The AR15 lower receiver is the legal heart of the rifle, being the only part that is legally “the firearm” and the only part that can’t easily be shipped to your door (in non-communist-oriented parts of USA). This article focuses on selecting a lower to start the build. Specifically on a “stripped” lower, or one that hasn’t had any additional parts added or attached (you’ll learn more by adding them yourself).

Lowers I have include: Daniel Defense, a lower created with a Ghost Gunner 2 (aluminum without coating/treatment post-cut), and some soon-to-be built Aero Precision lowers. Ideally I’d have tested a whole bunch of lowers from various manufacturers with thousands of rounds to give some more objective outlay of variables and performance. However, I suspect many blog/aggregator posts on the net that discuss which lowers to buy with affiliate links have similarly NOT performed any such analyses. So here we go!

Variables for lower consideration:

• price
• manufacturer reputation re quality, quality control (QC), tolerances
• materials
• manufacturing method: billet (cut/machined from block) vs. die forging (heated block pushed into mold)
• compatibility: mil-spec (if this label applied, SHOULD be cross-compatible re fit with other “mil-spec” parts)
• added features

overview based on cost categories

• lower cost (~<$100)
  • PSA
  • Anderson
  • Brownell’s - reported magazine fitment issues
  • KE arms polymer
• medium cost (~<$150)
  • aero
• expensive & super expensive
  • American Defense Mfg
  • Angstadt Arms - main focus seems on PCCs
  • Bravo
  • Black Rain Ordinance
  • BAD: Battle Arms Development
  • F1 firearms
  • Geissele
  • KAC: Knights Armament Company
  • LMT: Lewis Machine & Tool - MARS used by New Zealand Defense Forces
  • Radian
  • Seekins
  • Sons of Liberty Gun Works
  • Spikes
  • Noveske

materials & manufacturing process

main methods:

• billet: aluminum blocks -> machining
• forged: aluminum pressure-molds -> machining
• plastic: likely all injection-molded
• cast: molded aluminum -> machining

Billet manufacturing involves starting with 6061-T6 bar stock, which is created by by heating aluminum under compression to form a block. The heat & compression result in strengthening of the material (hardening). It is thereafter CNC machine cut to yield the final dimensions. 6061-T6 is noted for greater corrosion resistance vs. the 7075-T6 employed in forged (thought mainly due to latter’s higher copper content).

Forged manufacturing entails heating & hammering 7075-T6 aluminum under pressure to mold it into a final base shape, and then machining this molded aluminum via CNC to its final shape. This process is thought to generally impart increased strength due to internal grains of the metal following the shape of the part (aligned crystalline areas evident under microscopy).

Casting involves pouring melted A380 aluminum into a mold, and machining to finish. Not as strong vs. forged & billet due to possibility of internal imperfections such as cracks & small holes.

assessment & comparisons

There seems to be a consensus that both billet & forged are great for lower processes. Forged may afford higher strengths, but may be slightly harder to machine. The molding process with forged also results in higher cost for customization of the pre-machined base design.

Plastic lower manufacturing still is nascent but growing. Personally at this time I’d prefer getting aluminum non-plastic until they have been more thoroughly abuse-tested. While you might expect the industrially-manufactured molded plastic lowers to be stronger than FDM 3d printed ones, seeing the many 3dp breakages where the buffer tube attaches to the lower with unmodified designs gives me pause. This should be mitigated by some of the industrial plastic designs integrating with the stock (less of a breakage focal point). Pros of these could be reduced weight and cost savings. Some are offered at quite cheap prices, e.g. as of this writing KE arms lower with attached fixed buttstock and grip for $80.

Cast lowers seem less common now due to lowered costs & possible hole-defects in the cast material lending to lower strength properties vs. forged & billet.

This page has a decent breakdown of manufacturing methods, from which much of the above was synthesized.

features

Following are some specific features you might see advertised for some lower models. Features like full ambidextrous control sets are, as of this writing, only on the more expensive lowers.

• integral trigger guard: is the lower part of guard fixed, or a separate piece attached via screw/pin?
• upper interface set screw: there will always be a fit tolerance between upper & lower receivers. a set screw can reduce rattle
• threaded holes: these allow use of bolts/screws rather than roll-pins (former being much easier to install)
• magwell flare: flaring the magwell entrance might make reloads slightly faster for you
• ambidextrous controls
  • safety selector
  • magazine release
  • bolt release
  • bolt catch
• multicaliber: e.g. 5.56 & 300 blk
• finish: generally anodized aluminum (e.g. type II or type III), Duracoat, Cerakote
• flair (e.g. engraved memes, functionless shaping)

specific considerations

If you are just starting out building (as you probably are if reading this), then you probably want to start with one of the cheaper lowers. If you can afford the fancy lowers, then you can always build another platform after the first one! Learning the process with a cheaper one also reduces the chance of scratching/damaging fancier components on the next build. Furthermore, you’ll get more shooting experience improvement out of investing comparatively in components such as the optics, trigger, and barrel (so put money there first).

Aero available options all seem based upon two base models: m4e1 (~$125 from aero, ~$107 regular but $90 sale at Brownell’s) vs. gen2 (~$110 from aero, ~$94 regular but $73 current sale at Brownell’s). Both are forged & mil-spec, with an upper tension screw (reduce tolerance rattling with upper) & flared magwell. The m4e1 for ~$15 more has an integrated trigger guard, threaded bolt catch, and accepts short-throw safety selectors.

PSA’s offerings are a little harder to classify given their large quantity produced, many customized meme-flair engraving options, and inability to group their website offerings via the base models. The PSA AR-15 stealth lower seems the closest to a current non-novelty base model that they have. This ~$60 (from PSA) model is forged & mil-spec with a separate-piece lower guard. They have derivative memeified versions with Karens & ATF agents.

Anderson Manufacturing offers their “AM-15” with the guard either integrated or separate each around ~$81 from Anderson. These are seen significantly discounted at Brownell’s (non-integral guard version for $95 regular but $53 sale’s).

There are some scattered reports on online forums of customers getting out of spec lowers made by Anderson (e.g. holes incorrectly placed) and of PSA having QC issues, but these seem relatively rare (particularly in context of the seemingly large volume produced).

I personally went with the aero M4E1 just to make the build slightly simpler & easier with the integrated guard & threaded pin, as well as their reputation among builders as having better QC (re lower risk of having to deal with something being out of spec, even if the chance is low with other mentioned brands). For a subsequent build I might consider some of the more expensive options, mainly with consideration of more ambidextrous controls (to perhaps be covered at a later date).