July 6, 2022

Spotter Up

In Depth Tactical Solutions

Testing the lightweight FMK AR1 eXtreme AR-15 Lower

3 min read

This is a review of FMK’s 4.3oz Lightweight Polymer AR Lower “AR1 eXtreme”

FMK’s AR1 eXtreme composite lower is surprisingly durable, lightweight, American made, and fits like a glove to any of my MILSPEC uppers.

I grew up in Arizona so I know that anything used outdoors and is made of plastic will get destroyed from dry heat and a little abuse. I assume this is the reason I’ve never been a fan of polymer lowers. Regardless I decided to give the FMK AR lower a try because I was looking for an affordable lightweight lower to use on a 3-gun platform. I soon found out that this AR1 lower has given me a new perspective on “polymer”.



  • Frame: High Impact Proprietary Composite Polymer
  • Caliber: Multi-Caliber
  • Size: MIL SPEC
  • Weight: 4.3oz
  • Colors: Black and Dark Earth
  • 100% USA Made

MSRP: $59.95

Addtional details here: FMK AR1 Lower

*Accepts standard mil spec uppers up to .50 beowulf.

My initial concerns I’m sure are as most. How durable can it actually be and what kind of failures will I get from a lower that has the potential to flex? How long until I get a stress crack? How will it line up with my assorted uppers and will it take any work to fit parts?

I’m happy to say that for most of these questions there have been zero issues. I’ve put 800 rounds through one of the lowers and banged it around a bit, held the mag-well against surfaces while firing to see if the pressure would flex a failure but no problems came up.

FMK promotes a proprietary composite material which can withstand a beating and not get damaged as others would.

The only legitimate complaint I’ve had which isn’t a big one because it was an easy fix were the detent and spring holes being way too small for any mil spec detent to fit into. I thought it may have just been the ones I received but I read this was fairly common. Since the lower is polymer it really isn’t an issue since it took me less than 5 min to run a small drill-bit through the holes to widen them up. Just pick a bit slightly larger than your detent and you’re good to go.

Another positive is that the FDE matches pretty close to Magpul’s components which look great. Many other manufactures blend of FDE isn’t too close and is easily noticeable. FMK also gives you the option of custom engraving which I did not go for but I’m sure a few folks would.

I have quite a bit more shooting to do and will update later with a higher round count to see how far this guy will go. For now, I’ll enjoy having a lightweight lower to run and gun which costs next to nothing. Below is a video of one of the matches I ran the AR1 Lower in Northern Virginia.


Based on the very affordable price of $59.99 and a lifetime warranty, if you’re looking for a lightweight and durable polymer lower I would recommend this FMK A1 eXtreme to my own mother.

Let me know what you think, you can read more on the A1 eXtreme through FMKfirearms.com.



*The views and opinions expressed on this website are solely those of the original authors and contributors. These views and opinions do not necessarily represent those of Spotter Up Magazine, the administrative staff, and/or any/all contributors to this site.



1 thought on “Testing the lightweight FMK AR1 eXtreme AR-15 Lower

  1. The article is now 2 years old. Hows it holding up and how many rounds have passed through it? Would love to know or see an updated article.

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