Got started down this path in the 20-in. AR rifle thread, but it has carbine content, too, so I pulled it out into its own topic.
Originally Posted by OlongJohnson
It turns out there's more to "mil spec" receiver extensions than diameter, but it's damned difficult to actually find detailed information. I did my deep dive on this topic back in November, when I was shopping parts for a rifle build and saw a comment on Brownell's asking if their rifle extension had rolled threads. The answer was no, but I had to pull that thread and destroy the sweater.
It's easy enough to find that a "commercial" tube is expected to have a larger diameter, basically the major diameter of the thread form so the threads could be cut into it from the larger diameter, be made of a 6061 extrusion, and have the end be a piece of plate welded on.
The commonly available information will tell you that a "mil spec" tube is made of 7075 and has a smaller diameter. Most discussions stop there, because that's what you need to know to buy the right butt stock for whatever tube you have. But dig long enough, and you'll find some discussion of forged or impact extruded. Dig longer, and you might find something about rolled threads.
Most sources advertising mil spec tubes are machining them from 7075 bar stock. A few places advertise them as forged, but they are still machining or cutting the threads into them.
Along the way, I contacted BCM and was told that the threads on all their receiver extensions are cut. Searching the web, I found discussion from 5-10 years ago that named several companies offering receiver extensions with 7075 and possibly rolled threads. The only manufacturer that appears to still be a source of receiver extensions with rolled threads is Colt.
I'm not going to get into the whole forged vs. cast debate. That's covered ad nauseum elsewhere. In some places, you may read pedantic know-it-alls saying the TDP receiver extensions are impact extruded, not forged. That's nonsense, as impact extrusion is nothing but a particular forging process. This video shows a part similar in form to a receiver extension blank being impact extruded. Note that this process not only delivers superior material properties, it's a heck of a lot faster way to get a closed-end tube than drilling the middle out of a piece of bar stock.
The reason it's not commonly done is that the equipment is massively expensive; even tooling for processes like this is extraordinarily expensive. Anyone not supplying receiver extensions for .mil-size contracts will never make back the capital expenditure. It's much cheaper to just run the equipment you already have for a little longer and drill the hole. But a fully-machined RE starts as a piece of bar stock, which is an extrusion. The bar stock starts and ends with the same material properties the forging blank has before it's forged. The forged part ends up with refined grain and greater overall strength, at least for equivalent dimensions and alloys.
The other part of the discussion above is the rolled threads. If you really want to get into it, follow the "screwing to win" link in the quoted posts above.
This short video illustrates the basic concept of rolled vs. cut threads very effectively.
It should be obvious that the threads are the most highly-stressed location on a carbine receiver extension; the tube has the most leverage against that point in bending, and the tension of the castle nut against the receiver end plate is not insignificant. Anything that can be done to strengthen the threaded region is likely to be beneficial in overall robustness of the extension.
So I ordered a Colt rifle receiver extension and a Colt six-position carbine receiver extension from Specialized Armament. Received them today.
At the bottom of the rifle extension, on the inside, I can see the radial outward flow of the aluminum grain. I haven't found any other supplier advertising forged rifle REs.
The carbine extension has more thorough coverage of dry lube so no grain is visible. However, the slot for the receiver end plate tang cuts through the threads, and thus provides a cross-section of the material near the thread roots. It's actually possible to see the difference in material where it's denser adjacent to the thread form than it is directly under the peak. Shows up as a difference in the appearance of the anodizing when you get the angle of the light just right.
I thought it was pretty cool that I can actually see the evidence of these manufacturing processes and the resulting material properties in the parts.
At least for this one part, I'm not aware of any aftermarket supplier claiming to make a piece with the "Original Recicpe" features that I am able to verify are in the Colt parts. FN presumably does on its .mil contracts, but nobody seems to know on its civilian issue rifles, and anyway I haven't seen any FN REs for sale as stand-alone parts in the market.
I'm probably going to stick with Colt parts for future receiver extension purchases, as I believe them to be significantly better than any available alternative.
Reply With Quote
