Due to ATF final rule 2021R-08F, all Quarter Circle 10 Full Pistol Builds will no longer come with pistol braces installed or included.
Please contact us if you have any questions before ordering.

Component Compatibility

Listed here are known compatible configurations, either tested by our own staff, or reported to us as successful builds by customers. Our receivers are built to Mil-Spec standards, and will function as intended with Mil-Spec components, but with the vast variety of accessories and parts available, we cannot guarantee compatibility with every part on the market.



Our uppers are designed and milled to match the profile of the Vltor MUR 1 upper receiver. As such, the MUR 1 will have a very tight fit, but our lowers will be compatible with almost any upper receiver you choose to use in your build. Our blowback bolts do not have machined grooves for the use of a forward assist, and we recommend the use of a “slick-side” upper receiver. We can report proper function with the following upper receivers:

Vltor Modular Upper Receiver
GIBBZ Arms G4 Side Charging Upper Receiver
X-Products X-15 SCU Billet Side Charging Upper Receiver



The following triggers have been tested by Quarter Circle 10, as compatible and well functioning within our platform.

Mil-spec trigger group with round hammer profile (no notched hammers)
including enhanced mil-spec triggers like ALG defense.

Hiperfire Reflex

Hiperfire Competition (Formerly known as Hiperfire 24C

Geissele SSA

Rise Armament - All Models

Elftmann AR-9/ AR-45

KE Arms SLT-1

Timney AR PCC Trigger

Timney AR PCC Two Stage Trigger

We regularly test and evaluate new products for use in our weapons. Once approved, they will be added to the list. If you are a Trigger Manufacturer, and would like us to test and evaluate one of your triggers, please send an email to Technical@quartercircle10.com.

Bolts, Barrels and Buffers:

Our CLT9MM lower receiver is compatible with our 9mm bolt, which offers superior machining tolerances and engineering, and is designed specifically to work flawlessly with our barrels. Both CMMG and JP offer 9mm bolts that will also function right out of the box.

Because of the narrower feed lips in a Glock magazine as compared to a Colt-style magazine, our Glock Small Frame lower requires the use of a bolt that has the “Glock Cut” feature. Because of this, we also offer our Glock-cut bolts in all calibers. This is our proprietary bolt, built from the ground up to function with the Glock magazine, rather than simply modifed from existing 9mm bolts. JP also offers a bolt with the Glock Cut.

Proper Buffer and Spring Combinations:
Using the proper buffer weight and spring combination in sub caliber blowback AR platforms is critical. Your weapon will not function reliably, or safely, with a standard AR-15 carbine buffer and spring. Standard 5.56/.223 buffers are typically on the light side ranging from 2.9-4.5oz. With a straight blow back system, a heavier buffer and spring combination is essential.

QC10 designed an Adjustable Blowback Buffer Assembly for use with most pistol caliber carbines, suppressed or unsuppressed, including 9MM, .40 S&W and .45ACP.

We strongly recommend using our Adjustable Blowback Buffer Assembly with the Sprinco Red Spring Buffer Spring. Any standard length carbine or buffer tube will work. We use SB Tactical.

Quick change weight management allows use with multiple configurations.
Package Includes: 4 aluminum weights, 4 steel weights, 4 tungsten weights, 4 rubber spacers, buffer body and allen wrench.

9X19: 5-8oz buffer

.40 S&W: 7-9oz buffer

.45 ACP: 9-11oz buffer


When Using a Suppressor

9X19: 8-11oz buffer

.40 S&W: 9-11oz buffer

.45 ACP: 11oz buffer


Stocks, Magazines and other Accessories:

Any stock or arm brace that is compatible with a Mil-Spec buffer tube will work with our lowers.

For the Colt 9mm lower receiver we recommend the Metalform Magazine.

For the Glock Lowers we have found that the Glock OEM Magazines work best. SGM Tactical magazines have also proven reliable in most cases, and many customers like ETS Group magazines as well.

Any other parts or accessories that are manufactured to true Mil-Spec standards should be compatible. Unfortunately, we cannot test or even list every accessory that is compatible with our lowers, and common sense and product measurements should be consulted before the purchase of parts. If at all possible, try to find your desired part locally to verify a perfect fit, or check back with our constantly growing list of verfied compatible components and accessories.