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AR15 Type Receiver Modifications
This is a short page with instructions on a relatively benign modifications to the AR receiver that will benefit owners and gunsmiths. The first is a fairly simple alteration that is set up to prevent the loss of small parts that often come up missing when a stock is removed from the rifle. This modification is basically a retention system for the rear takedown pin detent and spring assembly. If you do not have this modification, the spring and detent can be lost when the stock is removed for maintenance or storage and transport in a small space. It is also common for gun dealers and small vendors to sell AR lowers with the internal parts installed, and performing this upgrade makes it easier to sell the lowers as mostly assembled with less risk of losing parts between the time the lower parts are installed and the time a stock is installed later on.
This picture shows a standard factory AR type lower (these from RIA). The parts shown below the receiver on the right are the ones that you can reduce the risk of losing when the stock is removed. Thus leaving you only to worry about the buffer retainer and spring. The receiver on the left shows the small screw being used to the spring, and in turn the detent and takedown pin, in place. If any of these parts get lost, the gun can be rendered useless. That is a real problem considering there are times a survivor may have to remove the stock for storage or transport.
Most of the tools you will need are shown here. Basically a tap in size ********, a tap handle, a Dremel or similar tool with a cutting disk and a pair of pliers with a wire cutter. You will also need your handy small screwdriver and it would help to have some tap fluid and touch up gun blue. Have one small fine thread screw on hand, it will not matter much whether it is a slotted, phillips, or hex head screw since you will be cutting the head off. The screw threads should match the tap.
Install the takedown pin as usual, complete with the detent and detent spring. The detent springs that come with the various part sets are often different lengths and even slightly different wire diameter, which will alter the number of coils. Either way, run the spring in until it stops and see how much is sticking out the back. Normally you would push it all in and it would be held in by the stock. In this case, check the length that is sticking out the back and cut off about 1/8" more than that much with the wire cutters. This should result in the spring being about 1/8" short of the opening when you put it back in. Pull the spring and detent out and remove the takedown pin.
Using a little bit of tap fluid on the tap, feed it in until just before the tap handle starts running up on the back of the pistol grip. Back it out slowly and carefully. Note that the tap will try to "wander" a bit when you first put it in, but it will straighten out as it goes deeper.
Once you get the tap back out, use a pipe cleaner to clean the metal flakes and chips out of the hole. Give it a few shots of spray cleaner/lubricant to get any other crap or gunk out of there.
You are finished with modification to the lower, now for modifying the bolt to become a plug for the detent spring hole. Just put the screw in and turn it to where you still have the head, plus a little under 1/4" of threads sticking out the back. It should have at least 1/8" of thread inside the lower, with just little bit of tension up against the detent spring.
Fire up the dremel (or similar moto-tool) with a thin ceramic cutting disk installed. Cut the head off the screw and flatten out the part that remains. Then cut a single slot in the part that is still sticking out. You will then be able to use a small screwdriver to turn the new headless screw until it is flush with the back of the lower reciever. This will put more tension on the spring and thus the detent and takedown pin. It it seems that you are cinching the coils up too tight and the pin will not budge, back the screw out, remove the spring, and cut a few coils off of it. If the screw/plug is long enough, you may grind some length off of it, but you need it to be around 1/4" long to make sure it holds under pressure.
You can use a little bit of touch up gun blue on the screw to make it look professional, then run it all the way in flush with the back of the lower. You are now done with this simple modification and have very little risk of losing these parts again during storage or maintenance.
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