Reloading Press: 6.8x43mm SPC
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.
6.8x43mm SPC
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.
| INPUT | ||
| 6.8x43mm SPC | ||
| Chamber Pressure | 54000 | psi |
| Barrel bore | 6.8 | mm |
| Case Length | 43 | mm |
| Chamber Bore | 10.7 | mm |
| Barrel length | 406 | mm |
| Bullet Mass | 115 | grains |
| Aspect Ratio | 3.6 | L/Bore |
| Burn length | 21.3 | mm |
| Projectile Caliber | 6.8 | mm |
| Total Accelerated Mass | 115 | grains |
| Expansion Ratio | 2 | expansion |
| Projectile Load | 1 |
The rifle develops appreciable damage at near pistol-length barrels, at least within the assumptions of the calculator. It would equal the injury of a .45 ACP even at pi- ratings in a barrel the size of a long revolver. Even so, calling it “pi-” does it little justice. It would earn a wound channel modifier of 0.7 if the scale were left open to any number, compared to the 0.5 of the 5.56x45mm, the 1.0 of a 9mm, and a 1.2 of a 0.40 S&W.
- 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.
You need a purpose-built upper receiver (or at least a barrel, chamber, and bolt change) to shoot this cartridge. Some rifles and uppers can be found below.- 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!
Building your own and putting the savings into optics is also an option.
I assume that since you seem to like 6.8 that you're on the 68forum? There was a decent thread taking apart Nathanial F's Firearm Blog post. He's biased against and cherry picks the data is the upshot.
I am a romantic about the .280 Brit, so when this round came around I embraced it. Made GURPS stats for both via the "that looks about right" method and played on. I was happy that my handwaving was close to the Tactical Shooting stats!
Most often though 30 rounds of 5.56 seems more effective at 5d than 25 rounds of 6.8 at 6d at the actual table. Darn it. I persist on taking the oddball when we're gaming a setting where resupply is viable.
Angus! Welcome back!
I'm not on th 68forum, but I probably should be. I'm not surprised that the post I cited got taken apart – I'll try and find the thread and link to it. Data cherry picking is nearly the US National Sport . . .
I think you're probably right about the relative value when rolling 5d vs 6d. In my games, I tend to use Armor as Dice, so that extra 1d injury appear every time, so it's a stronger case to be made for it.
As far as resupply, my players tend to make up for heavy ammo with lots of ST – Peter, I'm looking at you. 🙂 The only reason I'm not using a 6.8 rifle in the Castle of Horrors game I'm playing in right now (modern guys invade Castle Ravenloft with the serial numbers not-quite filed, using GURPS) is that I decided for no good reason to give my guy a quirk about only using FN weapons.
We trended for a long time towards game settings where no ammo just wasn't available unless it was the most common stuff made. It wasn't mass so much as availability.
I'm really an FAL man at heart. 😉 No availability issues with 7.62x51mm NATO most settings.
Oh, I think your drop to pi- is brilliant! I'd never get to implement it because what players I have are already complaining about complexity if the rules as written.
One can dream though!