New BCM Complete Upper on an M&P15 Sport lower (for now) - Any compatibility issues?

AR/M4 AR General Discussion
1

So I have decided to buy a new 16" BCM Complete upper!!! (Receiver, 16" ELW barrel, KMR, BCM BCG, MOD-0 comp.)

My current AR is the Smith & Wesson M&P15 Sport …

I will be taking of that stock smith upper and placing the new BCM upper on the M&P15 lower.

(FOR NOW … new lower build is coming next)

Any issue with compatibility placing that upper on this budget lower?

Buffer, trigger, fitment, etc. etc. ???

Just wanting to make sure I am good 2 go when the new BCM comes in and the M&P lower will do what I need in the meantime till I build the new lower.

Thanks so much for all the help guys! :slight_smile:

2

Other than tolerance stacking and variance I wouldn’t imagine so. No idea what buffer S&W is running though, and while a carbine buffer might not cause issues, an H1 or H2 is preferred by many.

3

Not familiar with tolerance stacking etc. … I would like to learn more about that though.

And ya I have seen here and there some end up buying a new H2 buffer with a BCM upper install. (I wonder if I should buy one to be safe)

4

You shouldn’t have any issues using the M&P lower. I would pick up an H buffer if your going to be running a lot 5.56 pressure ammo.

5

Should be good to go. I agree with Kain, if you have any reliability issues look to the buffer first but I think you wont have any issues.

6

And congrats on the new upper! You will be very happy when you get the box in the mail. I just picked up a 11.5 kmr upper that I’m waiting on.

7

I primarily run .223

So it looks like have a H / H2 / Etc. … if I do decide to get one ahead of time should I get the H or H2?

And only shooting .223 99% of the time you think I should be good with the stock M&P lower? (new to AR’s, buffers, etc.)

8

All uppers and lowers are built, or at least should be built to a particular spec. Now most manufacturers have a range that they consider in spec and from one manufacturer to the next there can be variance. Some spec on the high end, some on the low. What sometimes happens if you get two products that run on opposite ends they may not fit together easily or even at all. For example I have an RRA lower that is tight and will not fit an old bushy lower I have, nor a recent production BCM upper(at least not with out a lot of force.) But another RRA upper drops on fine, as does another BCM upper. This would be tolerance stacking.

As far as buffers go. I run H1s in mine, though I have thought of grabbing an H2 or even H3 just to toy with, and maybe stick in an old overgassed Bushy, but that neither here nor there. Again your upper should run with a carbine buffer just fine though.

9

I would stick with a H, if your going to be primarily shooting .223.

10

I concur. The only ammo that has given either of my middys issue while running an H buffer is Tula .223 55gr steel cased ammo, and that is pretty well known to be shit ammo. Wolf, Hornady Steel match, various .223 and 5.56 ammo has not been an issue.

11

So I need to buy the H Buffer or do you think the stock S&W M&P Carbine buffer may work fine?

(I may be buying a new complete lower that comes with an H buffer at some point so I would hate to buy one and I don’t need it and end up with 2 after I buy the new lower that comes with it)

12

I used a standard M&P lower with an H buffer for a long time with a BCM upper. It will work fine.

I have changed out the trigger, added an ambi safety (I shoot lefty), and an A5 over time, none of that is necessary though.

13

The stock carbine buffer will be fine, The rifle will run smoother (less recoil, bolt bounce) with a H buffer. I would run the carbine buffer for now until you get or build your lower.

14

Did you ever run it with the stock buffer? (or did you swap it out to the H right away with trying the stock smith buffer?)

15

I used a stock buffer for a while, too. It worked.

I have a used H buffer laying around, if you want it, PM me, I will send it free of charge.

16

Seriously?

Wow that would be awesome Zane!

PM sent

Looks like I should be good then … this buffer was the one thing I was worried about

I will be sure to pay it forward if someone else needs that H buffer when I buy the complete lower that comes with the H buffer.

Thanks again brother! :slight_smile:

17

And that’s what this forum is all about, generous offer Zane

18

No problem.

19

Yes VERY informative and awesome members here … shown by all the threads I have created over the last couple days and given TONS of killer info … and Very Fast replies by many members (I don;t get that elsewhere)

AND of course this absolutely amazing offer from Zane … Wow … very humbled by the offer brother, can;t thank you enough for putting that thought at ease foe me :slight_smile:

I am gonna hit the range with the new upper AND this H buffer now feeling much more comfortable in their not being any cycling issues.

Thanks again! :slight_smile:

20

When I bought my BCM upper, the only lower I had was a Rock River. Now I had read enough here to start wondering if I needed to get a better lower from one of the ‘good’ companies (BCM, DD, Colt, etc.). I have learned over the past 2-3k rounds that it seems to work fine, and as long as you have an in spec lower made from the proper material (ie. no polymer), it will usually work with no problems. Put it together and give it a try, I don’t think you’ll have any issues.