If you have a Colt LE6920 with a fixed front sight tower I think I would look for an AR with a low profile front gas block and a free-floated hand guard. The Ruger 556 MPR offers that as well as a nicer trigger. With an 18 inch barrel, a rifle-length gas system makes sense.
I have three ARs. One is a PSA assembled from a generic "multi" lower with a two-stage trigger and an 18" barreled upper with a 223 Wylde chamber, a free-floated hand guard, and a rifle length gas system. Some claim a significantly reduced recoil impulse with the longer gas systems. To tell the truth, I really don't notice much difference between this AR and my other two, which are 16" barreled carbines with carbine-length gas systems. The lower that I bought had a light weight "carbine" buffer and with the longer barrel and gas system, I had some failures to feed and a forward (2 o'clock) ejection pattern that I did not like. I switched to an H1 buffer which improved things considerably. But from a wear and tear standpoint, I think there is enough evidence to suggest that the longer gas systems are better, although the difference in wear on the rifle might not become apparent until many thousands of rounds are shot.
One of my other ARs is a Colt LE6920 which was the OEM2 version with a low profile gas block, to which I added my choice of furniture and a free-floating hand guard. The third is a Ruger AR556. But I have also looked closely at the Smith and Wesson M&P15 Sport II, and I have shot a few.
The only one of these rifles I have never had a malfunction with is the Ruger. The unlined, cold hammer-forged barrel of the Ruger is slightly more accurate than the chrome-lined barrel of the Colt LE6920 with 55 grain FMJ .223 Remington or 5.56x45 ammunition. With the Ruger I typically have 5 round groups of around 1 1/2" at 100 yards off the bench whereas the Colt is usually around 1 3/4" or slightly greater. Of course, the Colt has a 1:7 twist barrel whereas the Ruger has a 1:8 twist barrel, for what that is worth.
Most of the differences between the Ruger AR556 and the M&P15 Sport II have been discussed. The Smith comes with a Magpul polymer flip-up BUIS with small and large peep apertures. The Ruger rear flip-up sight is proprietary with only one peep aperture. I like the threaded Delta ring on the Ruger quite a bit. It makes removing and reinstalling the hand guard much easier. But it can be easily over-tightened and jam on the hand guard. Use some anti-seize compound on the threads on the barrel nut, and tighten only finger tight and you will never have a problem. I have never had it loosen during shooting.
Biggest difference between these two are the barrels, the Smith barrel having a slower 1:9 twist rate and being melonited. The Smith does not use a mil-spec trigger guard like the Ruger does, Its trigger guard is an integral part of the lower receiver. The Ruger uses a proprietary front sight tower with pins over the barrel instead of under, as on mil-spec "F" sight towers. Years ago, there was an issue with the Ruger pins walking out, but this seems to have been resolved. The Ruger front sight tower does not have a permanently fixed 1 1/4" sling swivel but instead has a socket for a quick detach sling swivel, which I prefer. Both have very basic polymer furniture but the hand grip of the Ruger happened to fit my hand better than the one that comes on the Smith. The M&P15 uses an A2 style hand guard with the finger notch, which I happen to hate. On the other hand, the M&P16 Sport IIs that I have shot had somewhat better single-stage triggers than the stock Ruger AR556 trigger. Both use "mil-spec" type single-stage triggers which are nothing to write home about, however, and this tends to be one of the first things that gets swapped out on my ARs.
The Colt SP-1 bolt carrier group with the unshrouded firing pin that Ruger uses on the AR556 has been much ballyhooed. I understand the issue that can potentially occur using this BCG with a notched hammer and a large diameter collar firing pin. This is an issue which should not arise if the owner knows what they are doing. Many firing pins are available that are compatible with this BCG. The other potential issues are deformation or breakage of the firing pin retaining pin (cotter pin) or peening of the firing pin collar. I inspect these parts carefully every time I clean the AR556 and thus far have seen no evidence of either. I carry at least one firing pin retaining pin with every AR I own, regardless of the type of BCG anyway.
IMO, the M&P15 Sport II and the Ruger AR556 are much more alike than they are different. Both are solid values if you are looking for another 16" barreled carbine with a carbine-length gas system and a fixed front sight tower. The most significant difference are the barrels. I can't speak for reliability of the M&P15 but those I know who own them speak of them well. My Ruger AR556 gives up absolutely nothing to my Colt LE6920 in terms of accuracy and reliability. But if I were you, I would go with the Ruger 556 MPR over either the AR556 or M&P15 Sport II.
, plus the upper makes them almost as much as a complete rifle.
