The 21st-Century Cartridge: Laurie Holland concludes his study of the 6.8mm Rem SPC

The three deer bullets tried. Left to right: 110gn Sierra SPT; 130gn Speer PSP; 130gn Hornady Interbond

Handloading the 6.8 Rem SPC is fairly straightforward, but does present challenges – mainly finding propellants that provide maximum performance from its small case. Also, since we’re stuck with manual bolt operation and straight-pulls lack the primary extraction cams found in conventional turn-bolts, the powder’s pressure/velocity curve has to suit these rifles if we’re to avoid a struggle to open the bolt after every shot. My SGC SSR-15 was of the ambidextrous variety with a shorter captive folding handle on its left and much more substantial grip directly attached to the bolt carrier on the other side. I consider pressures to be close to the acceptable usable maximum when the less efficient left handle starts to ‘stick’ and the right-side handle has to be brought into play. Fine-grained propellants are often needed to maximise the charge weights that can be physically accommodated, and a powder funnel with a five or six-inch drop tube allied to a slow pour can increase the usable charge weight by another grain or two.

This brings me back to magazines. Thin-walled mags are used to increase the internal width to accommodate the slightly fatter SPC cartridge, edge-to-edge welding replacing overlap joints. A beneficial side effect is that the inside length may also increase slightly. My PRI (Precision Reflex Inc) magazines allowed a COAL of 2.3in compared to the SAAMI maximum of 2.25. What’s 50 thou’ between friends? In this case it’s enough to let you use another 0.4 to 0.5gn of most powders for maybe 20fps additional MV. Such is the hunt for every last few fps, that this is used by American SPC fans to achieve an aggregate 100-150fps over the SAAMI specifications. In the event, I seated my three 110-115gn FMJ/OTM bullets to 2.28in, comfortably within the magazine’s length and none too far out of the rifling lands when chambered.

The SPC is a nice, well-mannered little cartridge suited to straight-pull AR-15 type rifles

Fortunately, the SPC uses its limited powder capacity efficiently and also proved to be tolerant over its fodder. Before I had the rifle rebarrelled, the information I’d seen suggested that powders at the fastest end of the rifle grades were suited to its internal ballistics, the two 4198s (Hodgdon and IMR) frequently quoted, these better known in the old .222 Rem with 50gn bullets. Hornady’s seventh edition of its reloading manual, the first to include the SPC recommended H Benchmark and Alliant Reloder 10x, a little slower but still well up in burning rate charts. Then I went onto Hodgdon’s online reloading centre facility to find data for seven Hodgdon and IMR grades. The fastest burners were still the 4198s, but at the other extreme there was data for H4895 for 50fps higher MVs and IMR-8208 XBR producing 100fps extra. XBR is an Australian manufactured ‘Extreme’ type which is fine-grained and marginally faster burning than H4895 – I had it on hand as it’s an excellent .308 Win powder with 155gn match bullets for long-range FTR shooting.  Translating this to Vihtavuori and Reload Swiss, N120 through 133 looked possible in the former range and new high-energy RS40 in the latter.

QuickLOAD provided its usual invaluable service with some 35 powders available in the UK from eight manufacturers appearing at least marginally suitable. However, many won’t consume the entire charge, some only 90 per cent of it – and that’s in a 24in barrel, so they’d be hopeless in a 16in. It’s necessary to accept some wasted energy potential, but I was unhappy with values below 95 per cent.

I tried 16 powders mostly with the 115gn Sierra and some of those recommended, especially IMR-4198, worked well, others less so. Whilst not producing the highest velocities, I took a liking to Viht N133, which seemed a good all-rounder. My one try with IMR-8208 looked promising with good velocities, but the speed king was the new Reload Swiss RS40, a high-energy (added nitroglycerine) type that also incorporates Nitrochemie’s Ei ‘deterrent’ chemicals, which control burning rate to allow a longer burn at peak pressures. I’ll let the results speak for themselves, but will add a comment on the 115gn Relcom FMJs. Initial COALs were all over the place due to huge variations in their base to ogive (BTO) measurements. Measuring and batching using a bullet comparator reduced three to five-inch group sizes to half that, an astonishing improvement with cheap, low specification products! As far as expanding deer bullets went, three such models plus some old unknown make of 130gn RNFMJ were tried with N133 and it proved possible to exceed 1,700ft/lb ME to meet the Deer Acts’ requirements. The 110gn Sierra Pro-Hunter could be loaded to magazine length, the longer Speer and Hornady 130s too, but they’re better suited to a longer COAL and single-loading into the chamber.
Conclusions? The SPC is a nice, well-mannered little cartridge suited to straight-pull AR-15 type rifles. It lacks the .223’s range of components, and the latter can be loaded with 69-77gn bullets that give better external ballistics than the 6.8’s 115gn models – but the SPC can exceed the 1,700ft/lb Deer Acts’ energy floor, making it more flexible for dual use range and field shooting.

WARNING: These components and loads performed safely in the author’s rifle: this cannot be guaranteed for other firearms. Good handloading procedures should be used working loads up from low starting levels while looking for signs of excessive pressure.