Rifle or Pistol Lower: The Decision That Comes Before the First Part

Before you buy a single part for a new lower, decide what the finished gun will be: a rifle, with a barrel of at least sixteen inches and a stock, or a pistol, with a shorter barrel and no stock. The choice is legal as much as practical, it determines which buffer tube and which rear furniture you buy, and on a stripped lower it is not recorded anywhere until you make it. This guide explains the two configurations, the rules that separate them, and how to choose.

The federal definitions

A rifle is a firearm designed to be fired from the shoulder with a barrel of sixteen inches or more and an overall length of twenty-six inches or more. A pistol is a firearm designed to be fired with one hand, with no stock. A rifle with a barrel under sixteen inches, or a firearm with a stock and a short barrel, is a short-barrelled rifle, which is a registered item under the National Firearms Act requiring an approved application and a tax stamp before it is made. We do not sell short-barrelled rifles and this guide does not cover building one.

The stripped lower's advantage

A stripped lower is transferred as "other" on the federal form, which means it has never been a rifle or a pistol. You may build it as either. If you first build it as a pistol, you may later convert it to a rifle by fitting a sixteen-inch barrel and a stock; the reverse, taking a receiver that was first built or sold as a rifle and making it a pistol, is treated as making a short-barrelled weapon in some readings of federal law, and the safe practice is to never put a stock on a lower that you may later want as a pistol. A complete lower sold with a stock is a rifle receiver from the day it is transferred. If you want flexibility, buy stripped or buy a complete lower with a brace rather than a stock.

Building a rifle

A rifle lower takes a carbine or rifle buffer tube, a stock, a carbine buffer and spring, and pairs with any upper of sixteen inches or longer. It is the simpler, cheaper and more useful build for most people: legal in every state that allows the rifle at all, accurate with a sixteen-inch barrel, and the configuration every accessory and every guide assumes. Barrel length is measured from the bolt face to the muzzle; a fourteen-and-a-half inch barrel with a permanently attached muzzle device that brings it to sixteen inches counts as sixteen, which is how many carbines are built.

Building a pistol

A pistol lower takes a pistol buffer tube with no stock, or a brace fitted to a tube designed for one, and pairs with an upper of any length. The usual builds are ten-and-a-half or eleven-and-a-half inch barrels in 5.56 or eight to nine inch barrels in .300 Blackout. Pistols are short, handy and loud, with more muzzle blast and less velocity than a rifle. As of September 2026 a brace on a pistol is treated as a pistol accessory under federal law, following the court vacatur of the 2023 rule and the government's decision not to appeal; the law guide covers the details, and a few states have rules of their own on braced pistols and on pistols with detachable magazines. Federal law also bars a dealer from transferring a pistol lower or a pistol to a buyer under 21.

Vertical grips and overall length

A vertical foregrip on a pistol changes its classification under federal law to a firearm that is neither a rifle nor a pistol, which can be regulated as an "any other weapon" if the overall length is under twenty-six inches. An angled foregrip does not. Keep vertical grips off pistol builds unless the overall length is comfortably over twenty-six inches, and when in doubt, use an angled grip or a hand stop.

Overall length and the twenty-six inch line

Overall length matters for both configurations. A rifle must be twenty-six inches or more, which every sixteen-inch barrel with a collapsed stock satisfies. A pistol has no minimum, but a pistol under twenty-six inches with a vertical foregrip becomes a different class of firearm, and some states treat any rifle-calibre pistol with a detachable magazine as a regulated item regardless of length. Measure from the muzzle to the rear of the buffer tube or brace with the folding parts extended, write it down, and keep the number above twenty-six if you want the fewest questions.

Uppers and the pistol-length gas system

A pistol upper is usually built with a pistol-length gas system, which is short, harsh and over-gassed by design so that it cycles with a suppressor and with the shortest barrels. The rifle lower and the pistol lower take the same upper, so a builder can own one lower and two uppers, a sixteen-inch rifle upper and a short pistol upper, and swap between them in seconds, as long as the lower stays in the configuration that is lawful with the upper on it: never a stock with the short upper.

What the choice changes in the parts list

• Rifle: carbine buffer tube, castle nut and end plate, carbine buffer and spring, a stock.
• Pistol: pistol buffer tube or brace-ready tube, the matching buffer and spring, a brace or a bare tube, and no stock.
• Both: the same lower parts kit, trigger, grip, safety and magazine catch.
• Pistol uppers with short barrels often need a heavier buffer or an adjustable gas block to run cleanly.

How to choose

If you want one AR for the range, home and hunting where the state allows it, build a sixteen-inch rifle. If you want the shortest lawful package for a truck or a home and understand the blast and the brace rules, build a pistol. If you are not sure, buy a stripped lower, build the pistol first and convert it to a rifle later, which is the one direction the law is clear about.

Next: the parts kit installation, the buffer and stock for the configuration you chose, and the controls. Browse lowers and uppers in the catalogue, and your state's rules on the city pages.

General information, not legal advice. Current as of September 2026. Laws change; check the official source before you act.