AR Build, Buffer Tube and Stock

I chose to replace my buttstock and lower receiver extension tube. The lower receiver extension tube, commonly known as the buffer tube, comes in two sizes: mil-spec and commercial-spec. As far as I know, all commercially available Bushmaster's use the commercial tube. Either will serve you fine, but I chose to swap mine out for a mil-spec tube, as I want my entire rifle build to be mil-spec. There is a detent spring concealed beneath the rear plate, and I have yet to find a way of removing the castle nut without destroying this spring, so I went ahead and ordered a new spring set while I was at it. Although either buffer tube is fine, they are different sizes, so you have to know which size you have before ordering a new buttstock.

First, the standard buttstock has to be removed. This is accomplished by sliding it all the way to the rear, and then lifting up on the lever. It can now be slid off the end of the tube. You can now tell if you have a commercial or mil-spec tube. The commercial tube has a slightly slanted rear, while the mil-spec has a vertical rear. Also, the threads on a commercial tube are at the same level as the tube, while on a mil-spec tube the threads stick up higher than the tube. This is actually why commercial tubes exist. The military specifications call for a tube that is threaded, and then the body of the tube is milled down below the thread level. Obviously, this is expensive. The commercial tube begins with the same radius as the mil-spec tube, and gets the same threads, but the tube is never milled down. Instead, the buttstock comes with a larger aperture. This is significantly less expensive to manufacture. The mil-spec tube is a bit stronger and a bit lighter, but you are unlikely to notice a difference. Again, I only chose a mil-spec tube because I wanted an all mil-spec build. This sort of simplifies things.

Once the buttstock is removed, the castle nut can be loosened. Hold the rear plate in place while you do this, as it is spring loaded. If you do it right, you can even get the detent spring out without wrecking it. I have generally been unsuccessful in this regard. Loosen the castle nut as far as it will go, and then gentle remove pressure on the back plate. The detent spring should come out, along with the (very tiny) detent. I recommend putting these in a ziploc ASAP. The next step is to remove the tube. This is done easily by unscrewing it, but there is a stop pin under the lip of the tube which is under spring pressure. Be sure to hold this in place with a finger while loosening the tube.

The new buffer tube is installed in the reverse. The most important thing to be careful of is the new detent spring. Be careful not to crush it. Once the new buffer tube is installed, it is time to add the buttstock.

For this build, I chose the Magpul ACS buttstock. I chose this for three reasons.
1) Magpul makes some tough, high quality products.
2) Superior cheek-weld.
3) Storage space.
4) much cheaper than the UBR stock.

Again, this is a "real life" build, and the ACS retails for around $100. The UBR is upward of $250. There are other storage compartment stocks out there, such as Troy's nifty Battle Ax stock. This stock has almost endless storage space, but is heavier, more expensive, and in my opinion uglier. Also, I don't want to carry a whole bunch of gear in my carbine, just survival basics. Weight is important.

The ACS is a straight forward install, but the polymer "bullets" provided don't work very well for raising the pressure plug. I used a slim allen wrench. The final product is quite handsome.