S&W MP15-22 Flex


The Grand Baboon
June 12, 2011, 10:18 PM
As you probably know, the MP15-22 is constructed out of mostly polymer. This isn't a bad thing, except that it's a two part upper. Depending on the position of the rifle (resting on bag vs. magazine) your groups land in different places. What can I do to reduce this flexing? I was thinking on putting a scope riser between the two sections to brace the handgaurd. Is there a better option?

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June 13, 2011, 10:51 AM

Not really sure what you are talking about here. When you say the upper is 2 pieces I assume you mean that the rail is not part of the upper receiver. This is correct on most ARs. However since the barrel does not contact the rail it doesn't matter if the rail has flexed at all. I have never had any of the POI shift you are referring to my my M&P15.

June 13, 2011, 02:09 PM
Can't say I've noticed a problem with my 15-22.....accurate little sucker it is!

The Grand Baboon
June 15, 2011, 11:06 PM
Here's a perfect example of what is happening.
I'm also finding that I just can't get two or three consecuticve groups to land in the same place. I shoot a group, adjust, shoot again, and find the group to be in the wrong place. It's shooting with a brand new Leupold scope, so I don't know what the problem is.

June 16, 2011, 02:42 PM
As far as I know, the upper on the 15-22 is one piece. Are you talking about the handguard being a part that is separate from the upper receiver? If you are, that is how all ARs are (well, almost all). I'm going to guess that you have your optic "bridging the gap" in that the rear of the scope is attached to the upper and the front part is attached to the handguard. If this is the case, that could very well account for the variance that you are getting in the POI.

"Bridging the gap" is a practice that is usually frowned upon for the reasons that you stated. Get the scope off of the handguard. If your scope needs to be that far forward, then you'll need to get some sort of cantilevered mount or rail.

June 16, 2011, 02:45 PM
I took a look at my 15-22. The only thing I can figure is this.

There is a small endcap on the rail that comes very close to the barrel. It is possible that this presses on the barrel a bit. But I don't see how this would have any effect inside of 200 meters. More than likely it is just shooter variable.

June 16, 2011, 03:22 PM

I have noticed this exact thing with my M&P 15-22. On different days (temperatures) and different shooting stances it would accurately shoot to a different POI.

I will send you a PM to a link on a different forum detailing what I am observing and testing to remedy the problem. I would post the link in this thread but I'm not sure if that is a conflict of interest or not with THR.

Suffice it to say I think it is a property of the thermal expansion of Nylon, which is about 3 times that of aluminum and the substantial flex in the rifle as a whole. If you clamp your barrel in a vice (between blocks of wood :D) it doesn't take much to flex the receiver relative to the barrel. It takes even less effort to flex the handguard.

If you are using an optic try removing the forend plug to free float the barrel and see if that clears up your POI shift. This isn't a good option if you are using the irons, as the flex in the forend will really move the front sight about. It is also advisable to not bear down on the rifle too much as it will shift your POI, a shooting sling would almost certainly move things around.

Good luck

The Grand Baboon
June 16, 2011, 11:04 PM
That must be it then. I notice that the problem is more exaggerated as I shoot more and more.

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