Testing, testing...

[stanc]

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

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.

[stanc]

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.

I fully agree with the need to plan for use of tracers. In all my discussions about alternative cartridges, nobody else has ever considered that need. However, I would prefer to go with the smallest brass case compatible with that requirement.

I see little reason to be concerned about the possible use of plastic/metal cases. I mean, they've been working on those for how many decades, and have yet to produce a viable product.

[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? :confused:

[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? :confused:

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

[stanc]

It's a LOT more complicated than that.

You're wrong. It IS just that simple. Why do you always want to make things more complicated than they are?

The current 7.62X51 magazine is not strictly a single stack as implied by your math.

No, it isn't. It's a staggered, double-stack mag. But, each stack has the cartridges lying atop one another, so the height of each 10-rd stack is 10 times the 0.473" cartridge case diameter. The drawing below shows how the rounds are stacked.

(Disregard the position of the top round. The drawing is of a double-column, single-feed mag, whereas military rifle mags are of the double-column, double-feed type.)

Changing the cartridge diameter changes the available number of cartridges in a non-linear fashion.

Not true. See above.

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

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

[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]

You could be right -- but we'll need more than a cartoon to resolve.

Now, why did I know you were going to say that?

Problem is, I was (and might still be) under the impression that there's a gap between the cartridges.

Perhaps that's due to the post by Harrison on the old forum, where he said that 6.5 Grendel rounds have a gap between them? Even if he was right, it's because of stuffing a .441" case into a mag with external dimensions originally made for a much smaller diameter round.

With mags designed for a specific cartridge, there is no gap in between rounds. I couldn't find a photograph of a sectioned magazine, but the Garand clip has its cartridges stacked in the same manner.

...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.

That's true, and was assumed in the mag capacities I listed in my previous post.

However, capacity for mags of the same height as 7.62x51 mags would be as I showed. You can see a good example of this in the 20-rd 6.8 SPC mag marketed by Pro-Mag. It has almost the same height as a 20-rd M14 magazine (but slimmer, as noted in the previous paragraph).

[stanc]

P.S. If the Garand clip doesn't convince you, here's a double-stack rifle mag, showing the rounds contacting each other.

[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.

[INDENT]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[/INDENT]

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....

[stanc]

No can do, Joe. I don't know the weight of either the 7.62x39 cartridge, or the 50-rd RPD belts.

[JASmith]

Try 281 grains for the 7.62X39 (about halfway down the page at http://en.wikipedia.org/wiki/Comparison_...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]

Okay, here ya go.

7.62x39 -- Ctg: 281 gr / Link: 43gr

100rds linked -- 4.63 lbs

[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?

[Tony Williams]

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?

I think you've summed up the problems. Penetration into the case isn't necessarily an issue if the case capacity is big enough, but getting a good BC with less dense materials is difficult in small calibres; the length/diameter ratio could become a problem with stabilising such bullets.

[stanc]

Okay, let me see if I have the whole picture right.

We need to get a worthwhile reduction in ammo weight, while having significantly better terminal effectiveness than 5.56x45 in close combat, with terminal effectiveness at long range -- and barrier penetration at all distances -- comparable (if not equal) to 7.62x51.

In addition, for psychological reasons muzzle velocity must be equal to that of 7.62x51 ball, when fired in barrels of the same length: 2700-2750 fps in a 20" barrel. Plus, it must be done using "eco-friendly" bullets, with steel and/or copper as core materials.

In order to get the necessary weight reduction, the cartridge case has to be of significantly smaller diameter and (probably) shorter length than for 7.62x51. On the surface, this appears feasible. The weight comparisons I did on the previous page show that 100 rounds of linked 7x46 provides a 25% weight reduction, and 6.8x43 cuts weight by 35%. IIRC, that's close to what the early LSAT goals were.

The bullet also must be substantially lighter than the standard 7.62 ball projectile, but be very streamlined, with a long ogive, so as to have a high BC for maximum downrange velocity and energy. Unfortunately, with a non-lead core, such bullets are extremely long, with undesirably high L/D ratios that may make a matching tracer (which is typically longer than the ball round) too long to be viable. Then there is the reduced powder capacity to contend with, too. If the bullet is shortened enough to solve those problems, then BC is greatly reduced. Plus, the lighter the bullet is made, the less effective it will be against both soft and hard targets.

I used to think this concept was doable, if just barely. But, after the last couple days, I'm beginning to think it may not be.

[Tony Williams]

I think that with some compromises, we could get there, although lead-free ammo certainly makes it harder.

The barrier penetration doesn't have to match M80 at all ranges, just be closer to 7.62mm than it is to 5.56mm.

Similarly, I don't think that the muzzle velocity, or energy retention at long range, need to match M80 exactly as long as they are in the same ballpark, so that there will be little practical difference as far as the troops are concerned.

[JASmith]

Take the 120 GMX shape, use voids up front to move the CG aft, different materials for improving penetration, etc., to reduce the weight to 100 gr and one gets a BC of .400 assuming the same shape factor.

This BC, coupled with a MV of around 2750 ft/sec for an 18.5" barrel (see http://ammoguide.com/?catid=456) and you track the 7.62X51 M80 fairly well for trajectory and wind resistance.

The bullet would have 90% of the sectional density of the M80, so would need some design effort (e. g., M855A1) to bring the barrier defeat and armor penetration up to the M80 level.

[stanc]

The barrier penetration doesn't have to match M80 at all ranges, just be closer to 7.62mm than it is to 5.56mm.

Similarly, I don't think that the muzzle velocity, or energy retention at long range, need to match M80 exactly as long as they are in the same ballpark, so that there will be little practical difference as far as the troops are concerned.

It seems to me that barrier defeat and wound trauma must be equal to 7.62mm, or both the troops and the decision makers will object to replacing 7.62x51 with a less effective round. If we were still talking about replacing 5.56x45 only, that would be a different matter.

Also, if muzzle velocity (and to a lesser extent, downrange velocities) don't match M80, it won't pass Joe's "that looks about right" test. It isn't possible to match the retained energy with a lighter bullet launched at the same speed, so I'm not concerned with that. If terminal performance is as good as 7.62mm, energy figures shouldn't be of much consequence.