Testing, testing...

[stanc]

Take the 120 GMX shape, use voids up front to move the CG aft, different materials for improving penetration, etc., to reduce the weight to 100 gr and one gets a BC of .400 assuming the same shape factor.

The bullet would have 90% of the sectional density of the M80, so would need some design effort (e. g., M855A1) to bring the barrier defeat and armor penetration up to the M80 level.

Using a void up front may be an option for the UK and Europe, but it's likely a no-go for the US Army, which seems pretty well locked into the exposed penetrator design of M855A1 and M80A1.

I don't know what different materials you're referring to. The only viable choices seem to be steel and copper. NAMMO's lead-free bullet is an FMJ with steel/steel core, and the RUAG projectile (like the US bullets) is steel/copper.

If I figured correctly, scale down the 130gr and 149gr versions of M80A1 to 7mm, and the resulting bullet weights are 101gr and 115gr, respectively. 6.8mm and 6.5mm versions are much lighter, IMO far too light to have a chance of achieving the necessary terminal performance.

[JASmith]

You likely wouldn't do a linear scale -- let the length be long enough to get 95-105 grains and you're good to go!

BTW Non-concur on statement that "US is locked in on M855A1 style" -- it just happens to be the current fad. Hopefully it won't last as long as the M80!

[stanc]

You likely wouldn't do a linear scale -- let the length be long enough to get 95-105 grains and you're good to go!

It just happened that a linear scale down makes a .284" bullet that gives a cartridge OAL of 2.5 inches, the length for which Murray designed the 7mm UIAC. As scaled, it also would keep only the boattail extending into the powder space.

For the smaller calibers, 6.5mm and 6.8mm, I agree the bullet can be lengthened. However, I don't know if 95-105 grains is heavy enough to give the desired terminal performance. Again, we need some testing, testing...

BTW Non-concur on statement that "US is locked in on M855A1 style" -- it just happens to be the current fad.

I wouldn't use "fad" to describe it, but I didn't mean they were locked in forever. But, they are clearly determined to use that bullet type for the foreseeable future.

Hopefully it won't last as long as the M80!

It'll last until there is a perceived need to replace it, and/or a leadership that pushes for a change.

[stanc]

I think that with some compromises, we could get there, although lead-free ammo certainly makes it harder.

I went back and reread what little info is available on the 7mm UIAC. It turns out that the 2650 fps published muzzle velocity is for a 16-inch barrel. (I was under the mistaken impression that it was for a 20-inch tube.) That translates to ~2750 fps out of a 20-inch barrel, with a lead-core, 130gr bullet.

So, even with the powder space used up by a "green" bullet of the same weight, it seems like it ought to still match (or at least get extremely close to) 7.62 muzzle velocities.

I'm not sure if a .422" diameter case is sufficient for the task, but it does look like the 7x46 is capable of producing the desired performance.

I guess that brings up the question, is a 25% reduction in ammo weight "good enough" to make the switch worth the costs?

[Tony Williams]

It seems to me that barrier defeat and wound trauma must be equal to 7.62mm, or both the troops and the decision makers will object to replacing 7.62x51 with a less effective round. If we were still talking about replacing 5.56x45 only, that would be a different matter.

I don't think so. The official view of both the US and UK armies (which is what we need to address, whether or not we think it is right) is that 5.56mm is adequate to around 400m; if all future engagements were to be within this range, 7.62mm would promptly be returned to support status. So the main focus of an intermediate round in replacing 7.62mm must be its long-range performance. From that perspective, improved penetration and effectiveness at short range (compared with 5.56mm) is a secondary advantage - although I believe a very important one.

I went back and reread what little info is available on the 7mm UIAC. It turns out that the 2650 fps published muzzle velocity is for a 16-inch barrel. (I was under the mistaken impression that it was for a 20-inch tube.) That translates to ~2750 fps out of a 20-inch barrel, with a lead-core, 130gr bullet.

Cris was also talking about a heavy, long-range sniper load of 175 grains at 2,400 fps. I'll have to ask him about this - I'll be seeing him next week.

I guess that brings up the question, is a 25% reduction in ammo weight "good enough" to make the switch worth the costs?

Two different scenarios: if LSAT proceeds, the same cost would be involved whatever calibre is chosen. If LSAT fails, there is still a chance of introducing a new calibre (although the likelihood drops sharply) through stressing the "combined advantages":

1. To reduce the weight compared with 7.62mm ammunition without losing long-range effectiveness.
2. To improve the effectiveness compared with 5.56mm ammunition without the heavy recoil of 7.62mm.
3. To provide greater tactical flexibility, with all riflemen being equipped with weapons effective at all combat ranges.
4. To save on procurement, support and training by halving the number of different small arms acquired.