How easy is it to accurately predict the angle of repose for bullets.  Is that angle enough to have a significant effect on the effective BC of a highly optimized bullet?

nincomp said:

How easy is it to accurately predict the angle of repose for bullets. Is that angle enough to have a significant effect on the effective BC of a highly optimized bullet?

"Predicting" the angle of repose is easy if you have a 6 DoF software and a complete set of aerodynamic parameters of the studied bullet.

Problem is that it is extremely difficult to have a complete set of aerodynamic parameters of a bullet without using a wind tunnel, and even wind tunnels have limitations.

CFD have made some great progress recently, but even RANS / LES approaches are not always reliable.

For example, when computing the Magnus moment, people are happy when the computation is giving them the good sign (positive or negative) of the moment, but we are very far to be able to compute the real value.

Speaking about angle of repose, the "angle of repose" of the M193 when fired from a 1-in-14" twist was around 2.5°, and dropped down to ~0.5° in a 1-in-12" twist, and this difference was enough to account for a few % of BC increase (from 0.116 to 0.124).

Gimme a break. Look at how much more slender the VKO is.

The first 5.56 mm NL ammo (brass case cut to 40.5 mm) were fired out of an EPVAT tube and a HK416 F just before Christmas.

Radar measurements confirm that the C7 of the bullet is 0.208 around Mach 2, and the bullet was perfectly stabilized.

As expected, the dispersion from the EPVAT tube was catastrophic (we fired the NL case out of a regular chamber), close to 10 MoA, along with the launch angle (above 10° in one case). The MV was ~905 m/s with a chamber max mean pressure of 305 MPa, versus 930 m/s and 320 MPa for the IMI 5.56 mm used as reference.

What wasn't expected was that the HK416 F-S ran perfectly with this low-pressure ammo, with decent (~3.5 MoA) accuracy (launch angle around 4° max), the same POI as M855 ball, a MV of 815 m/s and SD below 5 m/s...

The difference seems to be linked to the EPVAT chamber profile, we are going to slightly change the bullet profile to achieve a better bullet support during launch, and ~2 MoA dispersion should be possible from the EPVAT tube and the HK416 F.

We are also going to ramp up the pressure, probably up to 340-360 MPa, to get a MV around 850 m/s from a 14.5'' barrel. Or going "full throttle" and 400 MPa to reach 900 m/s from the same barrel length and more than 900 m of supersonic range from a carbine...

Do you have much knowledge of the 70s high velocity French experimental rounds....7.5 necked to 4.6.....Any idea of their speeds?

Emeric, thanks for info

But can you please remind us what is the end goal of the neckless brass case / round? Weight decrease? Or attempts to squeeze a longer and heavier ballistically optimized projectile into the old 5.56mm form-factor?

I'm not Emeric, but he's indicated it's the latter

Hi Max,

The objective was to check that we could use a "much better bullet" in a unmodified 5.56 mm weapon, and that we could launch this bullet at a sufficiently high MV.

It's just a preliminary step before seating this same bullet in a composite case, the ultimate goal is to be able to launch a lead-free bullet with a C7 CB above 0.20, at a MV around or above 850 m/s from a 14.5" barrel (so slightly better ballistics than a Mk262), with a cartridge weight below 9 g.

Thanks

Now that's what can be called a reasonable evolutionary approach.

EmericD said:

It's just a preliminary step before seating this same bullet in a composite case, the ultimate goal is to be able to launch a lead-free bullet with a C7 CB above 0.20, at a MV around or above 850 m/s from a 14.5" barrel (so slightly better ballistics than a Mk262), with a cartridge weight below 9 g

That'll be wonderful if it comes to fruition, and I sure hope it does.

Thanks for giving us the project update.