FMJ yaw and upset performance

[JASmith]

Joe, that doesn't seem like a viable option for Tony's goal. I'm pretty sure that the Brits consider hollowpoints (whether of the fragmenting type, like Mk262, or expanding, like Mk318 and "brown tip") to be in violation of the Hague treaty.

Stan,

You are correct. The 77 gr Matchking is a place-holder for the longer term solution. A convention-compliant bullet needs to be the core of the discussion. Also, our marksmanship guru points out that bullets of the MatchKing quality are not likely to be manufacturable in the quantities we need. Further, those manufacturing limitations will likely also result in less aerodynamic efficiency. Both the aero efficiency and terminal effectiveness goals push us to longer and heavier bullets with the 77 gr 5.56 being at the low end.

That's part of the reason I mentioned the slightly larger case. We can get the effectiveness of the high quality 77 gr bullet with a somewhat heavier but less efficient cooper-steel bullet driven at the same or slightly higher velocity. I can see this logic pushing us into 6mm...

Remember also that Tony is searching for a universal infantry cartridge to replace both the 5.56 and the 7.62 in infantry roles up to and including the GPMG. He has it right that the 6.5 120+ grain class of bullet is the minimum likely to be seen as a viable 7.62 replacement for the GPMG. Problem is that it stresses the assault rifle role and we still have the vehicle and aircraft-borne 7.62 inventory. The vehicle and aircraft-borne weapons community, given a choice, would likely opt for retaining the 7.62 but enhancing the ammunition.

[Tony Williams]

Remember also that Tony is searching for a universal infantry cartridge to replace both the 5.56 and the 7.62 in infantry roles up to and including the GPMG. He has it right that the 6.5 120+ grain class of bullet is the minimum likely to be seen as a viable 7.62 replacement for the GPMG. Problem is that it stresses the assault rifle role and we still have the vehicle and aircraft-borne 7.62 inventory. The vehicle and aircraft-borne weapons community, given a choice, would likely opt for retaining the 7.62 but enhancing the ammunition.

Yes, that sums it up well, but I'd make a few additional points:

1. The 5.56mm is an unreliable performer (in terms of penetration and terminal effectiveness) even at short ranges, so a bigger and more powerful round would be desirable even if there were no call for a significantly longer range. This is what led to the 6.8mm Rem. So any "ideal" assault rifle round would have more ammo weight and recoil, by definition.

2. The recoil of a rifle with an intermediate round in the 1,800-1,900 ft/lb energy zone (probably the minimum for a "universal" cartridge) doesn't feel much worse than 5.56mm, and feels a lot less than 7.62mm. I say this after firing HK416, HK416/6.8 and HK417 in quick succession. So it should still be controllable in burst fire if needed in a short range emergency.

[LR1955]

Gene:

I don't see any mention of wanting to use a VLD in Tony's posts.

FWIW -- a year or two ago, I asked on the IAA (cartridge collectors) forum about M80 projectile shape, and was informed by one of the knowledgeable individuals that it has a secant ogive.

Stan

Stan:

Tony said "Is it possible to design a low-drag 6.5mm FMJ bullet of around 120-125 grains which exhibits rapid and reliable yaw and upset on impact?"

This implies VLD all the way.

LR55

[LR1955]

To match the long-range (up to 1,000m) performance of the 7.62 M80, in terms of retained energy and other metrics, with a smaller and less powerful cartridge requires a bullet with a better ballistic coefficient than the M80's. The 123 grain Lapua Scenar seems to do that job very well in the Grendel, so it's a matter of getting close to the BC of the Lapua with a mass-produced FMJ military bullet.

As far as 77 grain 5.56's are concerned, I have the strong impression that no calibre smaller than 6.5mm is likely to prove acceptable as a replacement for 7.62mm, in terms of penetration and terminal effectiveness. The Chinese tried to make a universal cartridge with their 5.8x42 (in light and heavy loads), but their army is hanging onto their 7.62x54R MGs.

Tony:

This one shouldn't be too hard to figure out. How much better does the 6.5mm bullet have to be before it shows a quantum improvement in exterior and terminal performance?

Start out with defining what that quantum improvement is and the rest will be easy to solve -- on paper.

What must the BC and velocity be to match in exterior terms and then multiply by about 50% before anything can be proven in real terms. When that is solved, then look at powder technology and finally bullet construction.

LR55

[stanc]

To match the long-range (up to 1,000m) performance of the 7.62 M80, in terms of retained energy and other metrics, with a smaller and less powerful cartridge requires a bullet with a better ballistic coefficient than the M80's. The 123 grain Lapua Scenar seems to do that job very well in the Grendel, so it's a matter of getting close to the BC of the Lapua with a mass-produced FMJ military bullet.

According to Lapua's numbers, the 108, 123 and 139 grain Scenars, and the 144 grain FMJBT, have BCs in approx the same proportion to their weight. If the same length is retained for a 123 grain FMJBT, it should have a BC close to that of the 123 Scenar: 0.527

[Tony Williams]

Tony:

This one shouldn't be too hard to figure out. How much better does the 6.5mm bullet have to be before it shows a quantum improvement in exterior and terminal performance?

Start out with defining what that quantum improvement is and the rest will be easy to solve -- on paper.

What must the BC and velocity be to match in exterior terms and then multiply by about 50% before anything can be proven in real terms. When that is solved, then look at powder technology and finally bullet construction.

LR55

My approach is a bit different: I would start with the bullet, because its characteristics are the key to the success of any new round. There are two main aspects to this success:

1. Terminal effectiveness, in terms of straight-line penetration through various intermediate barriers (including car windscreens, a real headache for 5.56mm) plus rapid and reliable yaw and bullet upset on impact (I'd like to see the bullet begin to yaw within a couple of inches of penetrating a body, and completely turn over in order to travel backwards within 12 inches).

2. Exterior ballistics, in terms of a rate of velocity loss significantly less than 7.62mm M80 (probably around 0.50 BC).

With the bullet design settled it would then be easy to work back from the requirement to match the retained energy of the 7.62mm M80 at 1,000m, in order to decide what the muzzle velocity needs to be. The next stage would be to calculate the propellant load to achieve this MV in a specified barrel length, and therefore the case capacity, and finally the form of the cartridge.

This approach could be applied to any calibre, of course, but I think that the 6.5-7mm range looks the most promising. A 7mm round would probably show more reliable terminal effectiveness than 6.5mm at the cost of more ammo weight and recoil, and a steeper trajectory; you pays your money and takes your choice...

[stanc]

2. Exterior ballistics, in terms of a rate of velocity loss significantly less than 7.62mm M80 (probably around 0.50 BC).

With the bullet design settled it would then be easy to work back from the requirement to match the retained energy of the 7.62mm M80 at 1,000m...

We did some similar calculations on the old 65grendel forum. With the superior BC of a 123gr 6.5 Grendel load, it's easy to exceed 7.62 M80 energy @ 1000 meters. The trouble is that at the closer distances (<500 meters) where most infantry combat occurs, 6.5 Grendel doesn't come close to matching 7.62 M80 retained energy.

[Tony Williams]

We did some similar calculations on the old 65grendel forum. With the superior BC of a 123gr 6.5 Grendel load, it's easy to exceed 7.62 M80 energy @ 1000 meters. The trouble is that at the closer distances (<500 meters) where most infantry combat occurs, 6.5 Grendel doesn't come close to matching 7.62 M80 retained energy.

No argument with that Stan, but it really doesn't have to - at closer ranges it merely has to show a substantial improvement over the close-range calibre (the 5.56mm) in barrier penetration and terminal effectiveness, which should be no problem!

[RangerRick]

No argument with that Stan, but it really doesn't have to - at closer ranges it merely has to show a substantial improvement over the close-range calibre (the 5.56mm) in barrier penetration and terminal effectiveness, which should be no problem!

One thing that hasn't been discussed here in detail is barrel length. Is it possible to have one cartridge that will perform well in a 16 inch bbl for riflemen, in a 22-24 in. bbl on a squad machine gun like the M249, and of 28 in. (or more) in a heavy bbl version of the SAW for a platoon machine gun?

Designated marksmen could carry a rifle with 20-24 inch bbl with a PRS type butt and good optics.

What is the point of diminising return on bbl length in the 6.5 Grendel?

In a pinch, riflemen could break down linked belts to load mags or machine gunners could use magazines in the mag wells of the SAW.

[Tony Williams]

Barrel length is an interesting and somewhat controversial issue.

Clearly, if you adopt a general-purpose cartridge with excellent long-range potential, it makes sense if the gun you use it in is capable of exploiting that potential. Which is to say, has a barrel long enough to develop the velocity the round is capable of. On the other hand, the current trend with rifles is to use ever-shorter barrels in order to keep the guns compact for urban warfare and clambering in and out of cramped vehicles.

So there would seem to be three options:

1. Select an intermediate length barrel (say 16 inches) which isn't quite short enough to be really handy and isn't quite long enough for good long-range performance.

2. Choose a modular gun with a quick-change barrel and issue both short and long barrels - and hope that your troops don't get caught with the wrong barrel fitted if the tactical circumstances change suddenly.

3. Choose a bullpup, and for the same length gun enjoy a barrel eight inches longer than with a traditional design. This is by far the most logical choice, but the US seems to have something against bullpups...