The US Army’s Newest Caliber: .277 SIG FURY/6.8x51mm

Ever since the M16 and 5.56 NATO was adopted way back in the early 1960s the Army has been trying to replace them with something newer, better, and more capable. After 60 years, tens of millions of dollars, dozens of attempts, and who knows how many man-hours — they might have finally done it. Introducing the .277 SIG FURY or as the Army likes to call it the 6.8x51mm.

What The Hype Is About

You might have read or heard that in late April 2022 the Army announced that Sig Sauer was given a contract for the Next Generation Squad Weapon (NGSW).

This is big news because the Army claims that these two new weapons will eventually replace the M4 and M249s that have been the backbone of the Army for decades.

New weapons aren’t as much fun without a new cartridge, and with the modern battlefield changing how we fight — something new is, seemingly, required.

Officially known to civilians as .277 SIG FURY the Army is simply calling it 6.8x51mm for now.

Firing a 130-140gr bullet at 3,000 FPS from a 16″ barrel, this has the makings to be one hell of a cartridge.

Why Was The .277 SIG FURY Developed?

Back in 2017, the military conducted a study that found that soldiers needed a new cartridge to better defeat near-peer armor at range.

Because this is a huge study that is fundamentally changing American warfighting doctrine, the “Small Arms Ammunition Configuration Study”, has not been released to the public. So we don’t actually know much about what is in it.

Bottom line though, Army brass was convinced that we needed a new cartridge. Also, that cartridge should be 6.8mm.

Based on that study the NGSW program was founded about a year later.

The requirements were for a new rifle system and for a new cartridge. For the ammo side of things, it needed to be accurate to at least 610m and able to defeat near-peer body armory to 500m and it had to use a 6.8mm projectile.

Sig Sauer, along with several other brands, got working and a half dozen prototypes were quickly developed. Sig’s version won out.

6.8x51mm, 80,000 chamber PSI, 140gr bullets, with a barrel life of ~12,000 rounds. This isn’t like what we’ve seen before.

What Makes .277 SIG FURY Different

Right off the bat what stands out is the crazy high chamber pressure of 80,000 PSI. For context, 5.56 NATO has a chamber pressure of about 60,000. .308 Win and 6.5 Creedmoor both run about 62,000 PSI.

SAAMI, the organization that standardizes cartridges, recommends that no cartridge goes above 65,000 PSI — even big boys like .338 Lapua Magnum only run about 60k.

So for .277 SIG FURY to be an 80k PSI round… this is very different.

In order to meet those pressures, the case for .277 SIG FURY is a multi-piece construction made from brass and steel.

The case body is made from brass, although thicker brass than is commonly used in cartridges. But the base is made from stainless steel, this is where the magic is.

This dual-construction is what allows the case to handle the insane pressure of 80k PSI.

All of that pressure means this flings a 140gr round at around 3,000 FPS from a 16″ barrel. Keep in mind that 6.5 Creedmoor is normally around 2,700 FPS from a 24″ barrel.

Ballistically, .277 SIG FURY is kind of like a 6.5 Creedmoor on crank and does it from a barrel that is 8″ shorter.

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The Army’s New Guns: XM5 & XM250

With this new bad-ass cartridge comes new rifles to fire it. Since .277 SIG FURY is 51mm long, it cannot fit in a normal AR-15/M4/M16 platform.

Thus, the need for a new rifle. Basically, something AR-10ish sized.

The XM5, commercially known as the Sig Sauer MCX Speer, is basically Sig’s MCX rifle that was released in 2015. Except the XM5 is bigger, basically an AR-10 size rifle.

Using a short-stroke gas piston, a two-rod design for the bolt carrier, and no buffer tube — this might seem familiar.

If you think about it, the MCX/XM5 is really not too much different than an AR-18. At least where the gas and bolt system is concerned.

Sure, the MCX adds a forward assist and moves the charging handle to something like the AR-15/M4, but design-wise… this is basically a souped-up AR-18.

Life uh… life finds a way.

For something even bigger, the XM250 is a belt-fed machine gun that is meant to replace the M249 currently used.

I haven’t gotten to see the XM250 yet, but I have seen and even got to fondle Sig’s MG-338, the .338 Norma Magnum version of the XM250.

This is a beast of an MG and takes a fresh new look at how the MG should be built. With great little quality of life features like feeding on both left and right sides, a side-flip feeding tray, and an M4 style safety, this is pretty cool in every way.

But sadly, that’s about all we know about it right now.

What Does This Mean For The M4/M16?

According to the Army brass, the XM5 should be fielded with combat units starting late-2023. This is mostly constrained by ammo availability, Sig Sauer is producing the ammo for now but Lake City is gearing up for production also.

While the brass is super, super hyped about this new weapon system… I remain unconvinced that it will actually fully replace the M4.

The XM5 is about 2 lbs heavier than the M4, ammo weighs almost 3 times as much meaning the average grunt is going to carry a lot less ammo, and we have yet to see the hyper need for this magically new cartridge like the Army claims there is.

Russia’s invasion of Ukraine is the most modern war the world has seen for a long time. 7.62×39, 5.45, and 5.56 are proving to be effective over there. So does this critical need for a cartridge that can defeat armor at 500m really exist?

I don’t have all the answers and my crystal ball is in the shop, so we’ll just have to wait and see.

For now, Sig Sauer is only getting $20 million dollars from the Army for ammo, replacement parts, training, two new weapon platforms, and a lot more.

Sig will be delivering those soon, but that $20 million isn’t going to replace the M4. Not by a long shot.

Is The M4/AR-15 Obsolescent?

In a word: No.

The AR-15 is by far the most effective and efficient rifle ever made. 5.56 NATO is a good cartridge that offers low recoil, high lethality, and ease of use.

For everyone who isn’t trying to punch body armor from half a kilometer away, 5.56 NATO does the job.

While the XM5 or even the MCX might seem revolutionary and new, they really aren’t. At their core, these are basically AR-18 rifles that have been modernized and resized.

I would love to see the AR-18 finally get its day in the sun, but if you really want that design system so badly — get yourself a Brownells BRN-180 upper and slap that bad boy on your AR-15 lower.

Wrapping Up

And there we have it. This is what all of the excitement is about.

While the .277 SIG FURY cartridge is very cool and I’m really looking forward to getting my hands on some (Maybe in a Sig Cross??), for now, it doesn’t really change anything.

The same with the XM5, on the outside it might seem totally new, but really it’s just an oversized AR-18.

The XM250 is much more interesting, but sadly that isn’t something any of us can get our hands on… likely ever.

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