Hi,

Buddy has older CA Compliant M&P 15 with commercial size buffer tube, wants to replace with mil spec.



It seems like this can be done, though I'm not clear how since the threads are slightly different and I thought that went direct into the lower...

In any case does anyone happen to know what parts are required for the swap, hoping to avoid a lot of trial and error.

Found some info here on previous posts but still pretty unclear on a lot of it.

thanks for any help

The buffer tube threads should all be the same.

This might help you out.

Mil-Spec vs. Commercial Buffer Tubes Explained - Gun Builders Depot

Thanks for the reply, from the link, doesn't this contradict that?

">>Because the threads on commercial and mil-spec tubes are produced differently, there are slight variations in thread height and measurement. This usually means a commercial tube won't fit a 7075 T6 forged (mil-spec) lower receiver. And a mil-spec tube likely won't fit a commercial (6061 T6 billet) lower, either."

One of the things I'm confused about it, it sounds like a mil spec tube won't fit a lower that has a commercial fitting.... But I do see people doing this...

I have 4 AR's. CMMG, DPMS, PSA, and a Spikes lower. They will all accept any buffer tubes I have. I also have an adapter to put a AR buffer tube on a Mossberg Shockwave shotgun. It too will accept either a commercial, or mil spec buffer tube.

I have a few commercial, and mil spec buffer tubes. The stocks from commercial and mil spec won't interchange. That's the only difference I've found.

I might very well be wrong, and am sorry!

I just checked and I think everything I own is mil spec. I could have swore I had a commercial buffer tube in the mix.

Never heard of a non mil spec lower.

A milspec receiver extension (aka: buffer tube) will more than likely thread right into any lower. You will just probably have more threads showing behind the castle nut on a commercial one. Why does your friend want to change them out? More options in stocks for milspec than there are for commercial stocks I'm guessing?

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OK, going by memory, the thread spec on the lower and the receiver extension/buffer tube is 1 3/16 in by 16 threads per inch. This is a standard thread form, and is used on all lowers and receiver extensions/buffer tubes. The threads may be produced by different methods, there's no significant difference caused by how the threads are formed if the QC parts inspectors are earning their money.

The easy way to ID commercial v mil-spec tubes is diameter. If your tube is the same diameter it's full length, it's commercial. If it necks down (gotta save a few grams to make weight spec) just aft of the threads, it's mil-spec. Unless there's a Jones for a specific stock that's only made for mil spec tubes, there's no good reason to change.

All you need is a wrench for the castle nut, a tube and a set of instructions (including torque spec for the nut)-or good photos/measurements of how far the original tube is screwed into the lower. The tube has to be in the right place, to hold the buffer retaining plunger in place without binding. Be careful the plunger/spring doesn't launch while removing the original tube. Don't forget to stake the castle nut to keep it from loosening.

thanks for the reply and info, sounds great. I'll probably point him at something like this complete kit then that seems well liked and has everything.

Aero Precision Enhanced AR15 Carbine Buffer Tube Kit | Wing Tactical

thank you, yes more stocks

Since a previous thread on the Kalifornistan AR hinted that the buffer tube is unique, the kit in question might be the way to go. It doesn't appear that the buffer tube spanner (wrench) for the castle nut is included.

He should also pick up a copy of Patrick Sweeny's book Gunsmithing the AR-15.

">>Because the threads on commercial and mil-spec tubes are produced differently, there are slight variations in thread height and measurement. This usually means a commercial tube won't fit a 7075 T6 forged (mil-spec) lower receiver. And a mil-spec tube likely won't fit a commercial (6061 T6 billet) lower, either."

That's what happens when you buy the cheapest parts and end up with stuff that was originally (or should have been) scrapped but someone decided to sell it/use it anyway. Over decades, I've been occasionally horrified by what shows up in varioius name brand guns. When things get tight and cost cutting begins, it usually starts in the QA/QC department.