So I figured it's been too long since we've had a Caliber Discussion.

Previously, much of our discussion focussed on the GPC concept / the one 'optimum' Squad caliber for both rifles and SAW/LMG use. 

Now, I'd like everyones thoughts on what an optimized 2 Caliber configuration might look like. 

Scenario: After extensive testing, the US decides the 6.8 NGSW was a mistake, but still wants to replace 5.56 + 7.62 with newer cartridges and weapons. Since the Army has long struggled with developing small arms, it has taken the perfectly logical step of entrusting this development to internet firearms nerds ;-) 

The requirement is for a 5.56 and 7.62 replacement, with the expectation that Squad's will go with a 2-Caliber configuration - a light caliber for carbines, and a heavier caliber for the LMG. 

Beyond that, there are no fixed requirements. Level IV Armor penetration is not a requirement, as widespread use of Tungsten is deemed impractical. Army projectile will be the standard EPR steel/copper style used in M855A1/M80A1.

Case technology: Cases can be Brass, thin wall stainless steel (ala Shell Shock) or SIG high pressure steel/brass. Polymer and CT have been deemed not yet ready for fielding. 

If presented this, what 2 Cal configuration would you suggest? 

gatnerd said:

After extensive testing, the US decides the 6.8 NGSW was a mistake, but still wants to replace 5.56 + 7.62 with newer cartridges and weapons.

So, at least the US can save the M250 and replace the 7.62x51 mm with the 6.8x51 mm. The regular-pressure round is already more effective than the 7.62 mm NATO, and the high-pressure version is duplicating the .338 Lapua ballistics.

If the US wasn't the US, they could ask SIG to make a +P version of the .17 Fireball in order to launch a 30 gr EPR bullet at 1000 m/s from a 13" carbine and a little less from the 8" derived PDW (or a .20 +P VarTag, with a 45-50 gr bullet at the same MV).

Please don't encourage a certain commenter here.;)

Agree with 6.8 for LMG, also as SDMR.

As for light.... Stoner was right? .222 Remington ballistics is what the original SCHV AR prototype was looking at, but certain desired results needed more velocity/longer cartridge case. AK-74 has proven .222 performance is good enough for government work; so Shell Shock case just long enough to hold enough powder to deliver M855A1 projectile 2800fps out of an M4 barrel?

EmericD said:

So, at least the US can save the M250 and replace the 7.62x51 mm with the 6.8x51 mm. The regular-pressure round is already more effective than the 7.62 mm NATO, and the high-pressure version is duplicating the .338 Lapua ballistics. If the US wasn't the US, they could ask SIG to make a +P version of the .17 Fireball in order to launch a 30 gr EPR bullet at 1000 m/s from a 13" carbine and a little less from the 8" derived PDW (or a .20 +P VarTag, with a 45-50 gr bullet at the same MV)

6.8 definitely has a ton of performance going for it in the LMG / DMR role. Downside is its near identical weight and identical volume to 7.62. I recall you mentioning in the EVOLYS or .264 LICC thread that even with a lighter LMG, having ~7.62 weight ammo was a drag. 

So right now I'm leaning more toward a lighter weight LMG round. Especially with what we're seeing in Ukraine; it seems like the pretty near future 600-1000m targets may end up serviced with suicide drones...

....

In terms of the .17 and .20 Vartag solutions, the .20 is quite close to what I was envisioning (I'll do more of a writeup in a bit.)

In terms of .17 30gr, I don't quite like it with a 5.56 parent case. However I think a .17 VLD EPR could be very promising if it was paired with a smaller diameter case to further reduce ammo weight / boost magazine capacity.

For example, if the MP7's 4.6x30 case was lengthened to 4.6x36-x40, and either run at standard 62kpsi with a Shell Shock case, or 80kpsi with a SIG case, that could be really sweet. 

4.6x30 40gr brass case is listed at 7.1g; conceivably a lengthened 4.6x36-40 but made of a SS case + 30gr could end up in the 6-7g zone?

Come to think of it, as a baseline for that, if you have Quickload, do you know how the existing 4.6x30 would perform from a 14.5" barrel with a 30gr @ 55-62kpsi? 

Also do you happen to know what the magazine length is for the 40rd MP7 mag?

farmplinker2 said:

As for light.... Stoner was right?

I don't know, but what the current HIC is showing is that you rifle caliber need to use the smallest amount of resources, and produce a muzzle report (flash & sound) as low as possible. The Soviets did an excellent job with the 5.45 x 39 mm and the 7N6 bullet with a L/D of 4.5, so anything smaller with a longer bullet (L/D around 5) will probably do the job, as long as your main assets (LMGs) are using something more potent.

But the US being the US, they will prefer to distribute more evenly the firepower among the squad because it's more flexible and more resilient to battle casualties, so something like a high pressure 6 mm XC will be perfect for them.

You're probably right, us Yankees LOVE our long-range marksmanship capacities, whether or not we can actually use them.

gatnerd said:

I'd like everyones thoughts on what an optimized 2 Caliber configuration might look like. 

Scenario: After extensive testing, the US decides the 6.8 NGSW was a mistake, but still wants to replace 5.56 + 7.62 with newer cartridges and weapons.

The requirement is for a 5.56 and 7.62 replacement, with the expectation that Squad's will go with a 2-Caliber configuration - a light caliber for carbines, and a heavier caliber for the LMG.

If presented this, what 2 Cal configuration would you suggest?

Carbine 

   Caliber:  6mm - 6.5mm  (0.243" - 0.264" bullet diameter)

   Bullet Configuration:  Similar to the 6.8mm GP EPR.

   Case Configuration and Size:  Configuration similar to the 6mm SAW with shortened neck (minimum size), or the 6.5mm Super Z (maximum size).

LMG

   Cartridge:  .264 LICC

   Bullet Configuration:  Similar to the 6.8mm GP EPR projectile.

EmericD said:

...what the current HIC is showing is that you rifle caliber need to use the smallest amount of resources, and produce a muzzle report (flash & sound) as low as possible.

Basis for that conclusion?

gatnerd said:

6.8 definitely has a ton of performance going for it in the LMG / DMR role. Downside is its near identical weight and identical volume to 7.62. I recall you mentioning in the EVOLYS or .264 LICC thread that even with a lighter LMG, having ~7.62 weight ammo was a drag.

Problem is, if you're the US, you're not going to develop a "2 caliber system" unless you screw your "one caliber system".

If you field an Evolys (or a M250) in 264 LICC, the momentum to field your IW in 264 LICC will be too high to resist.

You could drive the 100 gr Wolf FMJ  (a strict lead-core FMJ with 7N6 shape, nothing really exotic) at >900 m/s from a 14.5" barrel and achieve a supersonic range >950 m, with the same downrange energy of the 7.62 mm NATO, in a 15.6 cartridge (6.5 g bullet + 6.5 g case + 2.6 g load), or less than the 7.62x39 mm.

Here is a test of this bullet impacting at around 710 m/s, or a distance around 300 m if fired from a 14.5" 264 LICC weapon.

https://www.youtube.com/watch?v=b_WxJoCU7Ps

Why would you want a second caliber for you IW ?

stancrist said:

Basis for that conclusion?

1- Small arms ammo are secondary factors on the HIC battlefield, you should dedicate minimal amount of resources to produce them because you are going to fire billions of them. Any resource not used by small-arms ammo (like money) will be used more efficiently on bigger systems.

2- Avoiding detection is the best way to avoid being killed, so if the enemy can't easily detect you with IR or acoustic devices when you are returning fire, you could expect to live longer. The number of ground or aerial sensors deployed in a HIC battlefield is outstanding, I've been told that people deploying laser designators using an uncommon bandwidth for "stealthy operation" were "instantly" detected and engaged with 120 mm mortars in less than 3 minutes.