To allow the upper to be removed, the groove is closed on each end so the pins do not leave the lower when they are pressed out.ĬMMG packs their AR-15 lower parts kits in color-coded plastic bags. The depressions keep the pin in place when fully opened or closed. The groove has two small depressions at each end. These have grooves machined into the side where the detent pin rides. The thick head pin is the front, flat head the rear. The dual push pin system of the M16, including the AR-15s that fit them, use nominal 0.250 inch diameter pins with machined heads. The rear takedown pin must be in place as you install the stock, there’s no way to go back and do it after the stock is on. I’m not a big fan of ambidextrous triggers, simply because my trigger finger tends to rest in the space occupied by the offside lever on such a design. The safety comes in your choice of regular or ambidextrous. You can invest in a standard AR-15 lower parts kit (also known as the LPK) or a premium trigger set. Assembling the AR-15 lower shouldn't cause a brain cramp, but there are a few tricks to make certain it doesn't rattle like a jalopy. Leave it alone and build on something else. Then to re-assemble you push the punch out with the pivot pin and rotate the pin to release the detent into the slot.This is one of the Colt rifles with the two-headed hydra takedown pin. Note, leave the punch in place, if you allow that detent pin to escape you will have to remove the handguard and barrel nut. Then you can push the pin out with a 0.25 diameter punch to keep the detent pin in place. However if you go in thru the slot in the pin with a straight angled blade dental pic with a with a bit of patience you can press that detent pin up into its recess and rotate the pivot pin 90 degrees. The front pivot pin isn't, it can be a bit of a PITA. The rear pin is easy peezy to get free simply by removing the handgrip. Learned this lesson once when I forgot to chamfer the pins on a rifle I had just put together. During live fire any forces produced by recoil are aligned with the direction of the barrel and are 90 degrees to the axis of the takedown pins. Hint, about the only force that would arise is in the event you dropped your rifle perfectly on a stick or rock shaped perfectly to pop the pin free. of force with my thumbnail to release either pin and for those concerned about pins popping loose think about where a force that would overcome this detent would have to come from in actual use. End result is that in only take a press with about 5 lbs. I also only apply the chamfer on the release side because there really isn't any need to do the full diameter. BTW, by small chamfer I do mean small, using calipers measuring in the range of 0.020 to 0.030 inch for the complete width of the ramp, meaning a chamfer defined as 0.014-0.021 x 45 degrees. It only takes a minute or two and makes a huge difference in how the takedown pins release. Since I don't like using a hammer on any of my rifles I have used a small 2mm diamond ball shaped burr in a Dremel to chamfer the detent holes on any rifle I build. Downside of this is that you are using a hammer and punch to free the pins and chances of a missed strike causing damage are rather high. What happens with usage is that over time the force applied by using a punch actually peens a small ramp into the takedown pin which allows it to release with less force. Because when engaged what you have without any chamfer is a sharp edge digging into the 45 degree tip on the detent pin. As a result there is no "ramp" to ease disengaging the detent pin. What is missing is a small CHAMFER around the hole for the detent pin. The reason tight pins are so common is that the holes in the takedown pins are NOT finished as they should be from an Engineering standpoint.
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