Do CMMG lowers fit BCM uppers?

CMMG lower + BCM upper: yes, they fit

Yes. CMMG AR-15 lowers fit Bravo Company (BCM) AR-15 uppers. Both are AR-15 size receivers. A real mil-spec keeps AR-15 receivers from different makers close, though not exact, and fit problems show up most when the upper and lower come from different makers. We checked them with the rules the AR builder runs.

One exception: the CMMG MK9 Stripped Lower 9mm does not fit the BCM upper. The lower receiver (9mm) cannot support a AR-15 Standard (5.56/.223/.300 BLK etc.) build.

Fit comes from the same rules the AR builder runs on every part you add. Build data only ranks the setups; it never decides fit. Checked November 10, 2026.

Our take

Most built setup: the CMMG MK4 .223 Remington AR Lower with the BCM upper.

About 1 in 10 CMMG lowers in our builds carries a BCM upper.

CMMG lowers

CMMG makes complete AR rifles, pistols and parts, and it's known for Radial Delayed Blowback, its action for pistol caliber ARs in rounds like 10mm and .45 ACP.

Founded 2002, Boonville, Missouri, Made in USA

BCM uppers

BCM makes mil-spec AR uppers, barrels and bolt carriers, and it test fires every complete upper and bolt carrier group before it ships.

Founded 2003, Hartland, Wisconsin, Made in USA

CMMG lowers and BCM uppers by platform

Each row is checked on the most built part of that platform from each brand.

Platform	Fit
AR-15	Yes, they fit
AR-9 (9mm) lower + AR-15 upper	No, they do not fit

AR-15: yes, they fit

Both are AR-15 size receivers. This multi caliber lower takes the upper's 5.56 NATO.

Checked on this CMMG lower

CMMG MK4 .223 Remington AR Lower$100.02

and this BCM upper

Bravo Company Mfg Upper Receiver w/ Laser T-Markings$129.00, out of stock

AR-9 (9mm) lower + AR-15 upper: no, they do not fit

The lower receiver (9mm) cannot support a AR-15 Standard (5.56/.223/.300 BLK etc.) build.

Checked on this CMMG lower

CMMG MK9 Stripped Lower 9mm$149.33

and this BCM upper

Bravo Company Mfg Upper Receiver w/ Laser T-Markings$129.00, out of stock

Most built CMMG lowers and BCM uppers setups

Ranked by how often builders pick the exact pair. Only pairs our rules pass are listed.

#1Yes, they fit12% of these buildsCheck fit
CMMG MK4 .223 Remington AR Lower$100.02 Bravo Company Mfg Upper Receiver w/ Laser T-Markings$129.00, out of stock
#2Yes, they fit6% of these buildsCheck fit
CMMG Lower Reciever Parts KIT$30.65 Bravo Company Manufacturing Standard Carbine Upper$775.35
#3Yes, they fit6% of these buildsCheck fit
CMMG MK4 .223 Remington AR Lower$100.02 Bravo Company Manufacturing Standard Carbine Upper$775.35
#4Yes, they fit6% of these buildsCheck fit
CMMG MK4 .223 Remington AR Lower$100.02 Bravo Company Manufacturing 5.56 NATO Mid-Length Bhf$880.95
#5Yes, they fit6% of these buildsCheck fit
CMMG MkG Stripped Lower Receiver Midnight Bronze$253.99, out of stock Bravo Company BCM® MK2 Standard 14.5" Mid Length$856.39
#6Yes, they fit6% of these buildsCheck fit
CMMG RESOLUTE MK4/ Lower Receiver ZEROED Trigger$558.99 Bravo Company BCM® MK2 Standard 14.5" Mid Length$856.39
#7Yes, they fit3% of these buildsCheck fit
CMMG RESOLUTE MK4/ Lower Receiver ZEROED Trigger$558.99 Bravo Company BCM® Standard 16" Mid Length Upper$973.40, out of stock
#8Yes, they fit3% of these buildsCheck fit
CMMG MK4/ Stripped Lower Receiver Charcoal Green$100.36, out of stock Bravo Company BCM® Standard 9" 300 BLACKOUT Upper$831.45

Watch the frame size. Builders also pair parts our rules say do not fit, like these two: Lower Receiver (9mm) cannot support an AR-15 Standard (5.56/.223/.300 BLK etc.) build.

What CMMG and BCM say

CMMG: CMMG's MkGs lower takes 9mm or .40 S&W Glock and SGM magazines and is meant for CMMG MkGs Radial Delayed Blowback uppers only.

CMMG: CMMG's MkG lower takes Glock and SGM .45 ACP magazines and is made for CMMG .45 ACP Radial Delayed Blowback uppers, and other brands' uppers may work but are not guaranteed.

CMMG: A CMMG DISSENT upper fits most of CMMG's BANSHEE, RESOLUTE and ENDEAVOR lowers only after you pull the buffer tube and stock and install the adapter plate that comes with the upper.

BCM: BCM M4 uppers are machined from 7075-T6 forgings with a Type III hard coat anodized finish.

BCM: BCM's M4 upper takes barrels with an M4 barrel extension, and the bore is cut slightly undersized for a tight fit, so you may need to warm the receiver to seat the barrel.

Check your exact parts Start a build All brand pairs

Fit is judged from the specs retailers list, which can be missing or wrong. Confirm with the manufacturer before you buy, and have a qualified gunsmith check any build before you fire it.

Other uppers builders put on CMMG lowers

CMMG lowers + CMMG uppersMost picked
CMMG lowers + Aero uppersSecond most picked
CMMG lowers + TacFire uppersThird most picked
CMMG lowers + PSA uppersFifth most picked

Other lowers under BCM uppers

Aero lowers + BCM uppersMost picked
PSA lowers + BCM uppersSecond most picked
BCM lowers + BCM uppersThird most picked
Smith & Wesson lowers + BCM uppersFourth most picked
LMT lowers + BCM uppersFifth most picked
Wilson Combat lowers + BCM uppersSixth most picked
Sharps Bros lowers + BCM uppersSeventh most picked
Griffin Armament lowers + BCM uppersEighth most picked

CMMG lowers and BCM uppers: questions builders ask

Will a BCM upper fit a CMMG lower?

Yes. CMMG AR-15 lowers fit BCM AR-15 uppers. Both are AR-15 size receivers.

Do I need a matched upper and lower?

A real mil-spec keeps AR-15 receivers from different makers close, though not exact, and fit problems show up most when the upper and lower come from different makers.

Why might a BCM upper feel loose or tight on a CMMG lower?

Some play between an AR-15 upper and lower is common, but it can get excessive when the two come from different makers, and a tension screw or an Accu-Wedge takes it up.