Cartridge assessment began with the 6mm PPC case, necked up to 6.5mm. The 5th
SFG soon discarded the fat PPC case due to poor magazine capacity and
insufficient reliability in the M4. Their attention then turned to the .30
Remington case, which is essentially a rimless .30-30 Winchester. Its head and
body diameters are larger than 5.56 (0.378 inch), but smaller than 7.62x39mm
(0.445 inch). This thoroughly obsolete cartridge was chosen as the parent case
because its smaller head diameter (0.422 inch) required less metal to be cut
from the bolt head compared to the PPC or 7.62x39mm cases, which improves bolt
service life. Several rebated-rim prototypes were created with an SPC body but
5.56's rim (0.378 inch) to utilize unmodified M4 bolts. After trials, it was
clear the full-diameter rim helped extraction as compared to the rebated rim
design.
Once the case dimensions were tweaked to fit and work in M4-compatible
magazines, the project team quickly turned their attention to bore size.
Derivative wildcats from 5.56mm to up 7.62mm diameter shooting bullets from 90
to 140 grains were subjected to a battery of tests, and a sweet spot emerged.
The 6.5mm bullets showed the best accuracy and the 7mm bullets were the most
destructive, but the 0.277-inch bullets showed almost the same accuracy and
trajectory as the 6.5mm and almost the terminal performance of the 7mm. When
necked down to 0.277-inch and shooting 115-grain bullets, it provided the best
combination of combat accuracy, reliability and terminal performance for up to
500 meter engagements. This cartridge was deemed 6.8 Remington Special Purpose
Cartridge (SPC), because 0.277 inch is 6.8mm in metric and .30 Remington
provided the parent case.
Numerous articles and Internet rumors have suggested that the SPC designation
means 6.8 is good only for Close Quarters Battle (CQB), but not distant targets.
This is incorrect, and contrary to the intent of the project and capabilities of
the cartridge. The SPC designation was assigned based on the intended
integration into the Mk12 Special Purpose Rifle (SPR). The SPC was designed
from the ground up to provide increased energy, barrier penetration, and
incapacitation from the Mk12 SPR, from contact distance to 500 meters.
Based on their experience with 7.62x39mm, the project team set a velocity goal
of 200fps faster than the AK-47 ammunition from the same barrel length, with a
projectile that provided a better ballistic coefficient (BC) and terminal
performance. This was achieved very soon into the project using Sierra
115-grain and Hornady 110-grain Open-Tip Match (OTM) bullets. Using Ramshot
1660 powder for initial development, the team easily exceeded the 200fps goal.
Shooting from an 18-inch SPR barrel, these loads shot 2635 to 2650fps, 300fps
faster than the AK-47.
