Bullets, MVs and groups: Part two of Laurie Holland’s investigation into the 6.8mm Rem SPC

Southern Gun Co. SSR-15 rebuilt with a 24in 6.8mm SPC barrel

We have a small AR/M16/M4 compatible .270 calibre cartridge, the project inspired and driven by American military Spec-Ops users. It employs a slightly shorter case than that of the 5.56/.223, but it’s fatter – increasing the capacity – and has a longer neck. (Incidentally, it’s often said that the 6.8mm can use .223/5.56 AR magazines with a follower change – not so as the fatter cartridges make the magazine sides bulge.) Bullet weight has risen from the 5.56’s 62-grain (77-grain Special-Ops loading) to 115-grain and diameter from 0.224 to 0.277in to increase wounding effects. The mix saw load development input from specialist ammunition manufacturer Silver State Armory (SSA) alongside Remington’s heavy lifting. Ballistically, the cartridge is suited to 16-18 inch barrels, additional length only garnering around 20fps per inch. The M4 carbine’s 14.5in barrel doesn’t reduce performance much, although the very short tubes used in Special-Ops weapons knock a couple of hundred fps off. In almost any situation and usable range, the 6.8SPC should outperform the 5.56 M855 in retained energy, likewise the opposition’s 7.62×39 AKs (Table two).

Hiatus

PRI 6.8SPC magazine – a clever redesign to squeeze the fatter rounds in whilst retaining the .223 magazine’s external dimensions

All that’s missing then is the cartridge’s nominal MV and ME values, and here we have a problem. Initial reports from field testing said 2,800fps from 110/115-grain bullets, and that’s what Remington promised in 2002. Then word circulated that the SPC had ‘pressure problems’ necessitating downloading, and there was a considerable hiatus before ammunition went on sale. Officially, the American SAAMI firearms industry regulator lists 6.8SPC as a 115-grain bullet at 2,725fps, 55,000psi Maximum Average Pressure, from a 1-10in twist 24-inch barrel, let’s say that’s 2,575-2,600fps from a 16-inch tube. Remington lists its various 115-grain bulleted factory rounds at 2,625fps, no barrel length quoted but likely 24in. Hornady shows its 100-grain GMX bullet ‘Full-Boar’ load, 110-grain V-Max and OTM rounds as 2,550fps; the 120-grain SST medium deer version 2,460fps but more realistically from a 16in test barrel. Table one shows what I actually achieved with Remington’s 115-grain ‘Premier Match’, and Hornady’s 110-grain OTM in my 24in Krieger barrelled SSR-15 comparing them to a .223 Rem handload from the same rifle and 7.62×39 factory ammo. The Hornady product produced MVs a bit down on those expected with an extra 127fps from the longer barrel (compared to +150-160fps), whereas Remington’s 115-grain Match load was over 50fps down on claimed MVs. Incidentally, Hornady’s 120-grain deer load should produce 1,612ft/lb ME in a 16in barrel, so it’d be UK deer-legal in a 20in rig, the extra 80fps or so MV taking energy to over 1,700ft/lb.

Chambers

The problems apparently arose with Remington’s SAAMI chamber specifications, its version given a short 0.050in freebore. Worse, incorrectly quoted dimensions saw the first chamber reamers produce an 80-degree cone angle in the throat instead of 45 degrees. Early rifles therefore produced excessive pressures with loadings that had worked fine in SSA’s load development and SOCOM field testing, exacerbated by the cone abrading bullet jacket material and causing rapid metallic fouling build-up at the back end of the barrel. Factory ammunition therefore has to be loaded to lower pressures as long as there is any possibility of it being fired in a rifle with such
a chamber.

7.62×39 CZ carbine. This (500 yard) range proved to be the maximum accurate distance on a cold November’s day

Even with the cone angle sorted, SPC fans regard the SAAMI chamber as sub-optimal, and with the cartridge having become an AR-enthusiast’s number, improved versions soon appeared. The main one has the cartridge name followed by ‘Spec-II’, (usually abbreviated to ‘SPCII’). The ‘6.8X43’ chamber is a similar proprietary form. They increase freebore to around 100 thousands of an inch, a little short of that in the case of the 6.8X43. Whilst the SAAMI specification calls for a six-groove one in 10in twist barrel, SPC aficionados recommend 1-11.25in with three groove or polygonal rifling. In concert, these changes allow loads that increase 115-grain bullet MVs by 125fps, moreover handloaders can use heavier charges than factory loads in any chamber with the correct cone angle. Returning to Table two, it’s likely SPCII chambered carbines’ external ballistics match those of 5.56/.223, but with significant terminal energy increases.

After its delayed launch, some SPC handloading features eventually appeared in American shooting publications, often using a custom CZ527 based rifle with a match quality barrel and the excellent H-S Precision bedding block stock from the ‘Varmint’ version of the Czech mini-Mauser. One such appeared in Hodgdon Powder’s 2007 Annual Manual with handloading results from the CZ bolt-gun, also reporting on performance in a Stag custom AR-15 semi-auto. I wasn’t impressed by the MVs, still less so by groups which averaged a bit above the inch at 100 yards. The author Frank W. James opined that 3/4 inch is as good as it gets. I suspect these tests used rifles chambered with early faulty reamers. In any event, they use the SAAMI chamber and understate the cartridge’s capabilities in a properly built custom rifle. My straight-pull SSR-15 had an SPCII chamber, so no problems there.

Before leaving the cartridge design, there is another noteworthy feature – primers. Hornady and SSA brass use Small Rifle primers as per the 5.56/.223, but for some unfathomable reason Remington chose the large version for this small case with its modest (under 30-grain) powder charges. In theory, this will ‘over-primer’ the cartridge, but my concern is that it leaves less metal in the small diameter case-head, weakening it and likely shortening case life through the head expanding and primer pocket loosening after fewer firings. The two types did allow an interesting experiment. Having ascertained that both have near identical internal capacities (water content weight in fireformed examples), I was able to see if MVs were affected. I loaded up 10 examples of each case with 115-grain Remington FMJBTs, 270-grain Viht N130, and one make of primer – CBC Magtech. There was little difference in the results.

Hornady / Magtech 7½ SR: 2,721 fps average; ES 43; SD 13; Group 1.7in

Remington / Magtech 9½ LR: 2,701 fps average; ES 37; SD 13; Group 1.5in