Reloading Press: .277 SIG Fury (a 6.8 Next-Gen Possibility)

The Reloading Press is a recurring feature here on Gaming Ballistic. In it, I use my ballistics spreadsheet (based on my original article Interior and Terminal Ballistics for GURPS, available for free from the digital version of Pyramid Magazine) to work through various cartridges, and note what kind of weapons use them. It’s a pretty detailed – perhaps overly so – look into the vagaries of projectile and cartridge design for firearms. It’s based on the cartridge because in most cases, while different weapons have pros and cons, quality and deficiency in the real world, these frequently compress into a fairly small amount of variation in the game (GURPS and Powered by GURPS products). While a firearms enthusiast may well have (or certainly WANT) to own or try fifty varieties of 5.56x45mm rifle…most of them boil down to “does 4d+1 to about 5d+1 depending on barrel length, with minor variation in Acc, weight, and a few other stats.”

Even so…sometimes you really wanna know. Or fling something odd downrange. And that’s where the Reloading Press comes in.

Well, that’s that. Recently I’ve been hearing rumblings about a new 6.8mm concept that the Army wants to field. This has included both an infantryman’s rifle, as well as a squad automatic weapon. That one, called the Next Generation Squad Weapon, has seen some fun videos. The competition got more publicity over the last few years, and finally something resembling cartridge data out there, in the form of the SAAMI specs for Sig-Sauer’s “.277 SIG Fury.”

This is not a new version of the 6.8 SPC. It’s not even close. So if you start to think that…think again.

.277 SIG Fury (6.8x51mm)

The .277 Fury is designed as part of a weapon system with the Sig-Sauer entry in the weapon competition. Rather than go deep on the variants…here’s a photo.

What’s up with the multi-part construction of the Sig Sauer case? It’s because the thing operates at a screaming-high 80,000 psi. Most huge-o-normous magnum cartridges like the .338 Lapua Magnum are in the 60,000 to 65,000 range (the .338 Lapua Magnum itself is just shy of 61,000 psi). This one seems like it’s in an entirely different ballpark. So much so it starts to seem like some of the discussion about TL9 Firearms and LP, ETC, and ETK propellant has relevance. The company had to redesign the case (which is otherwise similar in dimensions to the .308 Winchester case) to contain such pressure, using a steel rather than brass base, and three-piece construction.

The requirements of the cartridge seemed to have been a 3000fps projectile in 6.8mm caliber…out of a 16″ barrel. The .277 SIG Fury claims to deliver.

Let’s see…

Ballistic’s Calculator Inputs


Basic inputs for the calculator are as follows, selecting 406mm for the barrel length.

INPUT
	.277 SIG Fury
Chamber Pressure	80000	psi
Barrel bore	6.8	mm
Case Length	51	mm
Chamber Bore	11.53	mm
Barrel length	406	mm
Bullet Mass	135	grains
Aspect Ratio	5.3	L/Bore
Burn length	17.21564297	mm
Projectile Caliber	6.8	mm
Total Accelerated Mass	135	grains
Expansion Ratio	2.2	expansion
Projectile Load	1

The target barrel and set velocity get to where it’s supposed to get: 135gr/8.75g bullet at 3,005fps/916m/s from a 16″ barrel. I hope they develop some really nice coatings for that barrel, because at 80,000 psi…yowzers.

In any case, this projectile, which is much closer in mass to the usual 147 or 150gr 7.62x51mm NATO than the 62gr of the Mk318 Mod 0, or even the 115gr 6.8 SPC. In fact, it’s looking a heck of a lot more like a fatter 6.5 Grendel, and the length of the .277 SIG Fury seems in the Grendel’s ballpark. So it should be pretty good choice for range, too.

Notable notables, it looks like it’s about 7d+2 out of the muzzle of a rifle with a 16″ barrel, doesn’t fall to half its velocity to 700yds, has a max of just shy of 5,000 yds, and stays supersonic (and therefore accurate) out to 1,000 yards. This supports the contention seen that it’s better than .308. The lower mass and 916m/s velocity actually yields lower momentum (but not by much) than the 7.62 NATO round as well. Looking at building a rifle around the thing, let’s go up to a sniper-length 26″ barrel…for reasons to become clear later. At the low end, we’ll start at a barrel length low enough for this monster to do “only” what the 6.8SPC does…about 6d pi, or 21 points of damage. That happens at 213mm.

• The velocity is at the muzzle. The velocity with a 16″ barrel is tuned to match real-world data at 3000 fps, with the assumption that it is the lighter-weight 135gr projectile used to meet this spec.
• This projectile has a “native” wound channel modifier of larger than 0.7, which is the step from 0.5 to 1.0 on the SSR. As such, it will not ever drop from pi to pi- . . . if GURPS breakpoints corresponded to real-world ones…the 6.8mm projectile sits just over the happy end of one.
• It’s possible that the damage (penetration, really) is perhaps 10% high. That would still give a 16″ barrel about 24 points of penetration (6d+3), which ain’t bad.

The cartridge develops velocity quickly as the barrel goes from a quite-silly “less than 10 inches” to a more common 14.5 and higher. Even out of the 14.5″ barrel that is commonly seen on some M4s, you’re still punching at a higher calculated penetration rating than the 7.62 NATO. But that’s not where this one is going to shine. With 80,000 psi behind it, you can increase damage to over 8d with a longer barrel. Say…the 20-22″ barrels easily achievable in a compact carbine in a bullpup configuration.

The old Bushmaster M17S had a 21.5″ barrel in a 30″ rifle. The Desert Tech MDRX is about the same with a 20″ barrel. The M4 Carbine is 29.75″ long with a collapsed stock, and 33″ long with it extended. If we allow a 33″ rifle, bullpup configuration, you can have the same compact rifle as the M4, but with a 20-23″ barrel. In GURPSy terms, that’s 8d or 8d+1.

Platforms

So given that rifles not designed for it will probably tear themselves apart, the .277 SIG Fury is part-and-parcel with a purpose-designed system, shown in the photo here.

This one is suppressed, which I have to consider a good idea given how much pressure the round has to generate (though a conventional amount of very fast burning powder will do that nicely). Note the rifle suppressors are something like 7″ long, meaning that our notional 30″ bullpup can slap one of these on and still be shorter than the 40″ M16A4.

Do I have a thing for bullpups? Yes. Yes I do.

The weapon and its competitors (including one bullpup) are still in design, prototype, improve, and test phase. The squad weapon may well be the first out the door, and there’s some real innovation going on in this space too from a weapon usability perspective.

And we haven’t even gotten to the more odd of the rounds. The polymer-cased True Velocity entry, as well as Textron’s fully telescoped version. I guess we’ll have to watch and see!