Question about the M&P 15

RAIDENelder · Post by **RAIDENelder** » 01 Oct 2010, 04:40

This is a simple question that i think i even know the answer too but i want another opinion anyways.

Is the S&W M&P 15 have an "AR" Lower (i.e. will it work with my Beowulf upper from my last thread and the .308)?

I think yes but i am not sure, so i ask.

Anyways, thanks and as always thought an opinions are always appreciated.

Post by **jmz5** » 01 Oct 2010, 04:50

308 is different from the standard AR lower. The Beowulf will work with your standard lower.

sabotteur · Post by **sabotteur** » 01 Oct 2010, 04:52

The M&P 15 has a Milspec lower, so it will work with any milspec upper.

As for being compatible with a .308 upper, in theory yes, but unless you've figured out a way to get a .308 bullet into a AR-15 mag, you aren't going to be able to use it as the magwell is fixed for a ar-15 mag.

flyingirish04 · Post by **flyingirish04** » 01 Oct 2010, 05:49

sabotteur wrote: As for being compatible with a .308 upper, in theory yes, but unless you've figured out a way to get a .308 bullet into a AR-15 mag, you aren't going to be able to use it as the magwell is fixed for a ar-15 mag.

That isn't quite true. The AR-10 Lower and Upper is much larger and the bolt is much wider than an AR-15. There is no way possible, theory or otherwise that AR-10 uppers can ever work with an AR-15 lower, and vice versa.

sabotteur · Post by **sabotteur** » 01 Oct 2010, 06:43

Hmm....

I stand corrected. I guess I'm getting my facts of commercial vs. some of the stuff we have in development mixed up. I could have sworn that somewhere there is a interchangable lower out there that has an interchangable magwell. Must be some SBA program or something from my real job. I swear I am in information overload any more these days.....

My apologies and thanks for the correction Irish...

57FNLover · Post by **57FNLover** » 03 Oct 2010, 05:57

I.m curious too...
I own a S&W M&P15 along with a DPMS AR15.
Should the buffer spring and plunger in the lower be changed when changing the uper to another caliber?
I'm going to get a 5.7 upper for the DPMS and am wondering where to go with this....

SpaceCoyote · Post by **SpaceCoyote** » 04 Oct 2010, 11:19

flyingirish04 wrote: That isn't quite true. The AR-10 Lower and Upper is much larger and the bolt is much wider than an AR-15. There is no way possible, theory or otherwise that AR-10 uppers can ever work with an AR-15 lower, and vice versa.

This IS quite true. There seems to be a lot of confusion on this so I will lay it out.

AR10 (.308) receivers are NOT compatible with AR15 (.223) in any way. To complicate things further, .308 AR receivers and magazines from different manufacturers are NOT compatible like with AR15's. 308 AR's generally fall under 3 categories: 1. Armalite AR-10 style 2. DPMS Style 3. KAC SR25 style (none of which are compatible with ar15).

Caliber conversions on AR10 308 styles are generally limited to cartridges that are based on the .308 (ie .243 Win or 7mm-08) and can usually be done with a simple barrel change as the bolt face is the same.

Caliber conversions for AR15 .223 come in a wide variety and with wide degree of modification:
.223 to .50 Beowolf - change barrel, bolt & mag - or entire new upper
.223 to 6.8 SPC - change barrel, bolt & mag - or entire new upper
.223 to .300 Whisper - change barrel
.223 to 5.7mm - entirely different upper
.223 to 9mm - entirely different upper plus magwell insert
.223 to .22LR - too many different ways to list

This is obviously not an exhaustive list of every caliber conversion, just trying to provide examples.

srt-4_jon · Post by **srt-4_jon** » 04 Oct 2010, 11:24

sabotteur wrote:I guess I'm getting my facts of commercial vs. some of the stuff we have in development mixed up. I could have sworn that somewhere there is a interchangable lower out there that has an interchangable magwell.

http://www.mgimilitary.com/store/index. ... ucts_id=31" onclick="window.open(this.href);return false;

SpaceCoyote · Post by **SpaceCoyote** » 04 Oct 2010, 11:34

I am skeptical of the interchangeable magwell concept.

There is also the problem that different upper's for the .308 have different dimensions on the rear portion (opposite the magwell) where the buffer tube screws in.

Take a look at a pic of a 308 DPMS vs an Armalite and you will see what I'm talking about.

It's looks to me like the interchangable magwell would only work a DPMS upper. It might work with the Armalite upper but there will be a visible gap where that angle is different.

smpsmp · Post by **smpsmp** » 02 Jun 2012, 21:12

Talked to the Colt rep a couple of weeks ago at the shop and I asked him about the LE901, and the availability of lowers, and the magwells. He said as of now the gun is sold as a whole in a 308 set up, then they will release the 5.56 upper and the magwell to go with it. Eventually they will sell the lowers and magwells separate without having to buy a complete rifle or upper/magwell combo.

Post by **c1demo** » 05 Jul 2012, 15:58

Can you mount an M&P 22 upper on a M&P 15 lower? The reason I ask is I put a bumpstock on the M&P 15, and I thought I would try it with the 22 upper to see if it would cycle.

srt-4_jon · Post by **srt-4_jon** » 05 Jul 2012, 16:23

No you can't

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