AR15 Lower Parts Kit Assembly
If you’re reading this now, you probably read our Lower Receiver build articled featuring Polymer 80, 80% Arms, 5D Tactical, American Made Tactical, and AR15 Mold. If you’re like us, you probably rushed out to buy a kit. Now have concluded your milling session and into the assembly portion of a black rifle build.
The LPK Options
There are many LPK (lower part kit) options available on the market today. There are a few necessary things to look out for. We have used several varied LPKs for our builds to show the diversity and ease of use of all LPKs with different Lower Receivers.
AR15’s are going to vary significantly from AR10 (or more accurately called LR-308) in terms of the LPK. AR15’s tend to follow a very controlled and accurate account of dimensions and specifications set up by the military or sports competitors. These ‘Mil-Spec’ options mean a lower receiver and LPK will all have the same dimensions and tolerances with only slight modifications and differences between manufacturers. This is important because all the holes, parts, springs, and various pins will fit not only one lower receiver but all lower receiver… in theory.
How this differs from AR10 is that there is no ‘Mil-Spec’ equivalent for AR10’s and why they are better, and more accurately, called LR-308. Look for our AR10/LR-308 build in a future article.
So far, we have used and tested 4 different manufacturers on a range of lower receivers. These manufacturers include Aero Precision, American Made Tactical, CMMG, and Unbranded AR. Theoretically, each of these manufacturers should fit every single AR15 lower receiver made, including 80% or 0% kits. While we don’t disagree with this we did want to see if they would actually work on unconventional AR15 Lower Receivers.
Polymer 80 is a company with a moderately long history, in terms of polymer materials. Their lower receivers come with the idea that building them is quick, easy, and fun. While we had difficulties with the actual milling out of the Polymer 80 AR15 lower receivers, we were happy with the customer service and end result. Read about our build HERE. Regardless of the outcome of a milled lower receiver, we still need to make sure the LPK will fit or else it only has a nice paperweight.
One of the first LPK’s we received were from Aero Precision. Aero provides several options for LPK alone or with a buffer tube (the part that contains the bolt return spring and the stock attach to) for either a rifle or extendable stock. Aero Precision also manufactures their own rifles and parts, giving them a distinct edge in the market.
Remember what I said about AR10/LR-308? Aero Precision is one company that makes an AR10/LR-308 that is different than other companies. Regardless, it’s not relevant for an AR15 build but is something to keep in mind when ordering from them outside of the standard AR15 parts.
When we opened up Aero Precision’s LPK it was a bit of a jumbled mess. All the parts were in the same container. Iit was a bit overwhelming for a first timer to separate out what was immediately necessary and what would be installed later. We also knew that there are multiple resources available online in the process of installation of an LPK into a Mil-Spec AR15 lower. So, with that in mind, it is not our intention to give you a step by step process of the installation. Our purpose is to share the common issues and difficulties we experienced during installation.
With that, we began installed the trigger assembly first. The difficulty had, was finding the spring that was meant to go with the trigger assembly. First, it was installed backward and didn’t understand why it kept slipping out when we tested the tension in our hands. It’s important to note that one end is just the perfect width to fit into the cutaway. Once that was figured that out we took off running with no problems.
Next, came the bolt catch assembly. We knew this would be the hardest part of the build from our googled resource on how to assemble an AR15 lower. We set the pieces together and tried to force the rollpin though the hole. With Polymer80 the material expanded and moved fairly easily, but still required significant effort. In addition, we did not put the lower receiver in a block, for fear of warping the fragile material. With that in mind, we situated the receiver very close to our vice and slowly pushed the rollpin in with our vice; only far enough to get the bolt catch in. The rollpin was pushed a little too far, but we were able to muscle the catch in the rest of the way. With that, the rollpin has pushed the rest of the way in and we finished, which we thought to be the most difficult part of the build.
Moving on to the mag catch body we were filling with pride about our last accomplishment only to be torn down quickly. You see, Polymer 80 lower receivers are slightly wider than standard AR15 lower receivers, but the parts kit always remains the same. This meant only one thing, it was difficult for us to screw in the mag catch with the bolt catch installed. The bolt catch always sticks farther out of the side of the lower receiver. We were unable to truly screw the mag catch to the perfect position as it would strike the bolt catch. Taking a pen, we shoved the mag release button as far as it would go and carefully screwed the mag catch a few times. It was just enough to hold, but not enough to feel confident. As far as we were concerned, this was the best option.
We quickly moved onto the safety selector and hoped to avoid any more mistakes. Attempted to insert the selector switch into the receiver and were unable to pass it over and across the trigger assembly. The base of the receiver was not low enough. Deciding to forego complete disassembly we pulled out a router, removed the trigger group, set the lower inside the 80% Arms jig as best we could. Slowly, we removed some small sections of polymer from where the trigger assembly was set.
Despite this set back, it is important to note that it would have been much more difficult to do this with an aluminum lower receiver. Taking small bits of the polymer was both fast and relatively easy. The difficult part was not the milling, but setting the jig up and preparing for the milling processes. It is tedious.
With the extra material removed, moving quickly put the trigger group back into the receiver and the safety selector slid in with no issues. As we examined our handiwork, we realized our zealous drilling for the safety selector had made a slightly misshaped hole on the right side. With the polymer material and the problems in the past with the jig system, the hole was slightly too large. Despite this fact, we thought it best to move forward with the build, but it is something to pay very close attention to.
Each pin, spring, and piece fit into the receiver with little effort. The fire control pocket was built, and the safety selector was firmly placed. We attached the grip, pushing the safety selector spring into the receiver. The screw was tightened, and we have the selector a quick flip into ‘fire.’ Unfortunately, the safety was tough to move. The spring was pushing the safety up to the top of the oddly shaped hole previously drilled. We could easily get it into the fire position, but not back to safe. So, thinking on our feet, we removed the grip and clipped down the selector spring by 4 or 5 millimeters. In retrospect, this was a bad idea. The springs are designed to be used in a very precise way. By clipping the spring, we now changed the effectiveness of the spring as it fatigued.
Having installed the grip it was essential to move onto the next part, the buffer assembly. First, we put the buffer retainer spring and retainer into the hole. It fits tightly, but still moved freely. We screwed in the buffer tube and pushed the retainer down just enough. It was a perfect fit even though it felt a little heavy or difficult to screw in. Our worry was we were re-threading the receiver, but it came out perfect. After installing the buffer tube we added the rifle stock. It was at that moment we realized we had forgotten to put the takedown detents and springs in place. You see, the rifle stock actually holds the rear takedown pin spring and detent in place. So, leaving the buffer tube on we moved to the takedown pins.
As far as ease, the takedown pins are not easy but very straightforward. Put the spring in, put the detent on the spring, push the detent and spring into the hold, and the takedown pin. The front takedown pin requires a little screwdriver to push the detent and spring down, making room for the takedown pin. The rear takedown pin is easier. You insert the pin and put the detent in a small hole in the back of the receiver. Using the spring, you push the detent pin all the way through. With the Polymer 80 frame, the small hole needed to be reopened and cleaned of all polymer debris, on the end closest to the takedown pin. Pulling out a trusty paperclip, swirling it around, and attempted to push the detent pin all the way through there was success. We wish it had been done it sooner.
As we approached the last few pieces, a sigh of relief went up. This was easy but tedious. We were not expecting so many small issues and errors. Pushing the rifle stock on and making sure not to pinch the rear takedown pin spring, we used the screw to secure it.
The last piece was the buffer and buffer spring. Applying a small bit of oil to the spring and pushing it into the buffer tube was as simple as it gets. We had to be careful not to let the hammer stay up while inserting the spring or else the spring was bent too far to insert. The buffer came next and with only a little forced pushed right past the buffer retainer.
A large sigh of relief filled the air as we stood back, taking in the completed receiver. It looked great. The polymer didn’t scratch and still looked like it did when we opened the original box. It was hard to believe that this receiver was once sitting, waiting for us to mill it out and create such a mess.
Overall, the process was straightforward and with a little practice, it would become much easier. We knew the practice was coming soon, as we still had an 80% Arms, American Made Tactical, 5D Tactical and AR15 Mold build to get to.