Since I looked for the answer to this question for a very long time, I suspect that I'm not alone. This information was given to me by Pat Rogers. I don't know if these are his words or if he got them from somewhere else, but I owe him a debt of gratitude for providing me with this. I have shared it with a couple of other people, and they had the same "wow, thanks!" reaction that I did. That being the case, I decided to post it up here so that the other two people that read my blog can have that info, too. Thanks, Pat!
"Buffer Tube Basics
I know, the correct name is receiver extension tube... so now that we got one of most common mistakes out of the way, let's look at some common questions about receiver extension tubes. I do not pretend to know everything about these rifles, so if you see anything that is wrong, let me know.
This info is for carbine/short/collapsable tubes - all rifle/long/standard tubes are the same (as far as dimensions go)
What is a Mil-Spec Tube? This has to be the most common question. A milspec tube is just as the name implies, made to the specifications of the technical data package (tdp). The biggest concern is the diameter of the tube, the milspec part has a diameter of 1.150, the thread diameter is 1.1875
So the threads are bigger, are they rolled in? The thread are cut in with a regular old lathe. The narrower body section of the tube is reduced by cutting with a concave shaped cutter that "shaves" the metal down, sort of like a plane.
So what are tubes that are not Mil-Spec? During the original cloning of the military AR, the first commercial receiver extension tubes were made from an extrusion, the threads lathe cut and the body was not reduced. The common size for this is 1.170 and that has became the de facto standard for after market tubes.
So commercial tubes are bigger, are they stronger? No, the problem with them is that the threaded section is also 1.170, so the threads are not cut to full "height" and do not fully engage the threads in the receiver. Nearly all of the tubes that I have seen fail in the field broke at the threads or pulled the threads out.
Are there any other concerns with after market tubes? Well, there is really no set standard for size, so they can vary by manufacturer and even from lot to lot -- you can get combinations of after market parts that are tight or loose. Some of them are made with extrusions with an end plug welded in, the quality of the welding can run from pretty solid to pretty poor.
Are those the only two sizes? No, some manufacturers have come out with systems that do not use either the milspec or the aftermarket tube size... The early Choate stock is one example of a proprietory tube.
So what fits what? Milspec tubes work with stocks from Colt, Vltor, CMT, LMT/Crane, etc.
After market tubes work with RRA, BM, DPMS, etc.
A larger, after market stock will fit on a milspec tube, but will be loose and rattle -- a milsec stock will usually not fit on an after market tube... without a hammer.
Hey! My new Mil-Spec tube is too long? Some newer tubes made for the "enhanced" M4 stock are a little longer and have a small taper to the back to match the profile of the stock butt. It should not effect the use of a Mil-Spec stock however."
1 year ago