We’ve all the term before: Lower parts kit. LPK. Just “parts kit”. What is it, exactly? It’s super-important when it comes to building an AR 15. Although the stripped lower is technically considered the firearm of the thing, it’s the lower parts kit that makes it all work. Let’s break down exactly what’s in a lower parts kit, what each part is and what it does, and how it all works.
Fire Control Group
When most folks ask, “how does an AR 15 lower parts kit work?” They’re referring to the fire control group – all the other stuff in the kit is used for the magazine, the bolt, the buffer assembly, and the pins that connect the upper and lower (we’ll get give all those things official names later).
Back to the fire control group: This is the assembly inside the lower receiver that makes stuff go boom. In this system you have the following components and their functions:
- Hammer: The Hammer strikes the firing pin inside the bolt carrier group, causing a chambered cartridge (“round” or “bullet”) to fire.
- Hammer Spring: The hammer spring provides tension on the hammer when it is depressed down so that it slams forward, firing your rifle or pistol, when the trigger is pulled. The hammer springs rest on the trigger pin.
- Trigger: The trigger is the iconic “fun lever” that we all know and love. The trigger is the exposed, curved piece of steel sitting inside the open loop area of your lower receiver, above and forward of the pistol grip.
- Trigger Spring: The trigger spring attaches to the top of the trigger itself, providing tension against it when it is pulled. This tension allows the trigger to reset in the forward position after firing each round.
- Disconnector: The disconnector sits atop the trigger and catches the hammer so that it remains in a “ready-to-fire”, downward position, tensioned with the hammer spring so it can fire again. If firing the AR 15 and holding the trigger, the top of the disconnector will catch the hammer.
- Once the trigger is released, the disconnector will allow the bottom of the hammer’s notched fulcrum to catch on the sear of the trigger. The sear is the squared off protrusion of machined metal that sits atop and forward of the trigger.
- Disconnect Spring: The disconnector spring actuates the disconnector just slightly, so that it can catch the hammer with the trigger either depressed or released.
- Hammer and Trigger Pins: These pins are pressed through the lower receiver and their respective mechanisms (the hammer and trigger assemblies), holding them in place, where they can rotate freely inside the receiver. The ends of the hammer and trigger pins are visible – once installed, they’re flushed with the outside metal of the receiver itself.
- Selector Switch / “Safety”: The selector switch, commonly called the “safety” is the lever that sits flush against the left-hand side of the lower receiver. When engaged, the rounded portion of the lever prevents the trigger from being pulled. When disengaged, the selector switch rotates, exposing a flattened portion of metal that allows enough clearance for the trigger to be pulled.
All That “Other Stuff”:
Beyond the fire control group, the average lower parks kit includes a ton of other parts, like the hammer catch and button, the trigger guard, springs for the bolt catch, buffer retainer, magazine catch, and roll pins to secure all that stuff to the lower itself.
While any decent lower that wasn’t built by caveman will suffice for “that other stuff”, it takes a good, quality lower to ensure your fire control group functions flawlessly. Those two critical areas – the notch in the hammer fulcrum, and the sear area of the trigger itself – require precision machining and finishing to ensure your AR cycles properly.
Our Premium Lower Parts Kit
If these two areas are poorly made, your trigger may not reset, your AR might not fire, or your whole weapon may just feel gritty and tight. If you buy a lower parts kit or a used AR, and your trigger feels nasty, look at these two areas. They might require a little polishing and light sanding.
Our Premium Lower Parts Kit doesn’t need any of that. You can check our the numerous 5-star reviews, but rest assured this kit is perfectly built to be reliable when it counts most. Pair this kit with an AR 15 Punch Set to make assembly a breeze (trust us, you’ll want this for those roll pins and springs).