Lower Receiver Parts

When building an AR15 from a stripped lower receiver, the builder must consider what parts are needed and which parts may be an “upgraded” component. For a standard mil-spec build, there are lower parts kits (sometimes abbreviated as LPK) available, such as the Daniel Defense kit, which includes everything you will need to build a receiver right out of the box. A typical lower parts kit usually includes…

• A2 Pistol Grip
• Trigger Guard
• Pistol Grip Screw
• Pistol Grip Lock Washer
• AR-15 Pivot Pin
• Bolt Catch Plunger
• Bolt Catch Roll Pin
• Bolt Catch Spring
• Buffer Retainer Pin
• Buffer Retainer Spring
• Disconnector Spring
• Magazine Catch Spring      
• Magazine Release Button
• Two Pivot/Takedown Detent Pins
• Two Pivot/Takedown Detent Springs
• Rear Takedown Pin
• Safety Detent Selector
• Selector Detent Spring
• Trigger Guard Roll Pin
• Trigger Guard Set Screw
• Two Trigger/Hammer Pins
• Bolt Catch
• Hammer Spring
• Magazine Catch Assembly
• Safety Semi-Auto Selector
• Semi-Auto Disconnector
• Semi-Auto Hammer w/ J-Pin Assembly
• Semi-Auto Trigger
• Trigger Spring

If you’re building a basic receiver, and plan on using only mil-spec parts, the above list will work just fine. While nothing fancy, a basic lower parts kit will get your rifle on the road to functioning reliably at the cheapest cost. And essentially, they are all the same. The reality is that there are about 4 manufacturers that built LPK’s, then supply them to everyone else (similar to forged receivers).            

If you're looking to replace your trigger with something better than the mil-spec unit, then an LPK without a trigger, like the JP Enterprises LPK, will save you a few dollars and prevent any unused parts from accumulating in a drawer. 

As we have said in other articles, the beauty of the AR platform is that there is never a reason to leave “good enough” alone. Just like anything else on the rifle, any of the above listed lower parts can be upgraded for either enhanced functionality, aesthetics or for weight savings. The next few paragraphs will highlight some of the areas where a standard lower parts kit can be upgraded.

Pistol Grip

Most lower parts kits include a standard A2 pistol grip. Just like rifle stocks and handguards, everyone has a preference on what feels right when it comes to a pistol grip. While the A2 grip is light, it’s texture is relatively slick and it’s profile is fairly slim. For many, the A2 grip is good to go. For others, there are tons of grips available on the market to suit just about anyone's needs. The angle of the grip is one of the most noticeable changes from grip to grip. A steeper angle (more up and down), like the Wilson Combat/BCM Mod3 Starburst, may be a good choice if you're running an SBR, pistol, body armor or if you prefer a straighter wrist angle. A more laid down grip might be right for you if you like to have your wrist angled more forward towards the target. There is also a new grip on the market from HexMag that allows you to change the angle of the grip very easily so you can try them all and choose the right one for your style. Another feature to look for is the type of surface that is one the grip, as many can be too smooth to get a good purchase on. Most aftermarket grips will have aggressive stippling, traction bars, rough spots or even a rubberized coating (LMT/Ergo grip) to lock the grip in your hand. Other features such as storage compartments, different colors, replaceable back and front straps, finger grooves, etc., are found in a wide variety of options. 

Trigger Guard

We are discussing the trigger guard immediately after the pistol grip for a reason. If you’ve ever handled a standard Milspec rifle, you may have noticed the gap between the front of the pistol grip and the rear of the trigger guard, or the lack of room for gloved fingers. While it may not seem like a big deal, after 2 days of competitive shooting or carbine classes at your local range, that tiny gap can get irritating to your fingers very quickly. Magpul’s polymer MOE trigger guard mates up to their pistol grips seamlessly. The Wilson Combat pistol grip has a small extension on the front that covers the gap with most trigger guards, but must be trimmed when using certain guards, like their own aluminum trigger guard (go figure). The idea of ditching the flat mil-spec trigger guard comes from wanting a better look and more room for your trigger finger, just in case you're using gloves or have tree trunks for fingers. The curved profile of aftermarket trigger guards flows with the lines of an AR rifle much better. This simple, cheap upgrade can make a world of difference. 

Pistol Grip Screw

A standard pistol grip screw is a 1/4-28x1" socket head cap screw, available at any hardware store. There are few things to consider even when looking at a simple grip screw. Grip screws are available in both hex and slotted varieties. The advantage of a slotted screw is that in the field, a standard screwdriver may be easier to find than a hex tool, making the rifle easier to maintain in adverse conditions. The hex screw however, is easier to install, as you can guide the hex screw with a driver easier into the pistol grip.

Keep in mind that some receivers are not threaded all the way through, and may require a shorter grip screw. Also, some pistol grips that have storage, such as the Magpul MIAD, come with a lower profile socket head cap. These allow clearance for the storage compartment. In the case of the Magpul MIAD, if you use a standard grip screw, and not the one lower profile one that comes with the grip, you won’t be able to insert the lube bottle fully. (We found this out the hard way here at Strong Side.)

Most pistol grip manufacturers include a screw and lock washer with the grip, but if they don’t, it's easy to track down. There aren't many upgraded screws out there, as it's just a screw. However, one grip screw has been very popular because it's a cheap addition that makes installation easy and sheds a few grams of weight...the V SEVEN titanium grip screw. While a small upgrade, it's better than the long, 'heavy' mil-spec screw and should provide clearance for in-grip storage. 

Pivot Pin and Takedown Pin

Mil-spec dimensions for the pivot pin and takedown pins are .250”. While there are some slight variances, finding pivot and takedown pins that are exactly .250” will ensure a tight fit between the upper and lower receivers. Because the pins are fairly hefty, a popular place to lighten up a rifle is to upgrade these pins to lightweight units. V Seven lightweight pins are available through Strong-Side Tactical and have exceptional finish as well as weight savings. To ease installation, there is a small hole that you can use to get the pivot pin installed much quicker, plus a small dimple on the end to help ease disassembly. If you have a loose upper to lower receiver fit, things like the JP tension pins are a good choice to tighten up your receivers, but adds some time to disassembly. A tight fitting receiver set is important to squeeze out all the accuracy possible out of your chassis. 

Bolt Catch

The Bolt Catch assembly is comprised of the bolt catch, spring, plunger and roll pin. One of the weaknesses of the standard AR15 rifle is that a standard bolt catch requires the shooter to break their shooting position in order to use the charging handle or lock the bolt back. An extended bolt catch, such as the Phase 5 bolt catch latch enables the shooter to use their right hand to lock the bolt back and drop the bolt. Because assembly of the bolt catch is one of the easiest times to scratch your lower receiver, it’s better to install an enhanced bolt catch when first assembling your lower receiver, versus doing it later down the road and having to drive the pin out and then back in.  Another way to take the roll pin out (without needing a special punch) is to punch the roll pin forward, then cut the pin and punch the rest out. There are 'skinny' punches out there that will make the removal quicker. Make sure you have a steady hand, Dremel at your own risk! A billet receiver makes this task much easier, as many of them have set screw pins instead of roll pins. 

There are also lightweight bolt catches that can help save weight, while enhancing the look of your lower receiver. V SEVEN makes two new bolt catches that are lighter than the mil-spec part and are available in different materials. The 8620 bolt catch is made out of an alloy and is a good value upgrade part. The S7 tool steel version is also lightweight, but very strong since it's made out of a more rigid material. The 8620 catch has the traditional line pattern on the paddle, while the S7 part has a diamond/hatch pattern for an aesthetically pleasing look. These two patterns will match the different magazine release buttons as well. 

Safety Selector

A standard mil-spec safety selector is designed for right handed use only, where the shooter’s right thumb enables/disables the selector. There are four areas to consider when upgrading the standard safety selector.

• 90° (long throw) vs 57° (short throw) : A mil-spec safety selector is the standard 90° configuration, where the safe position is in the 9 oclock position, and fire is in the 12 o’clock position. Some users prefer a 57° orientation, which allows less rotation of the thumb to enable the selector so you don't have to 'break' your grip to activate the safety. When on "fire", the safety selector arm can also be used a place to rest your thumb. 
• Ambidextrous : Some safety selectors offer levers on the both sides of the rifle, enabling the rifle to be ambidextrous. If you are a southpaw or prefer ambidextrous capability, an ambidextrous safety may be for you. Some right handed shooters prefer this setup as well, so they can activate the selector lever with their trigger finger. 
• Lever Type : With the wide variety of safety selectors available, there are many types of levers. Some are wider, offering the shooter more surface area to engage the lever. Another option is the length of the lever, which comes in short or long with the V SEVEN products. The texture of the lever can also be found in both smooth and aggressive patterns. One of our favorites here at Strong Side is the V Seven Ambi Hybrid, which allows the best of all the above considerations.
• Weight : If a lightweight build is what you're after, the titanium core safety selectors form V SEVEN are a great way to lighten your receiver without sacrificing durability and reliability. These lightweight safeties are available with aluminum or titanium selector levers and in Ion Bond black or raw titanium finish.  

Magazine Catch

The magazine catch is a fairly straightforward part, as it’s simply a button to release the magazine. Some shooters prefer a slightly larger magazine release button, such as the V Seven Extended Magazine release button. These can offer faster reload time, and are an inexpensive way to enhance the ergonomics of your AR. Another option is to go with a lightweight assembly, which can be had in either aluminum or titanium, in a black or silver finish. These can help shed weight from your rifle while looking much better than the mil-spec alternative. 

Buffer Retainer

The buffer retainer is what holds the buffer inside the buffer tube so the buffer and spring doesn't shoot out of the lower upon disassembly. The retainer also makes contact with the buffer tube to help hold it in position. This is a straightforward part, so there aren't a ton of upgraded options. V SEVEN offers a titanium buffer retainer that is a tad lighter and a silver color, but it is not going to be noticeable. When using a JP captured spring, the retainer is not necessary, as the JP spring is a captured unit. However, some people like to have the best of both worlds, so they trim the tip of the retainer to be flush with the inner diameter of the buffer tube. Using a short retainer means that you still have the pressure against the buffer tube, but you can easily slide your JP captured spring unit out for cleaning or disassembly. 

Trigger

A standard lower parts kit includes a mil-spec trigger assembly (or fire control assembly as some manufacturers call it.) As stated in our Triggers Article, the trigger is one of the most important upgrades to your rifle. Most mil-spec triggers break around 9lbs or higher, with a gritty feel at first. In regards to smoothness, it’s luck of the draw if you get a smooth trigger or a gritty one in a standard lower parts kit. The surfaces will smooth out over time, but there is much to be desired. If you plan on deploying an upgraded trigger, such as the CMC drop in trigger system, be sure you choose a lower parts kit that does not include a mil-spec trigger, as you can save $30-40 and not have any left over parts.

Springs, Detents & Roll Pins

All of the springs, detents and roll pins needed for assembly are included in a lower parts kit. With some billet receivers, the roll pins aren't needed because they use a set screw style pin. Many upgraded parts will also come with the springs and detents included. In our experience, you can never have enough spare springs and detents, as they somehow end up shooting across the room or getting lost on the floor or carpet. Springs and detents are generic and there aren't any upgrades with these parts. Some of the detents can be smoother than others, but it's a luck of the draw. If the detent makes for a rough feel, try using some very fine sand paper and/or a polishing wheel to smooth out the engagement surface on the tip of the detent and make sure there is some oil/grease in the channels.  

Customize

Again, the beauty of the AR platform is customization. If you plan on upgrading any of the lower receiver parts, it makes more sense for you to hand pick the parts, versus ordering a complete lower parts kit. A well planned build will leave you with fewer left over parts and save you money in the long run. As with other parts, you may spend more money up front but it will more than likely save you the money in the end because you won't have spare parts and the part you always wanted is on your rig from the start. Mil-spec parts will definitely get the job done, but there's always room for improvement.