real fast AR question.

[bongfoo]

hello all. i'm just wondering. hwat is the difference between a mil spec stock and a commercial stock? is it length, or diameter? material thickness? anyone shed a little light on the subject for me?

[dieselpower]

the outer diameter of the tube itself...basically. i will update this with more info in a second.

Commercial sized tubes are made of aluminum that is pushed through a hole, think of it like a kids Play-dough machine, you load raw aluminum on the chamber and it is squeezed out a hole and comes out in the shape of a receiver extension tube. next they cut threads in the end of it. This is why the threads of a Commercial tube are the same size as the diameter of the tube. In between the threads valleys the metal is half as thick, since the threads were cut. There is no way to cut a thread as deep as the threads in the receiver, so there is a small gap inside between the receiver and tube, this is why the tube will wiggle until the tubes nut is tightened against the receiver end plate. Make no mistake there, just because its not wiggling doesnt mean its tight. I have actually with my own eyes seen a UTG tube pulled straight out of a lower. The firearms sling was snagged on a tree limb, the guy grabbed the tube and pulled with all his weight, thinking the limb would break....wrong ....the tube pulled out of the firearm. So not only is the tube weaker since the threads are cut, the shape of the threads doesnt really fit the threading on the receiver.

Mil-spec calls for the receiver extension tube to be forged, with rolled threads, meaning the threads can't be cut out of the tube, they must be formed by compressing the tube. This is why a Mil-spec tubes threads are higher then the body. The threads are actually made by forcing the tubes metal into mountains. This maintains the same thickness, and the threads can be compressed to the same height as the receiver threading. With a quality tube you can actually feel the threads joining while screwing it in. On a Commercial it just spins on like nothings there.

So just by looking at the threads you can tell if a tube is Mil-spec or Commercial. Be advised though, China has started making plastic Mil-spec sized Air-soft grade tubes. They feel very light, but like metal and are mil-spec diameter. Buy quality. There is a reason that Mil-spec tube is selling for $15 on e-bay....when real mil-spec tubes sell for $45 on AR15 manufacturer websites.

I am sure someone will provide a link to a commercial and mil-spec tube drawing.

[reidnez]

It's the diameter of the tube (technically the "receiver extension"). Commercial tubes are compatible with commercial stocks, mil-spec tubes with mil-spec stocks.

The vast majority of parts these days are mil-spec, that is what I would recommend buying so you have the widest availability of parts. I don't know why anyone saw the need to design a "commercial-spec" in the first place.

[tomd1584]

[dieselpower]

Notice on TomD's drawing the Mil-spec threads are LARGER then a commercials threads, but the tubes outer diameter of a Commercial is larger.

Commercial tubes are weaker both due to the threads being cut out of the tubes body and the fact they are shorter.

[shark92651]

Notice on TomD's drawing the Mil-spec threads are LARGER then a commercials threads, but the tubes outer diameter of a Commercial is larger.

Commercial tubes are weaker both due to the threads being cut out of the tubes body and the fact they are shorter.

Actually the threads themselves are the same on both mil-spec and commercial or else they wouldn't both fit into the same receivers.

[dieselpower]

Actually the threads themselves are the same on both mil-spec and commercial or else they wouldn't both fit into the same receivers.

Common Misconception. The threads on a Commercial are in fact shorter then a Mil-spec. A Commercial tube would need to have an outer diameter of 1.185 to be the same as a Mil-spec.

They do have the same pitch and threads per inch, but not the same shape and not the same height.

[dieselpower]

one the left is the receiver,

the top right is a Mil-spec tube. Notice the threads are mountains and are higher then the tube. The "meat" of the tube remains the same. When screwed in the thread walls will contact the walls of the receiver and have a large gripping surface.

The bottom right is a Commercial tube. Notice the threads are channels cut into the tube. This yields two things, first the "meat" of the tube is thinner, which will suffer from stress, secondly the tops of the threads must be slightly shorter then the receiver threads, due to thread and material +/- tolerances. This yields a gap between the tops and bottoms of both channels. This in turn yields less gripping surface, and is why a Commercial tube wiggles more then a Mil-spec until a nut provides force.

[The Director]

What's the purpose of a commercial tube then?

[dieselpower]

What's the purpose of a commercial tube then?

They are cheap to produce in mass quantity.

[shark92651]

I understand the differences in how they are made, but you may be surprised how many times people ask us if a commercial tube will fit in a "mil-spec" receiver, this is why I made the comment that I did. My point is that the thread pattern is the same for both and the outside diameter of the threaded part is the same.

[pyromensch]

dieselpower,
i was aware of these factors, however i felt the need to copy your description, and save it for future info,
thanks