Testing, testing...

[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.

[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/i...62mm_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.

[stanc]

I got the value from Gary's ammunition: http://www.inetres.com/gp/military/i...62mm_ammo.html

Note that the value is for 78 ft from the muzzle

Yes, that info is the same as in TM43-0001-27.

I couldn't find a call-out for barrel length.

Nor does the TM give it. However, giving MV from 24" barrel is pretty much industry standard for high-power rounds, so I think it's logical to assume (despite the proverbial warning about assuming anything) that the data is for 24" barrel. And IIRC, for the last 100 years, the standard barrel length of .30-caliber infantry rifles and machine guns has been 24 inches (except for the M14, at 22 inches).

The difference between 2700 and 2750 ft/sec is within the normal spread of chronograph testing.

Chronographed velocities can vary widely, depending on the operator skill, ambient temperature, ammo manufacturer, and other factors. IMO, it's better to use the nominal velocity, as listed in the manual.

When this came up earlier, I ran ballistics tables to determine what muzzle velocity would give 2750 fps velocity @ 78 feet. The result was just a hair over 2800 fps (more specifically, 2805 fps).

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.

That's prudent, but it will make a cartridge bigger and heavier than absolutely necessary. Remember, the only advantage of this concept is the reduction of weight and bulk relative to 7.62x51. The bigger and heavier you make it, the less that advantage.

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.

IMO, the only areas that need to show "significant" improvement over 7.62 M80 are cartridge weight and bulk. Indeed, they are the only areas that can possibly be significantly better.

Muzzle velocity should equal M80, for reasons previously stated.

Terminal effects must be very similar, if not equal, to those of M80, again for reasons previously stated.

Downrange velocities, trajectory, and wind drift should be comparable, if not equal, to M80. The closer they match, the better, partly for psychological appeal, and partly to permit use of optical sights with 7.62 ballistic reticles.

[JASmith]

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's on the money!

Frankly, I was using a looser meaning of the definition -- one more in line with Gene's 50% plus up. We could start with numbers of rounds per kilogram and/or numbers of rounds in a magazine the size of the current 20 round magazine. Don't know if 50% is the right threshold.

Yes, it would need to be demonstrated to the standard you called our attention to.

[stanc]

I was using a looser meaning of the definition -- one more in line with Gene's 50% plus up. We could start with numbers of rounds per kilogram and/or numbers of rounds in a magazine the size of the current 20 round magazine. Don't know if 50% is the right threshold.

There is no "right" threshold. Any figure you set will be arbitrary, neither right nor wrong.

Don't bother trying to set a threshold. Just design and develop the cartridge, then compare its weight to that of 7.62 M80.

It's a simple matter to compare the number of rounds in a mag the size of the current 20-round mag. A stack of 10 rounds of 7.62 will measure 4.73 inches. Divide by 0.441" and 0.422" case diameters, and you get the following:

• 7.62x51 -- 20 rds
• 7x46 ----- 21 rds
• 6.8x43 --- 22 rds

Not a very "significant" difference, is it?

I'd say it's better to compare weight of 100 rds of linked ammo.

Individual links don't weigh much, but 100 of them do add up. Also, links for the .422" case can be smaller and lighter than those for a .441" case. The latter will likely be about midway in size and weight between the 6mm SAW link (which is a near perfect fit to the 6.8x43 case) and the 7.62 M13 link.

Another possible basis for comparison is the volume occupied by 100 rounds. Last I saw, 200 rounds of linked 7.62x51 is packed in a .30-caliber ammo can, 100 rounds in each of two heavy paper cartons (one carton per bandolier). If you can somehow determine how many 100-rd belts of the alternative cartridges can be packed in cartons, in a .30 caliber can, that may show an advantage. Just intuitively, I doubt that a .441" cartridge will permit more than two, 100-rd belts per can, but it might be possible (emphasis on "possible") that three, 100-rd belts of a .422" round could fit.

Yes, it would need to be demonstrated to the standard you called our attention to.

What standard is that? Did I miss it?

[JASmith]

...It's a simple matter to compare the number of rounds in a mag the size of the current 20-round mag. A stack of 10 rounds of 7.62 will measure 4.73 inches. Divide by 0.441" and 0.422" case diameters...

It's a LOT more complicated than that. The current 7.62X51 magazine is not strictly a single stack as implied by your math. Changing the cartridge diameter changes the available number of cartridges in a non-linear fashion.

What standard is that? Did I miss it?

Check Tony's explanation of significance in today's post #140 at 6:01 PM.

[JASmith]

You could be right -- but we'll need more than a cartoon to resolve. Problem is, I was (and might still be) under the impression that there's a gap between the cartridges.

'Sorry 'bout complications, but that's the way life is. Simplify where possible, but make sure nothing important gets left out in the simplification.

In the way of complications -- if the magazine is purpose-built for the cartridge and, as you point out, the cartridges are stacked as efficiently as possible (the reason for the double contact between bases), then the magazine thickness also decreases.

Read it earlier today. Didn't see any "standard" in it then, don't see one now.

(sigh...) Oh well, another indication of not being on the same page.

I hope we get it worked through!

[stanc]

I'd say it's better to compare weight of 100 rds of linked ammo.

Individual links don't weigh much, but 100 of them do add up. Also, links for the .422" case can be smaller and lighter than those for a .441" case. The latter will likely be about midway in size and weight between the 6mm SAW link (which is a near perfect fit to the 6.8x43 case) and the 7.62 M13 link.

Some figures for your consideration.

7.62x51 (147gr bullet) -- Ctg: 392 gr / Link: 65gr
7x46 (130gr bullet) ----- Ctg: 292 gr / Link: 50gr
6.8x43 (110gr bullet) --- Ctg: 263 gr / Link: 35gr

100rds of linked 7.62x51 -- 6.53 lbs
100rds of linked 7x46 ----- 4.89 lbs
100rds of linked 6.8x43 --- 4.25 lbs

Obviously, 100 linked rounds of Tony's proposed 6.5-7x45 would have a weight somewhere in between 7x46 and 6.8x43.

[JASmith]

Just for grins, run the numbers for the 7.62X39....

[JASmith]

Try 281 grains for the 7.62X39 (about halfway down the page at http://en.wikipedia.org/wiki/Compari..._AK-47_and_M16)

I would also interpolate the clips for the disintegrating belt so one could get a rough estimate of apples to apples comparison.

[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.

I got to thinking (Which is always dangerous! ) about that. Did a rough sketch of a 6.5mm "EPR" projectile, modifying the 120gr GMX with an M855A1-type penetrator. (See attached.) I don't know how to make a close estimate of what the bullet weight would be reduced to by using steel where higher-density gilding metal was previously, but it would almost certainly weigh less than 120 grains, perhaps 115 or so?

Note that, compared to M855A1, the 6.5mm penetrator is much longer relative to bullet length, and the base slug is much shorter. Also, at 1.403" length, the base of the 6.5mm bullet will extend rather deep into the cartridge case, reducing the powder volume. The solution would seem to be shortening the tail end of the projectile, and thereby increase powder space.

However, shortening the bullet's rear will further reduce weight, to maybe 105 grains? That would also reduce the BC, possibly impacting trajectory and retained velocity/energy. But, it'd enable MV to increase, which is good. A bad point is that this shortening would shift the center of balance forward, which would likely delay yawing in soft tissue.

I haven't done a similar drawing for 6.8mm and 7mm, but going by this one, it looks as if you may be right about a "green" 6.5mm bullet not being heavy enough. Thoughts?