Is The Rail Not Standard? MBUS Problem

[McE]

I bought a Magpul Gen 2 Back Up Sight (front) for my 15-22P.

Yes, it's an authentic Magpul. No, it is not a PTS/airsoft/etc version.


I can't secure the sight to my rail. I have enough looseness and wiggle to make it unusable.

The sight's base is a fixed piece of polymer with a bolt running across it, which seems odd to me that it isn't designed to 'squeeze' onto the picatinny rail like 99% of other rail products that are more clamp-like with two separate pieces to allow lateral size adjustment.

The Magpul sight slides onto my rail very loosely, and the bolt doesn't change that other than sitting in one of the rail slots. Every review on the sight has said it secures tightly, so I have to wonder if there's something unusual about my rail.

Do the M&P 15-22's have an undersized width picatinny rail? Anyone else have problems with this same sight system on their 15-22? Could this be specific to the M&P15-22P?

 

[gkrook]

I've had 2 different sets of Magpul Gen 2's on mine and they both fit a bit snug. My 15-22 was purchased late last year, if that offers a hint of production generation.

 

[Majorlk]

I have three 15-22s and never had a problem with attaching anything. The rail is supposed to match Picatinny specs.

Have you actually tightened the through bolt on the sight?

 

[Rbrtito]

I put magpul sights but the pts just cause I doubt I will use them the only thing is I had to slide the front sight on from the rear forward not sure if that is normal or not but other than that there secured on the rail

 

[mn_doggie]

I bought a Magpul Gen 2 Back Up Sight (front) for my 15-22P.

Yes, it's an authentic Magpul. No, it is not a PTS/airsoft/etc version.


I can't secure the sight to my rail. I have enough looseness and wiggle to make it unusable.

The sight's base is a fixed piece of polymer with a bolt running across it, which seems odd to me that it isn't designed to 'squeeze' onto the picatinny rail like 99% of other rail products that are more clamp-like with two separate pieces to allow lateral size adjustment.

The Magpul sight slides onto my rail very loosely, and the bolt doesn't change that other than sitting in one of the rail slots. Every review on the sight has said it secures tightly, so I have to wonder if there's something unusual about my rail.

Do the M&P 15-22's have an undersized width picatinny rail? Anyone else have problems with this same sight system on their 15-22? Could this be specific to the M&P15-22P?

Nope. The 15-22 rail is standard. I've mounted lots of different stuff from the crappy Chinese fake Magpuls to Aimpoint red dots and they all worked fine.

Since the sights you bought are real Magpul, give them a call. They will be glad to help you.

 

[McE]

Alright, I put the Magpul sight onto a scope mount base (two separate pieces) for my Marlin 336, and it is snug and secures as it should, so the sight is not the problem.

When I have time later, guess I'll have to take some calipers to the rail on my 15-22P and see what's going on.

 

[Gopher Slayer]

Yes, the 15-22 rail is slightly smaller than standard 1913. A lot of aluminum handguards made for the ar15 are either a little bigger or smaller as well.

Anyone who has ever shared an optic with a QD mount between an ar15 and a 15-22 will tell you the same. They always have to go one or two more notches on the tension adjustment.


I'll ask the same question as Major...did you tighten the through-bolt?
Use the right screwdriver so you can get it as tight as it needs to be.

 

[Muss Muggins]

My 15-22 came with the MBUS, but they're the standard Magpul set. I just installed a similar set on my DPMS LR-308 and had to slide them on the end of the rail as an earlier post by Rbrtito described. I've got all kinds of stuff attached to the various rails, and nothing is loose. I've got the same red dot on the DPMS and the 15-22, and a quick check revealed no difference in how far down the screws are tightened. No clue about your issue.

 

[McE]

It's not an issue of tightening the bolt. The piece that sits on the rail is one piece of polymer without any real flex to it, and the amount it would need to flex to secure on this rail is unreasonable.

Like I said, these Gen 2 MBUS do not have the usual lateral tightening that most other rail devices have to clamp on to the rail. This just slides onto the rail from the end, supposedly snugly, then secured with a cross bolt.

 

[RV4driver]

Here's some measurements from rails I have...
M&P15-22 .818
.832
.829
.831
.832
.826
.829
So you can see, the 15-22 is smaller than all the others. I took an MBUS from Magpul off another rail and tried it on the 15-22, it was tight on the one I pulled it from and very loose on the 15-22, but I was able to tighten it without any problem. The area in the vicinity of the crossbolt is undercut so that it will flex. Just get a little more aggressive with it than you would with other rails. No biggie.

 

[Majorlk]

It's not an issue of tightening the bolt. The piece that sits on the rail is one piece of polymer without any real flex to it, and the amount it would need to flex to secure on this rail is unreasonable.

Like I said, these Gen 2 MBUS do not have the usual lateral tightening that most other rail devices have to clamp on to the rail. This just slides onto the rail from the end, supposedly snugly, then secured with a cross bolt.

Yes, the issue IS tightening the bolt! It takes very little flex to secure the sight. It takes all of 30 seconds, if you're slow.

But then, what do the dozens and dozens of us who have done it know ...

 

[Muss Muggins]

Here's some measurements from rails I have...
M&P15-22 .818
.832
.829
.831
.832
.826
.829
So you can see, the 15-22 is smaller than all the others. I took an MBUS from Magpul off another rail and tried it on the 15-22, it was tight on the one I pulled it from and very loose on the 15-22, but I was able to tighten it without any problem. The area in the vicinity of the crossbolt is undercut so that it will flex. Just get a little more aggressive with it than you would with other rails. No biggie.

All I can say is that I have the same set of sights that we're all talking about on a 15-22 and a DPMS LR-308 with an aftermarket quad rail. The receiver rail is part of the receiver on both rifles. I just spent 10 minutes trading them back and forth, front on the back, back on the front (I think that's 8 different combinations, if my probability calculations are correct). Fit was nearly identical, and either set on the 15-22 locked up just fine.

Of course, the OP is talking about a pistol, but I still have a hard time believing that rails are not rails. My Glock Laser Light fits the rails on my Glock, my 1911, my 15-22, my Stoeger Double Defense, and my DPMS.

 

[tacticool22]

Any pics?
You can always put an aluminum pistol length hand guard on.

 

[cjt50]

OP,

It has been my observation on numerous different rifles, (SCAR, ACR, XCR and a trunk load of AR's) that the picatinney rails are all slightly different in size. That's why quality QD mounts have an adjustment to deal with these differences.

As far as the MBUS is concerned, give it quarter turns till it's snug. Don't be afraid to crank down on it, it wont break. You'll be good to go.