This is my set up and it is considerably lighter with the CMMG upper installed. Mostly because of the Surefire Light and aluminum quad rail on the 5.56 upper.
Welcome, I was recently introduced to dedicated 22lr uppers for the AR15 by imschur and I like the CMMG Sierra I purchased better than any of the current crop of dedicated 22lr tactical rifles I've owned or seen. It'll mount right up to your AR15 lower in just seconds and is generally of better quality than the clones. Check them out.
A CMMG upper is what Drew talked me into. Actually he just needed to tempt me with a photo, I'm a sucker for 22s. So far the CMMG is the best tactical 22 I own followed by my Puma PPS. The other tactical22s I own I rate in order: Colt M4 OPS Remington 597 VTR GSG-5 SIG 522 GSG Kalashnikov
Drew, I hate the really crappy Ncstar folding sights I bought for my new CMMG upper and was thinking of these MBUS as a replacement. Would you recommend them, knowing that if they don't work out for me it'll be no big loss? They look SO much better than the Ncstars.
The AK74 muzzle brake is for the 5.45x39mm caliber. Will the larger 7.62 caliber bullet of your Mosin fit through the exit hole? My experience with Richard's micro fit stocks is that they come semi-inletted requiring you to finish the inletting. Stock inletting experience will be helpful.