The Reloading Press is an at-least-weekly feature here on Gaming Ballistic for 2016. Each week it looks at some interesting real-world cartridges and presents them with hopefully-useful information in GURPS Format.
The 6.8x43mm SPC cartridge was introduced as an attempt to increase the lethality of the M16 and (especially) M4 pattern rifles. The cartridge is more-or-less designed to fit into the same magazine length as the 5.56x45mm NATO standard bullet (but it’s not true that they use the same magazines – they don’t), and the lower receiver of the AR15/M4/M16 platform can be used unmodified with the new cartridge. The bolt, chamber, and barrel of course must change, and that can be done with an upper receiver swapout.
Whether or not this was the true design goal, one outcome of the design process that led to the 6.8x43mm is that the test velocity of the barrel is achieved with a 16″ barrel, rather than the 20″ standard for the 5.56x45mm. That means that the M4, with its 14.5″ standard barrel, is much closer in barrel length to the optimal barrel the cartridge is designed around.
Ballistic’s Calculator Inputs
Basic inputs for the calculator are as follows, selecting 406mm for the (test) barrel length.
|Total Accelerated Mass||115||grains|
- The velocity is at the muzzle. The velocity with a 16″ test barrel is tuned to match real-world data at 2575 fps, the highest energy load listed on the Wiki page. I do this because GURPS damage is based on kinetic energy, so no GURPS PC worth their salt will choose anything less than the highest energy if pure penetration/lethality is at issue. If you’re doing supppressed subsonic shooting, that changes.
- The pi to pi- range is where the damage drops from piercing to small piercing. For very short barrels (smaller than 177mm in my model), the bullet is slow enough that it will not yaw and fragment, and so it’s only good for pi- rather than pi. This is defined as 600m/s in my model, which is somewhat arbitrary.
- It’s possible that the damage (penetration, really) is perhaps 10% high. That would still give a 20″ barrel about 6d-1, and a 14.5″ barrel 18 points of penetration, or a full 5d out of an M4 platform.
- Half the diameter of the two rounds would be 6.59mm
- Half the bullet weight between the 62gr and 168gr standard marksman’s bullets for each platform is 115grains, which is where the standard bullet comes in
- The standard AR-15 length magazine holds 25 rounds of 6.8mm SPC; the AR platform magazines typically hold 30, while the 7.62×51 hold 20.
- The 7.62 is about 3400 Joules, the 5.56 about 1760J. Average that to get about 2,550J, where the actual round comes in at about 2,300J.
- The Barrett REC7 is the only 6.8 SPC rifle statted up for GURPS, found (of course) in GURPS Tactical Shooting. For a list price of $2,800, one can be yours.
- I would love to get my hands on a Desert Tech MDR in 6.8 SPC, but they’re not out yet. They’re also supposed to cost between $2,000 and $3,000. But it’s a bullpup, so mrowr.
- Stag arms makes a piston-driven platform in 6.8x43SPC that’ll run the buyer about $1,100, but you need to have a lower receiver to put it on (about $300, and that’s the part that counts as a “firearm.”
- LWRCI went the entire nine yards, redesigning the magazine well and contractiing with Magpul to make the only polymer-case magazines for this cartridge. The rifle itself is about $2,200 complete, and each magazine costs a darn-reasonable $25.
- The magazines for these rifles are quite finicky in my experience. Of the four 25-round steel magazines I own, two will not feed reliably. Barrett and Magpul make good ones, for $50 and $25, respectively.
- You can find some of the above, and many others, in this article. They’re all probably very expensive, though – this round never caught on as much as marketing hoped it would!