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What XCR Caliber and Chasis are you?

  • XCR-L in 6.8x51

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  • XCR-M in 6.8x51

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  • Nyet! XCR-M in .308 is fine!

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Discussion Starter · #1 ·
I have a thousand bucks that says Sig Sauer is going to win the Next Generation Squad Weapon competition (whether that means the MCX Spear is going to be a standard military rifle or a SecOps weapon is a different discussion). Sig's 6.8x51 round is what's being fired from that beast and the LMG that they are going to go with it.



My question is, can this new caliber be adapted to existing multicaliber rifles like the XCR (or the AR-15 for that matter)? The new round isn't 6.8 SPC, it's a new cartridge that generates internal pressures of 80-100K PSI (FYI, 5.56 pressures are at 55K PSI). Making a new barrel or bolt isn't that big of a deal and civilian rifles don't have to be limited to bullpup lengths, but does anyone know if the XCR's upper can handle that kind of pressure? Can any commercially made upper do that or do we have to wait for Sig to deliver to the civ market? And last, which version of the XCR would the barrel conversion be for, the -L or -M?
 

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Boils down to dimensions of the bolt and whether it can fit in the existing -M upper. Also, are the magazine dimensions the same as LR/SR25?
 

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Any rifle designed for 308 can relatively easily accommodate .277 Fury. Things to note though are that the breech may need a different profile and the ammunition is not reloadable. The Cross for example with a different bolt head, and barrel shoots Fury rather well.
 

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I am interested in seeing Sig's upcoming 6.5mm Fury variant using the hybrid case and higher pressures. It seems like that will be a better long range round, but if the military selects the 6.8 version, I wonder if any other Fury calibers will really become widely available, or if there will be a 'standard caliber' effect based on military procurements.
 

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I've always liked the idea of a 6.7-6.8mm round in a 51mm cartridge. It always looked like its the sweet spot for sectional density in that cartridge. The higher pressure just makes it so much more compelling. The higher sectional density would allow it to have a more consistent velocity over its flight. It may be enough to have less drop than the 6.5mm. (haven't looked at the drop tables yet.)

edit: After doing some napkin math, Tom is correct. A 120gr .265 bullet @3185fps will have less drop and more fps at 2000 yards than a 140gr .277 bullet @2950fps.
 

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A 6.5 bullet will fly better than a 6.8. given the case volume, powders etc... What makes the Fury work is the very high chamber pressure. the cullet diameter was dictated in he ask, it wasn’t the first or optimal choice. Essentially you get a single caliber that can perform over a very wide range of use cases. You can’t reload but the military doesn’t do that and so doesn’t care. The same round can be used in rifles, carbines, light and medium support wepones, DMRs and so on. Simple effective cartridges with simple logistics.
 

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A 6.5 bullet will fly better than a 6.8. given the case volume, powders etc... What makes the Fury work is the very high chamber pressure. the cullet diameter was dictated in he ask, it wasn’t the first or optimal choice. Essentially you get a single caliber that can perform over a very wide range of use cases. You can’t reload but the military doesn’t do that and so doesn’t care. The same round can be used in rifles, carbines, light and medium support wepones, DMRs and so on. Simple effective cartridges with simple logistics.
What's bbl life like with that kind of chamber pressure?
 

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What's bbl life like with that kind of chamber pressure?
I haven't seen any hard numbers. Mostly of the time, authors just say something like "Better than you would expect."

The closest I found to an attempted answer: NGSW Update: Push for the 6.8mm Continues
Barrel life at this point is nothing but conjecture. If forced to make an educated guess, I’d say that if barrel life at standard pressure is 2,500 rounds, barrel life at high pressure will likely be 2,000 rounds.
I do wonder how he came up with the 80% number.

I would like to know:
How expensive is the ammo (3 piece shell)?
Will it be reloadable?
 

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I haven't seen any hard numbers. Mostly of the time, authors just say something like "Better than you would expect."

The closest I found to an attempted answer: NGSW Update: Push for the 6.8mm Continues


I do wonder how he came up with the 80% number.

I would like to know:
How expensive is the ammo (3 piece shell)?
Will it be reloadable?
LOL...2500 rounds? That's insane.
 

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We’ll get to see over time, but barrel life isn’t simply measured in rounds fired. It’ll really be x rounds until accuracy drops below and acceptable level. Some calibers for example hold 1/2 MOA for the first 600-1000 rounds then drop to sub MOA for the next few thousand and so on as they degrade. I’d be really surprised if the military will accept something with a barrel life under 10k-20k rounds for their purposes. Note too that heat is as much a factor as velocity/pressure. An M4 barrel should survive 7500 rounds, but fire it on full auto for a couple of minutes and it destroys itself. Fire slow, occasionally and it’ll last well over 10k rounds. An example of this is the lifecycle for M16/M4 bolts. Suggested replacement is every 5k rounds.
 
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If you're spending other people's (tax) money, then price isn't really a factor. Burn out barrels and use expensive ammo that can't be reloaded--no problem.
Good point....when has government ever been efficient?
 

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Discussion Starter · #14 · (Edited)
Boils down to dimensions of the bolt and whether it can fit in the existing -M upper. Also, are the magazine dimensions the same as LR/SR25?
Of course, a 51mm bullet won't fit into a gun with a magwell built for 45mm long bullets. Missed that detail.

What's bbl life like with that kind of chamber pressure?
From what I understand that the barrel handles most of the bullet's combustion. Does the receiver need any particular reinforcment to handle 80-100K PSI of pressure from the cartridge going off? I don't know what the MCX Spear or the other NGSW guns are made of but would an aluminum upper suffice to deal with that kind of chamber pressure and recoil?
 

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Of course, a 51mm bullet won't fit into a gun with a magwell build for 45mm long bullets. Missed that detail.


From what I understand that the barrel handles most of the bullet's combustion. Does the receiver need any particular reinforcment to handle 80-100K PSI of pressure from the cartridge going off? I don't know what the MCX Spear or the other NGSW guns are made of but would an aluminum upper suffice to deal with that kind of chamber pressure and recoil?
Yes, b/c that's' chamber pressure....you'd simply have to engineer the bbl and bolt/carrier assembly to be able to withstand the pressure. The upper is effectively just a housing for it all to ride in and mount to.
 

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The whole semi-automatic system may need to be redesigned. Pressure is potentially twice 7.62 NATO.
 

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It’ll likely be piston driven with the gas volume/pressure supplied to the piston via a regulator. Most likely a controlled redirect vs bleed off. The barrel/bolt assembly takes the pressure while recoil will be managed though the mount. Pressure has little to do with recoil. It’s all about equal and opposite forces. The bolt unlock timing, weight and weapon weight all reduce ‘felt’ recoil. The belt fed .277 Fury machine gun is a giggle to shoot and the prototype rifles are apparently quite popular with he test teams. It is designed to fit in the 308/6.5CM cartridge envelope. The case is 51.2mm long, exactly the same length as a 7.62x51mm cartridge. The bullet, projectile, is actually a 7mm diameter. Barrel wear is a factor or pressure and the high twist rate.
 
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