Novice-to-Novice: Lower Receiver build AR15 Mold 

A few months ago we received a lot of interest in the process of building a firearm. We thought long and hard about going through the paces of a building a firearm, seeing as we are complete novices. It dawned on us, if a novice shows the problems, maybe they can be avoided. So, we jumped on the AR15 band wagon and got to work. 

Now, before anyone attempts to build a gun there are a few considerations to make. The first is cost. Building a firearm is much more expensive than actually purchasing one. Even buying an unfinished lower receiver is about the same price as buying a complete one. When building, we quickly learned that it isn’t about saving time or money but about really making it your own. 

The second consideration is ability. While it may not be extremely difficult for someone mechanically inclined it can be challenging with no prior experience. One mistake can make a firearm incredibly unsafe to operate. If you aren’t a stickler for details than building a firearm may not be for you. 

The last major consideration, when deciding to build a firearm, is why. If you are building one to stay off the grid, look elsewhere to learn. We do not promote building firearms in order to ‘pull one over’ on the government. If you want to make something you are a part of and proud of, this is the type of article for you. Building your own firearm deserves bragging rights and gives a builder an incredible sense of accomplishment.  

Lower Receiver Considerations: 

According to the ATF, a firearm is the lower receiver. In most rifles, the lower receiver and upper receiver can be made from the same piece of metal, or polymer. Building a firearm that requires a one-piece receiver is not only extremely difficult, but requires extensive knowledge and tools. About 15 years ago companies started making lower receivers that needed to be milled or have pieces removed, in order to not be considered a firearm.  

Why AR15? Well, it’s simple. The AR15 platform is modular, meaning, the lower receiver is a completely separate piece from the upper receiver. This gives us a few advantages. There is no need for a plethora of tools or skills in order to produce or manufacture a lower receiver. In addition, you can quickly change the upper receiver (or lower receiver) if you run into any problems. This changing of the upper receiver also allows users to change the caliber of weapon they have. An AR15 can come in a 5.56/223, .458 big bore, 6.5, 6.8, 300 Blackout, 7.62×39, as well as a few odd-ball calibers. While the .308 looks similar to the AR15, the lower receiver has different dimensions making this caliber an AR10 rifle. Keep an eye out for our possible, future AR10 build. 

When starting with an AR15 lower build the first consideration is material. Lowers are manufactured in either aluminum or polymer. Polymer is usually used when weight is a strong factor in the type of hardware you put into a rifle. It also is more scratch resistant than aluminum. Aluminum is considered to be a stronger material and can withstand some extreme weather conditions and heat, without sacrificing durability. Each have their own pros and cons. 

Our Build 

AR15 Mold 

AR15 Mold is the first product we have seen that takes a 0% lower receiver build kit approach. They have designed and produces a system that creates a polymer lower receiver without the need for milling, drilling, or multiple tools. In essence, their system is the perfect survival system, as it can be stored a raw materials and assembled many years down the road, or after 30 years of prolonged storage. 

In addition to being a polymer material, which can be built as polymer only, they have designed and included a steel brace which reinforced the entire receiver where it needs it the most; the rear takedown pin, grip, and buffer threading. This means that as a polymer it can ‘probably’ take more abuse than a fully polymer counterpart like Polymer 80 (read out Polymer 80 build here). We have yet to test this out but it sounds like a fun process. 

Additional Materials 

There is not a need for additional materials outside of a wrench, hammer, flathead screwdriver, and liquid gasket kit. Most of this can already be found in most homes and a liquid gasket kit is only a few dollars at an auto store. 

The Build 


The kit is delivered ready to pour, excluding the reinforcement piece. The reinforcement pieces it comes with are pre-bent, but do require some extra muscle to bent into a perfectly appropriate shape. This process took us nearly an hour, on our first try, and still wasn’t 100% perfect. 

There are no direction included, instead there are quite a few online videos showing the process and taking about cleaning and assembly of the mold. We watched about 2 hours worth of videos before moving on to disassembling and reassembling the mold. Many of the pieces are very small and either fit loosely or incredibly tight. We coated the pieces with a wax (AR15Mold recommends car wax, we used hair wax) and started to put it all together. 

We recommend that the reinforcement brace be as narrow as possible before assembling. During our first pour we did not do this and regretted it. Because of the wider brace the entire mold was also difficult to reassemble. There are precautions about certain pieces being put in a specific way to avoid breaking the mold. Heed these warnings! 

The most difficult part was the gasket. It needs to be put on in a very specific area. After assembly the gasket material is likely to come out of the edges. While not a bad thing it is not a good thing either. Use the excess to over as much of the seam as possible. 

The Polymer 

Their kit comes with Part A and Part B. I nicknames these Part Annoying and Part Beautiful. Part A is a very thin liquid that can really splash while Part B is pretty thick and sticky. This is a 2×1 mixture meaning 2 parts A for every 1 part B. We poured each part into a cut and weighed it out. There are directions on this process written directly on the Part A and Part B cans. 

We were called to mix for 90 seconds after these two parts are combined. The entire process should actually be completed in 5 minutes or the polymer is too hard to pour. On our first try we decided to forgo the dyes and move straight to the mixing and pouring. 90 seconds after mixing and stirring we were ready to pour. The cups actually got pretty got from the chemical reaction, hotter than expected. 

The Pour 

This part is fun. We put the mold into a cardboard box and began pouring into the tiny hole. A funnel would be appropriate at this point. Ever few seconds we stopped pouring and rocked the mold back and forth while tapping it. We did this to remove any air bubbled that might be trapped inside the poured polymer. This process went: pour, tap tap rock, pour, tap tap rock. 

We poured until polymer came out of the buffer tube hole. The plug is incredibly difficult to put in and we needed to actually hammer it into place. This forced even more polymer out of the buffer tube portion of the mold and got polymer all over our hands and hammer. 

There was quite a bit of excess polymer in the cups. They say that you can get 6 pours out of the entire kit, but with proper measuring you could easily stretch that out to one or two additional pours. We felt a sense of shame and loss as we threw the rest of the polymer away. 

The Wait 

70 minutes until the polymer is dry enough to remove the mold. We watched an episode of Star Trek – Deep Space Nine which wasn’t very good. It also wasn’t long enough and our patience started to thin. 


Once our wait was over we moved on to removing the receiver from the mold. The side pieces took quite a bit of prying to separate and we were a bit worries we may damage the mold. Approaching this step cautiously we were excited to see what would happen but wanted to make sure to preserve as much material as we could. The side popped off and we got our first look at a new lower receiver. 

Removing from the mold was more difficult than assembling the mold. We noticed that there were still a few air bubbles and we could see the reinforcement brace sticking slightly through the polymer. As we tried to take the buffer retaining pin out we just couldn’t remove it. It was stuck in there. 

For roughly an hour we pulled, hammered, and yanked on the piece. It was fiercely imbedded into the polymer material and we couldn’t remove some of the key components. As polymer dries it begins to expand and we didn’t want to damage any other necessary pieces of the mold. Additionally, we thought the polymer was squeezing the buffer retention pin piece so hard we couldn’t remove it. 

Out came the drill. We tried to drill from the side into where the pin would be located. It was our thought that doing so would allow us to pull the polymer material away from the pin. All we ended up doing was drilling into the pin a little. It still didn’t budge. We then drilled through the bottom and through the reinforcement brace. We weren’t sure how deep to drill and kept going. Finally, 10 minutes in the piece began to twist and was able to be removed. Unfortunately, we severely damaged the piece in the process and made our lower receiver inoperable. 

Second Build Fix 

Following our disappointment of our first build we started a second build and flipped the buffer retaining pin portion over while coating it with liquid gasket material. After 40 minutes we spun the retaining pin portion of the mold every 10 minutes. Upon de-molding the pin came right out without any hesitation. We googled the issue and found we weren’t alone. Many users had similar experiences and solved the issue with the gasket material. 


Overall, the entire process was pretty easy. It was actually easier that milling an 80% polymer receiver. We were impressed with the level of design that went into the AR15 Mold. They shared that an AR10 version is in the works and we’re just as eager to get our hand on that mold, as well. 

There takes no skill in this build. You do not need to be mechanically inclined to complete or use this kit. This makes it a perfect survival or SHTF option. If you didn’t care about bubbles or the brace in the final receiver you could probably get a 100% receiver completed from start to finish in about an hour. 

AR15 Mold also recommends baking the receiver to strengthen it. We did not do this and don’t plan to. The receivers are strong just as they come out of the mold. We did not feel it needed extra strengthening during our process. 

The Positives: 

  • No skill needed. 
  • Perfect for long term survival kits. 
  • 0% to 100% in 1.5 hours. 
  • Reinforced and strengthened with brace. 
  • Many color options. 

The Negatives: 

  • The brace is difficult to shape, even when partially shaped. 
  • De-molding is a delicate process. 
  • Easy to make mistakes. 
  • A lot of leftover material waste. 
  • Nearly impossible to completely avoid air bubbles. 

For the price, ordering the AR15 Mold kit will save you a ton of money over buying 5 complete, or 80% polymer lowers. The process is fun, easy, and can actually make a great family project (I’m looking at you Californians). No need to polish it up or make excessive changes because it comes out like it came off of an assembly line. 


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