outstanding hk quality. I'll mostly shoot 5.56 but I want the option to switch uppers. A match quality custom with another upper costs about the same as the mr 556. I'm ok with the fact the hk costs more but for the cost I'm hoping for versatility. I hope this helps make my question alittle more clear. Btw I've seen a few threads where owners have switched hk uppers to other makes of lowers but that isn't helping much. I hope I've intrigued you guys and I look forward to your replies. Thank you.