The .375 SOCOM Handloading Challenge

Having been a fan of the .458 SOCOM, Lane wanted to try the .375 SOCOM. He found the handloading project to be challenging but also rewarding.

March 13, 2023 By Lane Pearce

I violated one of my most seriously given “don’ts” while preparing this column. I’ve often cautioned, “Don’t use any load data found on the Web unless you can verify it from a known, reliable source.” My only excuse is the round is a necked-down variation of a wildcat that was loosely derived from a SAAMI-approved parent cartridge. And the Web data was a Quick Load spreadsheet that had the actual components and performance criteria I intended to use.

So why is the .375 SOCOM so interesting that I chose to violate my own advice? I already had a .458 SOCOM upper for my Rock River Arms lower, so I was familiar with its origins. Similar to the straight-walled .50 Action Express pistol cartridge, the SOCOM case is 0.3 inch longer, and the rim and head diameter are smaller. The .458 caliber was achieved by simply bottlenecking the case.

The .458 SOCOM is not a SAAMI-standard round, but Hodgdon’s reloading data center provides load data and indicates max operating pressures up to 39,400 CUP. Another attraction of the SOCOM round is that standard AR-15/M16 magazines can be utilized without modification.

Your immediate thought might be, “Why do you need a .375 SOCOM?” Well, I met Buddy Singleton, owner of Southern Ballistic Research (SBR), several years ago at a SHOT Show and became interested in the .458 SOCOM. I purchased an RRA AR-15 and the SBR upper, and then I prepared a reloading column. Later, Buddy offered me a good deal on a .375 SOCOM upper assembly, CH4D reloading dies, some SBR factory ammo, and a 100-count bag of brass.

Since I wasn’t going to be able to verify the .375 SOCOM Quick Load data with any of my usual reliable sources, I resorted to using my trusty old Powley computer. The Powley device was designed with two important criteria: 1.) It addressed only the then-current IMR propellants; and 2.) The predicted loads were based on yielding safe maximum pressures of approximately 44,000 CUP. And since there’s no direct correlation between CUP and psi values, you must use the predictions cautiously.

The first step is to determine the case capacity by weighing a fired case before and after filling it with water. A friend who worked at Hodgdon once told me about an easier way to do this. H335 has the same specific density as water. To avoid the mess and bother with filling the case with water, wiping off the excess, and maybe spilling it onto your electronic scale, just gradually add this free-flowing spherical powder until you reach the base of the case neck.

I performed this process using a few cases and determined 46.5 grains as the average capacity. The Powley tool indicated 40.0 grains as the proper charge weight of an as-yet-unknown propellant. I intended to load Speer’s 235-grain Hot-Cor SSSP flatbase bullet, so the ratio of charge-to-bullet weight was 0.17.

The sectional density of the Speer bullet is 0.240, so after closely aligning these two values on the next scale, the Powley computer indicated the “F” powder positioned between IMR 4198 and IMR 3031.

Since there is no IMR “F” canister powder, the instruction sheet directed me to increase the predicted charge by 5 percent and use that amount of the slower burn rate IMR 3031. Bingo! Right away I had the recipe, but the Powley’s pressure criterion exceeds the unofficial limit specified for the .375 SOCOM cartridge, which is about 35,000 psi. What to do?

I reduced the recommended charge by 10 percent and worked up carefully. From everything I read about the cartridge’s performance, I expected to obtain 2,050 fps from an 18-inch barrel with the 1:12 twist. I eventually recorded 2,074 fps using 42.0 grains of IMR 3031 without any obvious pressure issues. As you can see in the accompanying chart, that load delivered 1.32-inch three-shot groups from the bench at 100 yards.

I tested a half-dozen other propellants with similar burn rates, again starting with estimated reduced charges and working up to 2,050 to 2,100 fps. A couple recorded 2,150+ fps, so I backed off the charge accordingly. The most promising ones are included in the chart.

That unknown optimum “F” powder is Hercules HiVeL No. 2, which was discontinued in the mid-1960s. I happen to have some, so I tested a few rounds and found it would have been quite compatible.

Since they were derived from a handgun cartridge, a Large Pistol Magnum primer was carried over to the SOCOM cartridges. I tried both Winchester and CCI primers in my handloads with good results, i.e., minimum velocity dispersions and no pierced primers. Don’t substitute rifle primers because the primer pockets are too shallow, and the primers will protrude. That will likely cause a headspace problem when the bolt is released or could result in a slam-fire.

Most of the powder charges were compressed, with some of the slower propellants being compressed quite a bit. I rolled a cannelure on some of the Speer bullets to allow a light crimp of the case mouth. After the first range session, I observed case neck tension was adequate to prevent bullet creep even if the charges were compressed.

Towards the middle of my project, I was able to have a couple of the hand-loads shown in the chart tested for pressure in SBR’s ballistic lab. They are marked with an asterisk. And with that limited pressure data, I was able to reevaluate the other handloads based on relative burn rates and recorded velocities. Although my load data is not fully corroborated by extensive lab testing, it’s much better than flying blind using typically unverified recipes found on the Web.