Testing, testing...

[stanc]

I agree with all of that.

Cool. So which do you want to advocate: 6.5mm, 6.8mm or 7mm?

[Tony Williams]

I don't have strong views - any would do, they just have slightly different strengths and weaknesses.

If we were considering lead-cored bullets then I would definitely go for 6.5mm. With non-lead bullets it may be that a larger calibre is needed to get a reasonable weight. On the other hand, we don't want the weight to be too close to the 7.62mm or it won't be worth changing. So maybe 6.8mm would be the right compromise, but probably with a slightly bigger case and certainly with space for longer, more aerodynamic bullets than the 6.8 Rem. In fact, the late 1940s .270 British looks pretty close: 6.8mm calibre, case length 46mm, case diameter 11.2mm (.441). Pretty much the same as Cris Murray's 7x46 UIAC necked-down slightly.

[JASmith]

...Replacing the 7.62mm with a lighter round of equivalent long-range performance is likely to be more attractive to the army (US or UK) since they are very concerned about reducing the huge burden carried by dismounted troops.
...

Tony,

We've come a long way in our community discussions!

Shifting focus to the 7.62 makes a difference, with the Grendel representing the minimum caliber and case volume that should be considered.

I'm also suspicious of the claims of superiority in 6.8. I could see the point if one is looking only at ranges of 300 meters and under combined with a fiat that the maximum size base diameter is 0.425 and that the cartridge fit within an AR15. These are all legitimate constraints for the testing -- especially if the most mature senior officials approve.

The focus on 7.62 replacement relaxes both the length and the body diameter constraints a bit.

I don't remember off-hand if the explorations in Military Guns & Ammunition looked at a 0.445" diameter body and a COAL of 2.81" but a cartridge meeting these constraints might be the best shot at an attractive 7.62 replacement for dismounted infantry. This longer overall length eliminates most of the debate over whether "green" bullets will be implementable. They exist and do well in sporting rifles with the longer cases. We will also need to contend with the consequences of the almost certain retention of 7.62X51 for aircraft and vehicle-mounted applications.

My take on the community discussions coupled with my own assessments gives me the following thoughts:

[INDENT]I am very sure 6.5 will be the optimal choice both in the analytic and test world. A reasonable study, however, would consider .25 caliber, 6.5, 6.8, and 7mm.

I am very sure that gelatin-based tests will show little or no practical difference between optimized bullets in these calibers and be close enough to that of the 7.62 M80 to count.

The M855A1 propaganda suggests that properly designed bullets in these calibers are also likely make choosing between the calibers difficult

How much a caliber improves the soldier's ability to hit in this family of cartridges will likely be the deciding factor. This favors the smaller calibers because

• Bullets of equal sectional density have slightly higher muzzle velocity in the smaller calibers for the same case volume
  
• Bullets of equal sectional density can get slightly better ballistic coefficients in the smaller calibers.
  
• Bullets of equal sectional density are lighter in the smaller calibers, reducing recoil.
  

[/INDENT]

Hope the summary is useful!

Cheers!
Joe

[Tony Williams]

Shifting focus to the 7.62 makes a difference, with the Grendel representing the minimum caliber and case volume that should be considered.

I agree, on both counts. I would say that the old British 7x43 represents the top end (or its modern equivalent, Cris's 7x46 UIAC).

I am very sure 6.5 will be the optimal choice both in the analytic and test world. A reasonable study, however, would consider .25 caliber, 6.5, 6.8, and 7mm.

I am very sure that gelatin-based tests will show little or no practical difference between optimized bullets in these calibers and be close enough to that of the 7.62 M80 to count.

The M855A1 propaganda suggests that properly designed bullets in these calibers are also likely make choosing between the calibers difficult

How much a caliber improves the soldier's ability to hit in this family of cartridges will likely be the deciding factor. This favors the smaller calibers because

• Bullets of equal sectional density have slightly higher muzzle velocity in the smaller calibers for the same case volume
  
• Bullets of equal sectional density can get slightly better ballistic coefficients in the smaller calibers.
  
• Bullets of equal sectional density are lighter in the smaller calibers, reducing recoil.
  

All true, but the need to plough through barriers and to maximise the wound channel for non-critical hits argue for the bigger calibres.

[JASmith]

...All true, but the need to plough through barriers and to maximise the wound channel for non-critical hits argue for the bigger calibres.

I see convergence in the discussions.

Regarding the bigger calibres, there is no question that the bigger calibers with similar bullet shapes will perform better against barriers and generate bigger wound channels.

The difference is that we are striving to define a cartridge that can replace the 7.62X51 M80 round while reducing weight and recoil. In this model, we don't need the best performing round, just the one that slightly beats the M80 in all categories save for the one or two that really count.

So, while I can't yet claim to know the optimum caliber/cartridge combination, the stretched Grendel (similar to the Czech-based designs you've been looking at) seems like an excellent starting point.

Remember that the longer range engagements will tend to be dominated by weapons other than rifles. My view is that, in spite of my own fantasies of long-range marksmanship, the purpose of the rifle in to 600 to 1200 meter range is as much to keep the adversary honest while bigger weapons are brought to bear.

Fortunately, the 50+ year-old design of the M80 gives us a fair bit of wiggle room. This means that we have an opportunity to reduce weight and recoil while improving hit probability and numbers of rounds in the magazine and still be able to demonstrate at least M80 equivalence given a hit.

[stanc]

If we were considering lead-cored bullets then I would definitely go for 6.5mm. With non-lead bullets it may be that a larger calibre is needed to get a reasonable weight. On the other hand, we don't want the weight to be too close to the 7.62mm or it won't be worth changing. So maybe 6.8mm would be the right compromise, but probably with a slightly bigger case and certainly with space for longer, more aerodynamic bullets than the 6.8 Rem.

Concur with all above, except the 6.8 Rem -- free of the M16 mag length -- can be loaded with longer bullets.

[stanc]

The focus on 7.62 replacement relaxes both the length and the body diameter constraints a bit.

I don't remember off-hand if the explorations in Military Guns & Ammunition looked at a 0.445" diameter body and a COAL of 2.81" but a cartridge meeting these constraints might be the best shot at an attractive 7.62 replacement for dismounted infantry.

Disagree. To paraphrase Tony, if a cartridge is too close in bulk and weight to 7.62 NATO, then it won't be worth changing. Diameter of 0.44" and length of 2.8" is, IMO, much too close to the 0.47" / 2.8" of 7.62x51.

How much a caliber improves the soldier's ability to hit in this family of cartridges will likely be the deciding factor.

Disagree. All of the intermediate cartridges proposed to replace 7.62x51 can be expected -- if only because of reduced recoil -- to improve hit probability over that of the 7.62 NATO round.

IMO, the deciding factor is probably more psychological: how close to 7.62 terminal effectiveness does the alternative round deliver. If a new cartridge doesn't get close enough to 7.62 in "punching power," it won't have emotional appeal.

Remember, rounds like 6.5 Grendel and 6.8 SPC were aimed at replacing 5.56 NATO, to improve performance of the carbine and rifle. The concept being discussed now is intended to replace 7.62 NATO, to reduce the ammo weight of the GPMG and DMR.

[stanc]

Regarding the bigger calibres, there is no question that the bigger calibers with similar bullet shapes will perform better against barriers and generate bigger wound channels.

The difference is that we are striving to define a cartridge that can replace the 7.62X51 M80 round while reducing weight and recoil. In this model, we don't need the best performing round, just the one that slightly beats the M80 in all categories save for the one or two that really count.

IMO, we do need the round with the best performance, all factors considered.

Remember that the longer range engagements will tend to be dominated by weapons other than rifles. My view is that, in spite of my own fantasies of long-range marksmanship, the purpose of the rifle in to 600 to 1200 meter range is as much to keep the adversary honest while bigger weapons are brought to bear.

Fortunately, the 50+ year-old design of the M80 gives us a fair bit of wiggle room. This means that we have an opportunity to reduce weight and recoil while improving hit probability and numbers of rounds in the magazine and still be able to demonstrate at least M80 equivalence given a hit.

As noted in my previous post, this isn't about the standard infantry rifle or carbine. It's about the machine gun and squad marksman rifle, the primary weapons for foot-mobile infantry in long-range engagements out to 900 meters.

[JASmith]

...Diameter of 0.44" and length of 2.8" is, IMO, much too close to the 0.47" / 2.8" of 7.62x51...

This is Grendel-class diameter - same as 7.62X39 and the 6.5x45 Czech within a 2-3 thousandths of an inch. Going down to say, the .422" of the 6.8 SPC / .25 Remington reduces the powder capacity a bit much, even if the COAL is the 2.80" allowable within the 7.62X51 envelope.

We will need this powder capacity to get adequate muzzle velocity -- especially if it becomes clear that the 7mm is the minimum caliber.

Even so, we will be challenged to show that a 6-8 gram bullet of any caliber can be as effective as the 9.5 gram M80 round -- this in spite of reasonably sure knowledge that these are highly capable bullet weights in the calibers we're discussing.

[JASmith]

...we do need the round with the best performance, all factors considered...

That's the catch -- optimization is all about trades within constraints.

We can't do significantly better than the 7.62 in every category. In fact, Tony and his correspondents have talked an awful lot about an 8 gram bullet at about 2750 ft/sec. This means that we are already trading away muzzle energy. There will be other areas where our quest for reduced weight will negatively impact one or performance parameters.

Our challenge will be to find the cartridge that does well-enough. Demanding that the cartridge be at least as good as the 7.62X51 in every category means that we rename the .308 Winchester, and merely use a better 9.5 gram bullet. We would then get the weight savings by counting on the adoption of the hoped-for plastic cases.

We've got to have and to identify legitimate trade space.

[stanc]

This is Grendel-class diameter - same as 7.62X39 and the 6.5x45 Czech within a 2-3 thousandths of an inch. Going down to say, the .422" of the 6.8 SPC / .25 Remington reduces the powder capacity a bit much, even if the COAL is the 2.80" allowable within the 7.62X51 envelope.

We will need this powder capacity to get adequate muzzle velocity...

Possibly. It depends on what you consider "adequate." The .277 muzzle velocity in the ballistics table ( http://www.65grendel.com/forum/showthrea...1#post6038 ) is a conservative estimate for the 6.8x43 case, based on available info and figuring an OAL of 2.4 inches. At 2600 fps MV, trajectory was very close to that of 7.62 M80, and wind drift was virtually the same.

Plus, Cris Murray's 7mm UIAC has a 46mm case length and OAL of 2.5 inches.

With these two examples, I fail to see any need to boost length beyond 2.4-2.5 inches.

Even so, we will be challenged to show that a 6-8 gram bullet of any caliber can be as effective as the 9.5 gram M80 round...

Perhaps so. We won't know until we do testing, testing...

[JASmith]

Muzzle velocities of 2600 ft/sec won't hack either the trajectory or the ho-ho test.

Your plots of a week or so ago focused solely on the 1 kilometer game. The .277 bullet in fact did not do as well even at that long range. These cartridges will need to do at least as well as the current M80 in the 0-300 meter and 300-500 meter games.

Any new cartridge will need to have at least 2750 ft/sec muzzle velocity to be competitive in the shorter game and to satisfy the TALAR (that looks about right) test that the senior decision makers are sure to apply.

[stanc]

Muzzle velocities of 2600 ft/sec won't hack either the trajectory or the ho-ho test.

Uh, what does a cake have to do with it?

Your plots of a week or so ago focused solely on the 1 kilometer game.

Of course their focus was on the 900-meter fight. That's the primary reason increased numbers of 7.62x51 weapons are employed.

The .277 bullet in fact did not do as well even at that long range.

:confused: Huh? Max trajectory height was only 6 inches difference relative to 7.62 M80. At 900 meters that wouldn't even be noticeable.

These cartridges will need to do at least as well as the current M80 in the 0-300 meter and 300-500 meter games.

No problem. The 300-meter trajectories are virtually identical to that of 7.62 M80.

Any new cartridge will need to have at least 2750 ft/sec muzzle velocity to be competitive in the shorter game and to satisfy the TALAR (that looks about right) test that the senior decision makers are sure to apply.

If that's so, then we may as well stop now, and eat ho-ho.

To get that kind of velocity will require either reducing caliber to 6-6.35mm, or increasing cartridge size too much. Either way, it won't be worth changing.

[JASmith]

I guess we're not on the same page...especially since "ho-ho" is the same as LOL and suggests my view that the senior folks would indeed laugh out loud if presented a "new" infantry cartridge producing a puny muzzle velocity.

[Tony Williams]

Your plots of a week or so ago focused solely on the 1 kilometer game. The .277 bullet in fact did not do as well even at that long range. These cartridges will need to do at least as well as the current M80 in the 0-300 meter and 300-500 meter games.

I don't believe it is possible for a significantly lighter, lower-powered cartridge to match every aspect of the M80's performance at every range: that would defy the laws of physics.

What a general-purpose round needs to do is to match the M80 at long range, while being significantly more effective than 5.56mm at short range. This should be possible given a bullet that is both aerodynamic and designed to yaw reliably on impact.

After all, we are constantly being told by the military that 5.56mm is good enough at up to c.400 metres, and the only reason for carrying 7.62mm weapons is to reach out to longer ranges than this. Personally, I'm not convinced by that - too many soldiers I've listened to would rather have 7.62mm than 5.56mm at any range - but other things being equal a new intermediate should be significantly better than 5.56mm in barrier penetration and wounding effect at short ranges, an advantage which would growth with distance until it was comparable with M80.

[JASmith]

Tony,

Concur...

Apologies -- I neglected to include the caveat that the statement applies to the trajectories. The M855A1 shoots a tad flatter than the M80 in the 100-300 game.

[stanc]

I guess we're not on the same page...especially since "ho-ho" is the same as LOL and suggests my view that the senior folks would indeed laugh out loud if presented a "new" infantry cartridge producing a puny muzzle velocity.

Joe, we were clearly not on the same page in regard to a "ho-ho" test. :o

As for muzzle velocity needed for a 7.62 replacement, we're actually pretty close. I've thought all along that -- again, for psychological/emotional appeal -- an alternative cartridge should at least equal 7.62 muzzle velocities (in same length barrels), and said so in discussions on the old forum.

I don't know where your 2750 fps figure comes from, but for 7.62 M80 book velocity of 2800 fps from a 24" barrel, I estimate that a 20" tube would give ~2700 fps.

Wilson Combat lists MV of their 6.8 SPC 110gr loads as 2700 fps, from a 16" barrel. That would easily be ~2800 fps from a 20" tube. With a longer, more streamlined bullet, loaded to 2.4-2.5" OAL, it would have trajectory and wind drift comparable to 7.62 M80.

Due to its smaller diameter bullet, it'd be a bit tougher for 6.5mm (be it 6.5 Grendel or 6.5 HPI-SPC) to get muzzle velocities that high.

IMO, the .42" diameter SPC case can work, although "stretching" it a couple millimeters as Tony suggests might be worthwhile.

Using a .44" case, as per the 7x46 UIAC, would be less desirable, but still usable at that case length and cartridge OAL of 2.5 inches. Going to 2.8" OAL seems neither necessary nor desirable. Put a 6.5x52 Carcano and 7.62x51 NATO side-by-side; the difference in size is almost unnoticeable. I don't know if the difference in weight is significant.

[Tony Williams]

A brief comment on case capacities: I would rather have a bit more than is strictly necessary, for two reasons: this provides a margin for accommodating very long bullets (e.g. lead-free tracers) protruding back within the case without raising pressures; and it also allows for the possible use of a polymer case (with metal base) which will be thicker and will therefore reduce capacity.

[JASmith]

...I don't know where your 2750 fps figure comes from...

I got the value from Gary's ammunition: http://www.inetres.com/gp/military/infan..._ammo.html

Note that the value is for 78 ft from the muzzle -- I couldn't find a call-out for barrel length.

The difference between 2700 and 2750 ft/sec is within the normal spread of chronograph testing. As with Tony's comment, we want to err a tiny bit on the high side in those performance parameters that we want to equal or best the M80.

Can't do it with everything, else we get some grotesque requirements. We also need to keep the parameters where we demand "significant" improvement to an absolute minimum for the same reason.

[Tony Williams]

Of course, we can debate the meaning of "significant"

Statistically speaking, it simply means a result which could not have been achieved by chance. That is, if you compare a new intermediate with the M855 in terms of their average barrier penetration or gel test permanent channel size, the new intermediate should be clearly superior.

Barrier penetration can be quantified more precisely, but gel test channels are more difficult since it's not just the volume of the permanent channel which matters but the distance at which it reaches its maximum width.