You have quite a list of options there. I’m going to list them here and give what I feel are honest benefits and minuses for each weapon.
LWRC M6A2-pistons systems are clean, yet heavier than a DI system.
LWRC M6A3-
LWRC REPR----love this weapon. 762x51 NATO. Don’t like the removeable toprail. Have seen some LWRCs that didn’t return to zero. I can’t stand to have rail systems that are out of whack. Consider the LMT .308MRP as an alternative.
NOVESKE----builds absolutely solid rifles. I like their joint venture w/Vltor on using a VIS upper, and you can put whatever stock AR15 barrel you want. Love having a continuous toprail for positioning your eotech or aimpoint anywhere on it and not losing zero.
LMT----if you’re thinking MRP in the AR15 platform, and thinking 6.8 like it looks you might be given all the other weapon systems listed, I’d stay away from this one. 1.10 twist just a little fast and non-spc2 chamber. 5.56mrp GTG. I know it sounds funny because my sign in is LMTRocks, but if they did a 6.8 in spc2 for the MRP, I’d sell my Grendel in a heartbeat.
BUSHMASTER ACR—a project that has been jumping manufacturers for a few years now. Magbushmasteremington. Not sure if specs evolve or de-evolve during the whole process.
FN SCAR LIGHT—F’N AWESOME. 'Nuff said.
FN SCAR HEAVY
ROBINSON XCR----barrel/bolt/mag interchangeability. If I had the $$$ I’d purchase one of these. I think it shoots 6.8spc/6.5G/5.56nato/7.62x39—not sure about endurance testing.
AK47-----uberreliability/lacking in accuracy.
SL8 Modified into a G36
PTR91----heavy. Too heavy for what it is imho. Did I mention heavy?
Sig 556—haven’t shot or handled one enough to know anything about it.
FAL—another piston driven system that has been around for a long time. There’s a very good reason it’s been around for a long time. It’s one of the most popularly fielded rifles in the world.
Kriss TDI----very new, if it breaks parts may be hard to get. low recoil, hi capacity magazines in .45acp and they’re compatible with your Glock 21, if you have one.
Unfortunately you mentioned you have to have 16.1" bbls----I’d quickly say look at the LWRC PSD in 6.8 because of the compactness and punch the 6.8 provides.