6mm MAX is an exciting new intermediate cartridge that combines AR15 small frame weight and portability with AR10 large frame performance. Developed by BC Precision Ballistics and championed by Sons of Liberty Gun Works, the 6mm MAX can move projectiles from 55gr to 105gr at incredible velocities while maintaining low pressures and higher reliability than other intermediate cartridges in its class. We discuss the 6mm MAX, ballistic performance, comparisons to relevant cartridges, and what you'll need to build a 6mm MAX rifle for yourself.
6mm MAX was developed with special operations forces and competitive shooters in mind. The cartridge bridges the gap in performance between the AR15 and AR10 platforms by improving ballistic performance of the small frame rifles, which are more handy to carry, allow for higher ammunition capacity, and have greater options for standard parts. Similar cartridges include the 6.5 Grendel, 6mm ARC, and .224 Valkyrie. While these other cartridges have had more time on the market to accrue industry support, they are missing a critical aspect of performance that 6mm MAX capitalizes on - Reliability.
Case Origins
6mm MAX is a bottleneck version of the .350 Legend straight-wall case. .350 Legend was originally developed for semi-auto shooters whose states required straight-wall cases for hunting. The straight-walled case allows for 35 gr of water capacity when necked down for 6mm MAX, which is similar capacity to the 6mm ARC (34 gr) and 6.5 Grendel (35 gr) while utilizing a smaller case diameter. For reference, the .223 Rem case holds 29 gr of H20. Shooters may also recall the similar 6x45mm (27gr), which used a 6mm bullet in a tapered .223 Rem case. Thus, the 6mm MAX has an equivalent powder capacity with the other, wider-case intermediate cartridges, and a greater capacity than cartridges with the same case base diameter.
Feed Geometry
Reliable feeding in semi-auto rifles is critical to cartridge success. The thinner case body of 6mm MAX keeps the center axis of the round closer to the bore during feeding, which results in more reliable feed geometry than the more unconventional case shapes of other intermediate cartridges. Feed ramps on the barrel extension have also been modified from the regular M4 ramps in order to assist feed reliability for longer, heavier projectiles.
Bolt Design
Bolt reliability has been a source of frustration for many competition shooters using the 6mm ARC cartridge. The 6mm ARC uses a 6.5 Grendel Type II bolt, which has the same outer lug dimensions as a regular mil-spec bolt, but material on the bolt face has been reduced in order to fit the larger diameter case. However, 6mm MAX uses a standard mil-spec 5.56 bolt, which provides for stronger bolt lugs and extractor. Thus, reliability of the 6mm MAX bolt has a tremendous increase over similar intermediate cartridges.
Cartridge Overall Length (COAL)
As the cartridge was designed with ultimate performance in mind, every aspect of the AR15 small frame design was analyzed to allow for the cartridge's highest potential. The 5.56 magazine typically limits overall case length to 2.26" COAL. However, to gain additional performance in 6mm MAX, the designers chose to use the already-available .350 Legend magazine design, which provides for a slightly longer 2.30" COAL and uses follower and feed-lip geometry already designed for the case body. This allows the case to have a slightly longer COAL in order to take advantage of heavier bullets up to 105gr. Note that polymer AR15 magazines will not accept the 6mm MAX overall case length.
Magazines
Users will notice the .350 Legend magazines have a straight body design, especially when compared to a 6.5 Grendel / 6mm ARC magazine that has an exaggerated curvature. The .350 Legend magazines typically come in capacities up to 20 rounds, although DuraMag has prototyped 30-round magazines that have been tested in Quantified Performance competition.
6mm Max Ballistic Data, Velocities, and Ballistics
One of the most unique aspects of 6mm MAX is the cartridge's ability to use a wide range of bullet weights at SAAMI pressure standards. Shorter rifles will see incredible velocities from lighter bullets, while longer barrels will benefit from the ballistic advantage of longer, heavier projectiles. A velocity chart for factory loads of 6mm MAX is shown below. This data was collected by BCPB on Oehler 85 instrumentation at 55kpsi.
Barrels chambered in 6mm MAX will typically come with a 1:8 twist, although there has been some development effort on barrels with 1:7.5 twist. As the cartridge continues to develop, we'll be able to see which option shooters prefer.
Comparing ballistic performance of other popular small frame options, we can review the 6mm MAX elevation and wind drift performance at distance against similar cartridges. 6mm MAX and 6mm ARC ballistic performance shows extreme parity, with performance between bullets showing little to no relevant difference. We also compare ballistic data of the popular 5.56 77gr SMK for reference on elevation and wind deflection. The 6mm bullet is a clear improvement in performance for the small frame gun. All data listed below was measured from 16" rifles and factory loadings.
Competition Performance
The 2023 Quantified Performance season finale saw two major competitors shooting the 6mm MAX in precision gas gun competition. Mike Russo competed with a 16" SOLGW rifle in the General Purpose division, while Keith Steffens vied for the Open division title with his 20" SOLGW rifle. Both competitors ended up coming out on top of their respective divisions at the conclusion of the weekend, earning the champion titles for the series. The 6mm MAX performed well for these shooters, and we look forward to seeing it in the 2024 season!
QP Open Division Champion Keith Steffens and the SOLGW Broadsword in 6mm MAX.
What You'll Need to Build Your 6mm MAX
In order to build your own rifle chambered in 6mm MAX, you'll only need 3 new items: a chambered barrel, .350 Legend magazines, and 6mm MAX ammunition. All other parts of the build can use mil-spec or aftermarket parts designed for the AR15, including receivers, bolts, gas system components, and other related bits that go into building the small frame gun.
Here's what we know is coming for the 6mm MAX:
Sons of Liberty Gun Works is building complete rifles and uppers
DuraMag has 20-round mags and is developing 30-round magazines
Major brass and ammunition manufacturers have agreed to begin production of 6mm MAX ammunition (we don't have word yet on which manufacturers)
BCPB Factory ammunition is available direct, with SOLGW also making options
Reloading brass is already available from BCPB, with other factory options said to be coming soon
Lee Reloading dies are currently available for reloaders
6mm MAX offers performance worthy of a large frame rifle in a small frame package. With improved reliability over other intermediate cartridges in its class, the 6mm MAX looks to be a clear winner for competitors and high-end professional users.
