EmericD said:

The bullet is longer (L/D ~5.15) with a slender nose (~2.84 cal).

A bullet like that is actually even lighter than my estimate, there being more nose cone and less shank. 

@Gatnerd: that is the exact image i got my estimate from. However, when I went to measure it again just now ended up at 5.1 caliber length, 2.67 cal nose cone, and 0.15 cal meplat. Still only gets you to 125.5 grains.

Perhaps you are using incorrect density?

For an EPR 8.5 is about right. That is assuming 40% by weight of steel (7.8), and 60% copper (8.9). I'll give you 8.6 even (30%/70%), for all the 1.5 grains that wins you. For 135 grains you need to go over 9.

What other posibilities are there besides tungsten containing AP and lead FMJ/OTM weighing 135 grains, and the EPR's mass is yet to be disclosed, or the EPR is also 135 grains but isn't lead/tungsten-free.

VPMudde said:

Does that mean that the 135 gr figure is only for the AP variant (due to the use of tungsten)? And that the GP projectile is actually a fair bit lighter than AP? Or am I wrong about this shape? 

There has not been an official spec ever posted; quite the contrary - the specs were originally shrouded in secrecy. Original speculation for the 6.8 HVAP that preceded NGSW was a 6.8 125gr @ 3300-3500fps as the goal.

Then Soldier Systems published SIG's specs for their .277 Furry load / NGSW weapons. Specs were 135gr @ 2850fps from a 13", 3000fps from a 16".

Later, Cobalt / MARS shared their project - a 140gr @ 3200fps.

Most recently, a video of TV's / GD's bullpup mentioned 135gr @ 3000fps from their 20" barrel at 60kpsi. 

So we know that these companies are designing their cartridges around firing a 135-140gr projectile. Probably this is the 'surrogate' projectile mentioned in the initial NGSW program filings. 

So if the 135-140gr is the Surrogate, its entirely possible that the steel + copper EPR 'GP' is indeed lighter, maybe 125gr as your model suggests. 

If it is 125gr, we'd then likely expect velocity to be around 3100fps, based on the 135gr x 3000fps.

If the 6.8 AP uses the same EPR construction of the XM1158, where the steel tip is replaced by Tungsten carbide, but otherwise retains the same size and shape of the 6.8 GP, then we'd obviously see a difference in weight, since WC is ~2x the density of steel.

That would be worth modeling I think - if the steel EPR is ~125gr, whats the weight of the same projectile with the steel swapped for WC?

gatnerd said:

Cobalt / MARS shared their project - a 140gr @ 3200fps.

Most recently, a video of TV's / GD's bullpup mentioned 135gr @ 3000fps from their 20" barrel at 60kpsi. 

So we know that these companies are designing their cartridges around firing a 135-140gr projectile. Probably this is the 'surrogate' projectile mentioned in the initial NGSW program filings.

It seems rather unlikely there are two government-provided surrogates.

The bullet seen in the Cobalt/MARS round is probably not the surrogate.

The SIG, TV, and Textron rounds have what is likely a 135gr MatchKing.

.270 CALIBER 135 GR. HPBT MATCHKING – Sierra Bullets

VPMudde said:

A bullet like that is actually even lighter than my estimate, there being more nose cone and less shank.

The original EPR design used a bismuth core. Maybe the XM1186 solved the high temperature problem that plagued the 5.56 mm accuracy?

gatnerd said:

That would be worth modeling I think - if the steel EPR is ~125gr, whats the weight of the same projectile with the steel swapped for WC?

The back of this here enveloppe says 160 grains and change (with a density of 11.3)

@Emeric: perhaps, but the old bismuth-tin core M885A1 was pretty much the same shape as the eventual copper core EPR, so I would hazard a guess that that particular alloy's density is, like copper, 8.9 g/cm³. Pure bismuth is slightly more dense (9.8), but being rather brittle it's not a good material to make bullets out of. If they actually did manage a bismuth core bullet that would get you very close to 135 grains, but not quite.

While these first-gen FCS look bulky, this is single greatest upgrade to lethality

I imagine Elcan being free of ITAR will corner large chunks of the market.

Quick question about the textron 6.8.... am i to understand that only CT rounds can use compressed propellant?

Or could it be used elsewhere -for example- to reduce the size of a 556 round?

And can compressed propellant be combined with high case pressures?

Thanks