Lower Build Kit
Quality, mil-spec AR-15 Rifle and Pistol Lower Build Kits.
What good's that shiny new upper you bought for your AR build, if it doesn't have a quality lower build kit to finish off the other half? Instead of guessing which parts to grab - the lower parts kit, the right buffer, a good pistol grip, and everything else - just grab it all n' go with one click. Our lower build kit options include all parts required to outfit a barreled upper assembly, turning it into a ready-to-roll AR rifle or pistol build.
What's in a Lower Build Kit?
Just about everything you need (minus some basic gunsmithing tools, like a castle nut wrench and roll pin punches) to build out the bottom half of your AR's two assemblies. A lower build kit includes the following components, which are all you need to make an upper receiver function with a magazine, trigger, and buttstock and/or buffer tube (if building a pistol):
- Lower Parts Kit (fire controls, pins, and springs)
- Standard pistol grip with screw and crush washer
- Mil-spec Carbine buffer tube (works with rifles and pistols)
- Mil-spec buffer and recoil spring (buffer weight depends on caliber)
- Forged or billet 80% lower receiver (requires an 80% jig to fabricate)
Q: What does "mil-spec" mean, here?
A: That means the parts included in your Lower Build Kit are fabricated to a universal standard of fitment for the AR platform. Over 95% of AR parts on the market (including receivers, buffers, and buffer tubes) are "mil-spec" in their fit and finish. The mil-spec identifiers here mean your buffer is made to fit in the most commonly sized buffer tube (diameter of 1.148") which is also included with each kit. These mil-spec tubes and parts fit virtually all upper and lower receivers, including all our 80% lowers. Here's a more detailed guide covering it all.
Q: Besides a Lower Build Kit, what else do I need to build an AR?
A: The lower build kits you see here allow you to complete an entire lower receiver assembly. Once you have a lower assembly with a functional receiver and trigger installed, you only need a barreled upper assembly with a bolt carrier group and charging handle. To make things even easier, we have complete build kits that include what you see here, plus an upper assembly. Want to build a custom AR piece by piece? Check out all our available AR parts. We have bolt carriers, handguards, stripped upper receivers, custom charging handles, and more.
Available in 5.56/.223, 9mm, .308 and more
No matter what type of upper assembly and caliber you chose to chamber, we have compatible lower build kits. The Classic Kit is perfect for completing a mil-spec 5.56 NATO, .223 Remington, 300 Blackout, or even a 6.5 Grendel build. All parts contained in this kit work with all those uppers and calibers!
Building a .308 AR? Grab this Lower Build Kit, which includes a billet .308 80% lower. And finally, for those building a 9mm AR (AR9), we've got you covered with this 9mm Lower Build Kit. It includes the appropriate buffer assembly and a 9mm 80% lower with the correct ejector.
DISCLAIMER: If you are new to the world of DIY gun building, you likely have a lot of questions and rightfully so. It’s an area that has a lot of questions that, without the correct answers, could have some serious implications. At 80-lower.com, we are by no means providing this content on our website to serve as legal advice or legal counsel. We encourage each and every builder to perform their own research around their respective State laws as well as educating themselves on the Federal laws. When performing your own research, please be sure that you are getting your information from a reliable source.