Lightweight BCM Build


None of us here at T.REX are military or law enforcement. But for whatever reason, people ask a lot what Lucas runs. So we decided to give a broader taste of some different options for different preferences and budgets by writing up some of our primary carbines.

Every rifle is a compromise. That 12″ long ultra-reliable, ultra-precision rifle chambered in 50 BMG that weighs 4 pounds loaded and kicks like a 3-month-old just doesn’t exist. It doesn’t cost under $500 either. The carbine featured below is just my personal attempt at balancing reliability, light weight, features, and price.

After deciding on an AR-15 in 5.56 NATO, my specific requirements were:

1. Only have one primary, so it needs to be usable for anything I might want it for, from home defense to hunting in my area (hilly Middle Tennessee).

2. Lightweight: Having done a lot of hiking, I value light weight. But I didn’t want to compromise reliability or (much) functionality to achieve it.

3. Low-light capable with very basic NOD-compatibility: bad stuff often happens in the dark

4. Can’t break the bank (relative, of course): Only having one helps a lot. At some point, more practice ammo is better than a more expensive weapon.


The heart of this carbine is a Bravo Company 16″ mid-length light weight upper receiver group.  BCM is not the cheapest; they’re not the most expensive; but the uncompromising reliability is there.  Many professionals depend on BCM ARs, and I figure that should make them a good choice for a civilian like me too.

While this is a lightweight build, I did want a fully functional upper— dust cover, forward assist, and brass deflector (nice for support-side shooting).

Some say, “Cleaning is optional; lube is not.” This has worked for me so far.

BCM_Lightweight_Cavarms_Primaryarms_4The handguard is an ALG Defense EMR V2 MLOK, made by Geissele Automatics.  It’s inexpensive for what it does ($145), and has a very cool method for taking the barrel nut to the correct torque while still lining up the handguard exactly with the upper receiver.  Slim enough to wrap my hand completely around, the handguard’s small circumference does mean less real estate for accessories, a downside which Geissele cleverly mitigated by spacing the MLOK slots every 2 hrs around the tube rather than every 1.5.  This allows compact accessories to still be mounted on adjacent slots.  More on Geissele later.

The freefloat handguard allows forward mounting of an IR laser.  I found a receiver-mounted (45-deg. offset) laser to create significant reflection and glare off, well, everything anywhere near the front of it— red dot, handguard, light, support hand…

12 o’clock forward mounting also means no windage offset is needed and the unit can be quickly toggled with the support hand.  While the Magpul MBUS polymer front sight is lighter and cheaper, the MBUS Pro’s more compact design allows it to sit just behind the laser, shielding its switches.  Facing the MBUS Pro toward the rear allows the WMLx-IR to fit next to it at 10 o’clock.

Light/illuminator and IR laser can be activated ambidextrously, though a modified thumb-over grip is necessary for support side light/illuminator activation.

BCM_Lightweight_Cavarms_Primaryarms_7 BCM_Lightweight_Cavarms_Primaryarms_8

The inexpensive ($200) LaserMax Micro-IR weighs half an ounce and has retained zero nicely so far (although adjustments are continuous— no clicks).  It’s a civilian-legal, FDA-approved (safe to ingest?) laser with <0.7mw output.  Lest that be thought weak, it’s visible beyond working ranges for the Gen 3 NODs I’ve seen and blooms like crazy up close.  Beam shape and intensity seem similar to the DBAL-i2 and ATPIAL-C.  LaserMax also offers the UNI-IR laser, which is more water resistant, though still not waterproof.  I went with the Micro-IR because it takes up less rail space and the battery can be changed without removal from the rail and thereby losing zero.  Because it’s not waterproof, this laser is the big compromise on this build.  The other serious NOD-related compromise, to my limited experience, is that the IR laser and illuminator cannot be toggled together with a single switch.  This setup is not for serious business.  But a professional solution would come with a much higher price tag and weight.

IR illumination and visible light are provided by the Inforce WMLx-IR.  The LED- (not laser-) based illuminator increases identification range and cuts laser bloom under NODs.  The visible light allows good ID out to, I’d say, 150 yds.

The Primary Arms Advanced Micro Dot (manufactured by Holosun) seems like an outstanding unit for the price.  The dot is less perfectly crisp and round than an Aimpoint, but better than the Vortex SPARC. Estimated battery life is over five years on medium brightness.


The Cav-15 MKII lower receiver is one of the more unusual elements to this build.  Its one-piece polymer design saves about 1 lb. over an aluminum lower and the hollow buttstock allows a little storage.  The first Cav-15 MKII to support this upper had been slapped together in the last days of Cavalry Arms Corp.  It came with long gaps in the ultrasonic welding.  After a thousand rounds it developed a crack down the front of the magwell, which did not affect function.  The current lower was a warranty replacement (at long last) from GWACS Armory.  It’s held up perfectly so far and I don’t expect that to change.  Nonetheless, along with reducing the length of pull by 7/8″, modifying the grip, and drilling for sling attachment, I reinforced the front of the magwell with JB Weld.


The complete lower group with H-buffer weighs in at 28.75 oz.  One downside of the lightweight receiver/stock is that the carbine balances a little farther forward than I prefer— at the front of the magwell (without mag).


I really like the one-handed adjustability of the Vickers sling.

The rattlecan paint job makes it less eye-catching against barrels (not!). It actually fits well in the woods of Tennessee.

Returning finally to the subject of Geissele, the modification that, to me, makes the biggest difference to the practical accuracy of a carbine is an improved trigger.  After a sling, light, and red dot sight, it’s the next upgrade I like on a carbine.  While I’ve never met him, Bill Geissele of Geissele Automatics comes across as a knowledgeable, professional, and humble engineer— a real gift to the firearms world.  If you want to know how to pick a trigger for your application, I don’t know of anything more instructive than this 30 min. video.  I run his least expensive offering in my carbine— the G2S ($165).


In closing, here are some weight specs, prices, and links:

Bare rifle weighs in at  5 lb., 9 oz.  Adding optics, sling, cleaning kit, etc. brings it to 6 lb., 15.25 oz.

Bare upper (ALG V2 rail, Phantom 5C2) 47.5 oz
Bolt carrier group 11.5 oz
BCM Mod. 4 charging handle 1.25 oz
Assembled lower, including H-buffer 28.75 oz
Sights 3 oz
Primary Arms advanced micro dot w/ mount 4.75 oz
Inforce WMLx-IR 4 oz
MLOK rail section .5 oz
LaserMax Micro-IR laser .5 oz
MLOK hand stop .75 oz
Vickers sling 6 oz
Lube, boresnake, spare CR123A and 1/3N batteries 2.75 oz

Prices and Some Links:

BCM URG ($576)
BCM Low Profile Gas Block ($45)
ALG Defense EMR V2 MLOK ($145)
CAV-15 MKII ($225 plus $30+ transfer)
Geissele G2S trigger ($165)
Primary Arms Advanced Micro Dot w/mount ($200)
Magpul sights ($130)
Inforce WMLx-IR ($200)
LaserMax Micro-IR ($209)
Magpul BAD ($29)
Sling, snake, oil ($65)

Total: $2025.  What with sales and assembling my own lower, actual cost was about $1600.  I’m content with that for a reliable, fairly lightweight carbine with a nice trigger and basic NOD compatibility.


Disclaimer: I’m no expert on anything firearms related, and this build only has 3000 rounds through it.