Lower Parts Kit vs. Lower Build Kit: What’s the Difference?
Posted by Kevin on
Lower Parts Kit vs. Lower Build Kit: What You Need to Build an AR-15 Lower
Quick answer: A lower parts kit (LPK) contains the small parts that go inside and on the lower receiver: trigger group, springs, pins, safety, bolt catch, magazine catch and usually a pistol grip and trigger guard. A lower build kit adds the stock or brace and buffer tube assembly, so it includes everything needed to finish the lower except the receiver itself. Buy an LPK if you already have a stock and buffer setup you like. Buy a lower build kit if you want every part in one box.
Neither kit includes the stripped lower receiver. The lower receiver is the serialized part of an AR-15, so it is a firearm under federal law and is purchased separately through an FFL. This guide breaks down what each kit includes, which one fits your build, and how to choose a trigger and buffer setup that matches.
What is in an AR-15 lower parts kit?
A complete AR-15 lower parts kit typically includes:
- Fire control group (FCG): trigger, hammer, disconnector, their springs and the trigger and hammer pins
- Safety selector with its detent and spring
- Bolt catch with its plunger, spring and roll pin
- Magazine catch with its button and spring
- Takedown and pivot pins with their detents and springs
- Buffer retainer and spring
- Trigger guard and roll pin
- Pistol grip with its screw and lock washer
A budget-friendly starting point is the complete mil-spec lower parts kit. For our own spec, the RTB true mil-spec complete lower parts kit covers every part on the list above.
Complete LPK vs. LPK without trigger or grip
Many builders plan to install an aftermarket trigger or grip from day one. Paying for parts you will throw away makes no sense, so LPKs come in a few versions:
- Complete LPK: includes a standard mil-spec trigger group and a basic pistol grip. Best for a first build or a budget rifle.
- LPK without fire control group or grip: includes everything except the trigger, hammer, disconnector and grip. Best if you are buying an upgraded trigger. Options include the BN AR-15 LPK with no FCG or grip and the CMC Triggers lower parts kit with no trigger group or grip.
- LPK with an upgraded trigger: pairs the small parts with an enhanced trigger, such as the RTB enhanced nickel Teflon trigger assembly with complete LPK or the Rock River Arms LPK with two-stage National Match trigger.
- Build your own LPK: pick each part individually with the custom lower parts kit builder.
Shopping for a trigger separately? Browse our AR-15 trigger assemblies and AR-15 pistol grips.
What is in a lower build kit?
A lower build kit combines a lower parts kit with the rear assembly that holds the buffer and stock. Rifle lower build kits typically include:
- A complete lower parts kit
- A buffer tube (receiver extension)
- A castle nut and end plate
- A buffer and buffer spring
- A stock
Pistol lower build kits swap the rifle stock and carbine tube for a pistol buffer tube, and many include a pistol brace, such as the Tactical Sports SBA6 pistol lower build kit. Check the listing before you order, since exact contents vary by kit.
LPK vs. lower build kit: side-by-side
| Factor | Lower parts kit (LPK) | Lower build kit |
|---|---|---|
| Trigger group, pins and springs | Yes (except "no FCG" kits) | Yes |
| Pistol grip and trigger guard | Usually | Yes |
| Buffer tube, buffer and spring | No | Yes |
| Stock or pistol brace | No | Yes |
| Stripped lower receiver | No (sold separately through an FFL) | No (sold separately through an FFL) |
| Best for | Custom builds and builders who already own a stock and buffer setup | First builds and anyone who wants everything in one order |
Buying the buffer tube and stock separately
If you start with an LPK, you still need the rear assembly. The simplest way to get it is a buffer tube kit, which includes the tube, castle nut, end plate, buffer and spring. Add an AR-15 stock, or save a step with a stock and buffer tube kit combo.
Match the buffer tube to the stock. Most aftermarket stocks fit a mil-spec diameter carbine tube, while commercial-spec tubes are slightly larger. Building an AR pistol? See our pistol braces and pistol buffer tubes.
Which kit should you buy?
- First AR-15 build: a lower build kit. Everything is matched, and nothing is missing on build day.
- Upgraded trigger planned: an LPK without the fire control group, plus your trigger of choice.
- Upgrading an existing rifle: an LPK with an enhanced trigger, keeping your current stock and buffer.
- AR pistol: a pistol lower build kit with a pistol buffer tube.
- Multiple lowers: multi-pack LPKs, such as the 3-pack BN AR-15 complete LPK, cut the cost per rifle.
- AR-10 / .308: buy a .308-specific kit such as the BN 308 / AR-10 complete LPK, since some AR-10 lower parts differ from AR-15 parts.
Building the whole rifle, not just the lower? Our AR-15 build kit guide covers the upper, lower and everything in between.
Tools that make lower assembly easier
An LPK can be installed with basic tools, but a few make the job easier and protect your receiver's finish:
- Roll pin punches and roll pin starter punches
- A small hammer with nylon or brass faces
- A lower receiver vise block
- An AR-15 armorer's wrench for the castle nut
- Painter's tape to protect the receiver around the bolt catch and trigger guard pins
Find them in our AR-15 tools section. Lost a detent spring on the bench? Keep an inexpensive repair or "oops" kit on hand for the small springs and detents that tend to fly across the room.
Getting the lower receiver
The stripped lower is the serialized, regulated part of the rifle. A complete lower receiver must be transferred through a licensed dealer, and state laws vary. Our firearm and lower receiver purchasing guidelines explain how the process works. Browse stripped AR-15 lower receivers or a custom option from our engraved lower receivers.
Lower Parts Kit and Lower Build Kit FAQ
What is the difference between an LPK and a lower build kit?
A lower parts kit includes the trigger group, springs, pins, safety, bolt catch, magazine catch and usually a grip. A lower build kit adds the buffer tube assembly and stock or brace, so it includes everything needed to finish the lower except the receiver.
Does a lower parts kit include the lower receiver?
No. Neither a lower parts kit nor a lower build kit includes the lower receiver. The stripped lower is the serialized firearm and is purchased separately through a licensed dealer.
Does a lower parts kit include a buffer tube?
No. An LPK includes the buffer retainer and spring but not the buffer tube, castle nut, end plate, buffer or buffer spring. Those come in a buffer tube kit or a lower build kit.
Should I buy an LPK with or without a trigger?
Buy a complete LPK if you plan to use a standard mil-spec trigger. Buy an LPK without the fire control group if you are installing an aftermarket trigger, so you do not pay for parts you will not use.
Will an AR-15 lower parts kit fit an AR-10?
Not completely. Many fire control parts are shared, but AR-10 / .308 lowers use some different parts, such as the bolt catch and magazine catch. Buy an LPK made for .308 lowers.
What tools do I need to install a lower parts kit?
Roll pin punches, a small nylon or brass hammer, a lower receiver vise block and an armorer's wrench cover most installations. Painter's tape helps protect the receiver finish.
Is a lower build kit the same as a complete lower?
No. A complete lower is a lower receiver with all parts already installed, so it must ship through an FFL. A lower build kit is only the parts, without the receiver, and you assemble it yourself.
Build your lower the right way
Start with a lower build kit for a complete, matched set, or choose an AR-15 lower parts kit if you already have a stock and buffer setup. Questions about compatibility? Contact our team before you order.