Hi. I'm trying to design a new high velocity round. Something that starts with about 4000fps and retains that speed extremely well.

I'm curious as to how much a high BC would help with this. Does the 762NATO with it's high BC, retain VELOCITY better than 556 at range? or does it not make much difference within 300m.

Lastly, does anyone have BC information for the following rounds?

762NATO, 147grain.

6mm cartridges, 90grain bullet

6mm cartridges with 55grain bullet

556NATO,  77grain

556NATO, 55grain

and lastly, some custom rounds

6.5mm creedmoor with a CETME/FABRL bullet , 75grain

6mm creedmoor with a CETME, 55grain

thanks very much

I believe that Sierra bullets, as well as some other bullet makers publish BC data on their products

Example:

https://www.sierrabullets.com/product/30-caliber-7-62mm-175-gr-hpbt-matchking/

Baseline 7.62x51 with sub 150g bullet has a terrible BC , you don't wan't to launch anything like that at 4000fps as you would slow to below 3000 by 300m 

In any case these days folks loading the .300Norma Mag with 90 grains of powder only get to some 3500fps with 150grain bullets. So 4000fps is not something you could expect from anything not light and possibly saboted to have larger bore than projectile for speed.

Here are Litz radar-tested numbers G7 BC for off-the-shelf rounds

.308  

M80A1 .-131g  -  0.173

M118Lr- 175g -   0.243

Sig Elite-140g -    0.167

Sierra FMJBT 150g - 0.207

Lapua Scenar 155 -0.221

Solids

Warner Tool Flatline155g  -0.258

Cutting Edge 140g MTH 140g - 0.202

6mm

Lapua 90g FMJBT - 0.185

Lapua 90g Scenar L - 0.211

Berger Target BT 65g - 0.143

5.56mm

M995 51g - 0.126

M855 62g - 0.158

MK318 62g- 0.126

MK262 Mod 0 - 77g  -  0.190

Berger Target BT 82g -0.225

6.5mm (current bullets used by the military in 6.5CM and .260Rem)

Lapua Scenar 123g  -0.251

Berger Hybrid 130g  -0.287

Lapua Scenar 136g - 0.285

S&B FMJBT 140g - 0.231

smg762 said:

Hi. I'm trying to design a new high velocity round. Something that starts with about 4000fps and retains that speed extremely well.

What would be the purpose of that design?
Such a high MV will cause a lot of troubles on the engineering side. It will hardly be ever worth it.

A 150 gr bullet accelerated to this speed means the ME is above 7000 J. That is massive. To have a ME in the range typical for rifles the bullet would need to be in the 40 gr range. But this would mean the energy retention would be bad.
 

smg762 said:

and lastly, some custom rounds

6.5mm creedmoor with a CETME/FABRL bullet , 75grain

6mm creedmoor with a CETME, 55grain

The i7 form factor of the FABRL and the CETME bulllet were reported both in the vicinity of 0.82 - 0.83 depending on the Mach number, so that means a C7 of ~0.185 for a 75 gr 6.5 mm, and ~0.160 for a 55 gr 6 mm.

I don't see the 6.5 mm Creedmoor reaching 4000 fps with a 75 gr bullet (that would be 3600 J of ME, a little too much for this round).

The 6 mm Creedmoor seems to be able to reach 4000-4100 fps from a 24" barrel with a 55 gr bullet (~2800 J), but maybe a 77-80 gr bullet launched at 3400 fps from the same barrel length would be a better choice...

Only 308win case based caliber i know that is actually used in practice with loads north of 4000fps is 22-250 and that is with very light weight bullets

Mr. T (MrT4) said:

Only 308win case based caliber i know that is actually used in practice with loads north of 4000fps is 22-250 and that is with very light weight bullets

Hodgdon Reloading Center is proposing 13 loads for the 6 mm Creedmoor and a 55 gr bullet, with MV ranging from 4005 fps (45 gr of IMR 4166) up to 4122 fps (48.5 gr of CFE 223), and the VihtaVuori website is also proposing 4000+ fps loads with the N540 and N550 powder and a 55 gr bullet.

That won't be my choice, but it seems that you can drive a very light 6 mm bullet above 4000 fps from a 24" barrel.

The UK MoD is in search of a light cartridge that could defeat lvl IV plates at a range up to 300 m, and the 6 mm Creedmoor, the 6 mm XC or the old .240 Cobra wildcat (the .220 Swift necked up to 6 mm & improved) could probably fulfill this goal, but with a 77-80 gr bullet and a MV in the 3300 - 3400 fps bracket.

In fact, the 6 XC or the 6 mm Creedmoor would be very close to the Russian 6x49 mm "Universal", and the .240 Cobra could be seen as a modernized 6 mm Lee Navy. 

6mm bullets in 55g range are typical flat base bullets with extremely poor BC

I googled different manufacturers and such light weights are extremely few in 6mm , as the BC is quite poor till some 90grain most of the offerings are around that with exception of the specialised varmint bullets.

I would think that better penetrator designs and maybe off the shelf 6.5 or upcoming 6.8 might be more interesting than a whole new caliber ,

264USA was something in the middle and important thing it they actually went of a mid-size magazine box in comparison to Ar15 and Ar10  https://soldiersystems.net/2017/04/04/jim-schatz-a-path-to-overmatch-next-generation-individual-weapon-system/

 .264 USA. The round is a 6.5mm caliber round, based on lengthened 7.62×39 brass, with very little case taper (0.26 degrees, almost half that of 5.56’s 0.5 degrees, and still considerably less than 7.62x51mm’s 0.35 degrees) but a very slender 17.5 degree shoulder. At 2.6″ long, the .264 USA is closer to 7.62mm than 5.56mm in size and weight, but it nonetheless would afford slightly smaller and lighter weapons and magazines..264 USA be capable of producing 2,875 ft/s with a 107gr lead-cored Sierra HPBT from a 16.7″ barrel, or 2,657 ft/s with a 123gr Sierra from the same length.

 if one wanted a nice round for an assault rifle a 6mm with 85-90g bullets would fit into existing magazines , Grendel based - 6mmAR Turbo40 is about the max you can squeeze into Ar15 mag box with 80-110grain bullets

224 Valkyre , 6ARC ,6.5 Grendel   , 6 ARC was for some reason designed around 108 grain bullet that is why its shorter than Grendel , on the other hand wildcats went for higher capacity and 95grain bullets as 108 is kinda heavy for given case volume.

I have been fiddling with my own 6mm wildcat (6SPARC ?)based on Grendel case

Green is 6SPARC chamber ,blue is 6ARC

Mr. T (MrT4) said:

6mm bullets in 55g range are typical flat base bullets with extremely poor BC

The AR-2 shape is also a flat-base design, but due to its very long ogive, the bullet works well at high Mach numbers.

I've just checked the reference paper, and the i7 form factor at Mach 3.5 (1190 m/s) is 0.72, increasing slowly with the decrease of the Mach number, up to a i7 of 0.86 at Mach 2.

So, a 6 mm / 55 gr bullet fired at 1220 m/s (4000 fps) from a 6 mm Creedmoor would stay supersonic up to 1 km, with a mean G7 BC around 0.169 (equivalent of a good lot of SS-109 ball ammo, but launched ~300 m/s faster).

I don't want to imply that this would be a good ballistic solution for a small arm, I'm just answering to the OP.

Sounds like a slick 55gr

Commercial 58g-60g 6mm bullets have low BC and would drop to subsonic at 700m  , according to JBM you could get to some 900m with 65-68g the first boat tailed designs in the caliber.