Hey everyone, happy Independence Day weekend! I have what might be a stupid question. Recently purchased a M&P 15 sport 2 (NJ) compliant. I ordered 2 different stocks to replace the factory one. A Magpul and a strike industries. I was under the impression that the sport 2 was Mill spec. But either one of the stocks fit. Way to tight. They won’t even go on. I am new to all this, but am I missing something? Thanks for any help.

Generally speaking the commercial buffer tube has a slanted back. But I was under the impression that all M&Ps had/have milspec tubes. Can you measure the outside diameter?

So I’ve been back and forth with strike industries about this. I ordered the mil-spec stock and it does not go on. At all. The buffer tube is cut straight so to my knowledge it’s mil-spec. The bottom of the buffer tube (where the adjustments are) is square. As to the strike industries and well as the Magpul stock are rounded. So is it the buffer tube or the chance that 2 stocks are either made wrong or what. I’m confused. Thanks for the help. I’m at work so can’t measure it right now.

I have three M&P Sport II's, and all three have milspec Magpul stocks installed.

Doesn't NJ require the stocks to be pinned and non-adjustable? Not sure if that requires a different stock than a normal milspec or not...

If you have a milspec stock and try to put it on a commercial receiver extension (aka: 'buffer tube'), it won't fit. Conversely, if you try and put a commercial stock on a milspec receiver extension it will be a sloppy fit. Most, (but not all) commercial receiver extensions have a slant back to readily identify them as such. Since you tried 2 different stocks that you believe to be of the milspec configuration(s), my guess is that you have a commercial receiver extension on your AR.

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Hoping this might be easier to read.

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Dave Brown
SWCA #3279

I would take it in to a LGS, they will be able to help you out easily...

There's a very easy, non-technical way to identify a mil-spec receiver extension for the carbine. Just aft of the threads that screw into the lower receiver, the outside diameter will neck down for the length of the extension to save a few grams of weight to help meet weight spec for the complete firearm. You'll be able to see this change in tube diameter with your eyes. If there is no change in tube OD, you have a commercial extension.

When they were first introduced my former employer bought a slew of M&P 15's . They all came with commercial receiver extensions. But, that was some time back. Unless you're swapping stocks, which extension you have won't matter.

@WR Moore I see what you are saying. Looks like the buffer tube stays the same the whole way through. So that would make it a commercial spec correct? Being its NJ compliant, are there any stocks that I can use other than what it came with, or do I have to replace the buffer tube and spring as well. Thanks for the advice.

I have no clue about what the Peoples Republic of New Jersey will allow you do do with the stock. Logically, if you're replacing a pinned cheek piece/stock with another design and it's also non-adjustable, I'd think that would be legal. But you need to check with the NJ State Police as logic and statute law aren't necessarily the same. If legal, you should be able to fix the stock length where you want it.

As to sources of stocks that will fit commercial tubes, check out Brownells. Midway might also have some. Mag-Pul makes a bewildering array of AR accessories. If none of those float your boat, then changing the receiver extension to a mil-spec one is possible. But, the extension has to be installed correctly. Look at how the current tube lines up with the spring loaded recoil spring retention pin before you remove the current item. A replacement extension has to go in the same. You'll need a spanner wrench to remove the castle nut.

OP, how much did you pay for it? I'm probably going to post that question, because in my area the price went up from $659 to $820. I don't know when the price increased, but I wonder if the Highland Park, IL shooting on July 4 increased its demand in particular.

MILSPEC all the way.

I always thought the Milspec tube was a smaller diameter than the commercial tube.

It is, but not where it screws into the receiver. That's full diameter like the rifle receiver extension. Or at least that's the way it's on the drawing. I've seen some examples where the threaded section shouldn't have passed QC, but on something like that they don't inspect every part.

When the RFP for what became the M4 came out, it specified a weight, with fully loaded 20 round magazine, of 6 pounds. Colt saved weight every where they could, including reducing the tube outside diameter except where it threads into the receiver.