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Subject: Lengthening Assault Rifle Rounds when Increasing Calibers
Roman    3/25/2007 11:25:51 AM
In debates over what would be an optimal round for infantry forces, it is often brought up that the optimum is thought to lie somewhere between 6mm and 7mm calibers. Perhaps the most often mentioned round is the 6.8mm Remington SPC. The round, however, is 6.8 x 43mm, so its trajectory would likely be worse than that of the 5.56mm. This might be compensated for by its other benefits, but would it not make more sense to use longer rounds, in order to achieve better trajectories? For example, we could take the length of the 7.62x51mm NATO round and create a 6.3x51mm round. This would have the same mass/volume of projectile to mass/volume of propellant ratio as the 5.56x45mm round (assuming round proportions would stay the same, which they well might not [I am not an expert], but the principle holds regardless), so it would have a very flat trajectory, but would lie in the 6mm-7mm caliber sweet spot and still be significantly lighter than the 7.62x51mm round. What do you think?
 
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flamingknives       3/25/2007 11:29:33 AM
It would need a longer action, hence a heavier rifle, be more difficult to extract (thinner than the 7.62mm case and longer than the 5.56mm case.

Increased velocity would come at the expense of more recoil.
 
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Roman       3/25/2007 11:35:04 AM
I can see why the rifle would have to be longer, but why is a longer but thinner case more difficult to extract (I am not familiar with how rifles are designed...)?
 
As to the higher recoil than 5.56mm, that is unavoidable. It would still, however, be less than the 7.62x51mm recoil.
 
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Roman       3/25/2007 3:14:39 PM
I thought some more about this and it occured to me that since the length to width ratio is the same in a 5.56x45mm as in 6.3x51mm, I don't see why the case would be more difficult to extract.
 
Still, I guess telescopic ammunition might be a better way to go, but that too seems to have some disadvantages, such as the fact that the greater width means that fewer will fit in a magazine. Assuming the normal shape of military assault rifle projectiles, how much extra volume of propellant can a telescopic round pack into the same space and how much wider than a normal round of the same caliber does a telescopic round need to be?
 
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doggtag    lengthening the cartridge? or just the bullet?   3/25/2007 3:30:15 PM
Hmm.
No mention of the 6.5mm Grendel?
http://www.6mmbr.citymaker.com/i/Reader%20Photos/Grendel_308-65-223web.jpg" width=150 border=0>
6.5 Grendel®

Development | Products | Ballistics | Reloading Data

With the addition of the 6.5 Grendel® to the product lineup at Alexander Arms, the history of no compromise design, engineering and innovation continues. The 6.5 Grendel® provides an extreme range capability for hunting, competition and tactical applications at ranges way beyond those previously achievable with this class of weapon. The 6.5 Grendel® has the flexibility to move from lightweight varmint bullets in the 90 grain class, which offer superb accuracy for competition and small game shooting, to mid weight 108/120 grain competition bullets and then on to 130 and 140 grain bullets, ideal for longer range, tactical shooting.

The 6.5 Grendel® is challenging the status quo in Military and Law Enforcement units around the world. First unveiled in May 2003 at the Blackwater Training facility in NC, the 6.5 Grendel® out-shot the 7.62 NATO at range with half the recoil. Still supersonic at 1200 yards, the 6.5 Grendel® delivered superior external ballistics to the 7.62 NATO. Utter reliability, superior external and terminal ballistics than the current state of the art, outstanding accuracy in a lightweight M16/AR-15 platform it is what appears to be the pinnacle for what may be achieved in the M16/AR-15 chassis. The 6.5 Grendel® is not a series of compromises, but rather the perfect marriage of mechanical function, internal, external and terminal ballistics all working in harmony.

Shooting a 123-grain Lapua Scenar with a ballistic coefficient of .547 and a muzzle velocity of 2600 fps delivers outstanding accuracy out to 1200 yards. At 600 yards, tennis ball size targets are no match for this flat shooting round. For extreme accuracy, formidable terminal ballistics and long range applications, the 6.5 Grendel® from Alexander Arms is unbeatable.

Compared to the 5.56 the 6.5 Grendel® with roughly twice the lead mass gives you the potential for twice the mass of fragments, and if maximum fragmentation is coincident with maximum temporary cavity, the terminal ballistics are quite convincing indeed all in a package that shoots flatter, with 50% less felt recoil than 7.62 NATO M80 ball.

Again, Alexander Arms is ahead of the curve. Currently in testing with the US Military for widespread adoption, the 6.5 Grendel® seems assured a place in history.

-----
(marketing hype over)
 
There are scores of websites and magazine articles touting the perks of this cartridge compared to the current crop.
Some have even suggested that a dedicated military-grade round (with an AP core) could surpass 7.62 in several areas (the flatter overall trajectory and maintained velocity of the longer bullet as compared to typical military bullets helps give the Grendel its very favorable BC).
And there are also proponents who suggest that, with the right bullet weight and shape (and possibly a minor adjustment to the cartridge), the Grendel could possibly fulfill the roles that currently take two calibers, the 5.56 and 7.62.
 
I'll withhold my final judgment until I see a crew served MMG in 6.5 show up another GPMG in 7.62.
I see that it would have to show marked improvement in controllability off the tripod or pintle (bipod mode like the old M60 did, and because it would be looked at as a SAW replacement),
superior on-target effects at various ranges,
less crosswind interference,
superior penetration of various forms of cover,
and perhaps even more important, it would have to be shown to operate more reliably in whatever MG design.
It's obvious that belts of 6.5 Grendel would be lighter than 7.62, but would the extra weight as compared to 5.56 belts (and box satchels (is that what they're called?) as used with the SAW) be more than compensated for by using the better ammo, as we'd be carrying less rounds for the same weight?
 
I'd suggest taking a number of the typical targets than 7.62 can r
 
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Horsesoldier       3/25/2007 10:42:40 PM

In debates over what would be an optimal round for infantry forces, it is often brought up that the optimum is thought to lie somewhere between 6mm and 7mm calibers. Perhaps the most often mentioned round is the 6.8mm Remington SPC. The round, however, is 6.8 x 43mm, so its trajectory would likely be worse than that of the 5.56mm. This might be compensated for by its other benefits, but would it not make more sense to use longer rounds, in order to achieve better trajectories? For example, we could take the length of the 7.62x51mm NATO round and create a 6.3x51mm round. This would have the same mass/volume of projectile to mass/volume of propellant ratio as the 5.56x45mm round (assuming round proportions would stay the same, which they well might not [I am not an expert], but the principle holds regardless), so it would have a very flat trajectory, but would lie in the 6mm-7mm caliber sweet spot and still be significantly lighter than the 7.62x51mm round.

What do you think?

6.8 Rem SPC is pretty much identical, ballistically, to 77 grain Mk 262 ammuntion, except that it is pushing about 600 more foot pounds at the muzzle and about 300 more at 300 meters.  Performance is close enough that you can use a 5.56mm reticle ACOG with 6.8 Rem SPC without needing to do much in terms of hold-overs and the like.
That said, current development of alternate rounds is pretty much an American show and is pretty much driven and limited by the modularity of the AR-15 platform -- rounds have to get through the M16/M4 magwell to be viable.  About the most extreme use of this (in term so ballistic efficiency) is the 6.5 Grendel mentioned by Dogtagg. 
 
6.8 Rem SPC and 6.5 Grendel are good rounds, but suck if you stack them up against x51mm relatives or similar rounds.  6.5 Grendel underperforms compared to .260 Remington, for instance -- same bullet caliber and same bullet weight, but a couple hundred feet per second slower coming out of the shorter Grendel case.  So if you only look at it that way, a round like .243 or .260 Remington, etc., is better . . . I guess.
 
The problem I see is defining "better."  Switch to 6.8 Rem SPC or 6.5 Grendel and you get some real strengths -- greater retained energy for instance, and some irrelevant ones (as I've mentioned before, 6.5 Grendel is great for 600+ meter shooting, and when Joe can't even acquire the target at that range and, if he does see it, engages it with a service rifle guaranteed to shoot 4 MOA . . . so what?  Most military issue service rifles can't consistently capitalize on that performance, so you're pouring $$$$ into nothing that could be better spent better owning the 0-100 and 0-300 range environments).  They also cost you rounds in the gun, rounds carried as a basic load, and recoil, slowing follow up shots.  But, the minuses on all those are pretty mild -- 25 or 26 rounds in the gun versus 30, maybe losing a magazine or so in terms of basic load, modest enough recoil to work for CQB work.
 
All the minuses are magnified if we switch to a full-power x51mm case length.  Bulking ammo means even with relatively light bullet weights (100-130 grain, say) you're still talking about less ammo carried and fewer rounds in the gun.  There's a valid argument, in my opinion, that 6.8 Rem SPC or 6.5 Grendel (or the other couple of rounds that live in that general neighborhood) pluses and minuses are preferable to 5.56mm's pluses and minuses, though it's just shuffling those pluses and minuses in any case.  I would say the downsides of a x51mm case length outweigh the pluses, however.
 
Which is not to say that the overall length of an M16's magazine well should be the set in stone max limit for a service rifle round.  It appears likely that it will remain so for the current generation of weapons simply because of economic inertia and the fact that stretching an intermediate assault rifle round a bit longer overall does not seem to deliver anything markedly superior to what you can get with the M16 mag well OAL restriction. 
 
 
The next generation of ammunition/weapons seems likely to go with caseless or cased telescoping ammunition.  Current research is still looking at the 62 grain 5.56mm projectile as the bullet component, but those technologies might open the door for more serious debate about what caliber and bullet weight to use, as they'll be a blank slate without existing weapons systems to limit dimensions, etc.
 
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Roman       3/26/2007 9:28:44 AM
Interesting posts doggtag and Horsesoldier  
 
So the next caliber change will only likely happen when we transition to a different type of ammunition, such as caseless or telescopic. This makes me wonder what is the reason that the transition to telescopic ammunition has not started already --> unlike with caseless I was under the impression that there are not any inherent problems with caseless ammunition outstanding.
 
BTW: Does anybody have any info on the exact dimensions and weights of the various components (projectile, case, propellant) of ammunition? I would like to calculate how much weight and space (both in terms of width and length) you could save with telescoped ammunition of the same caliber as current rounds and need the mass and dimension data for that. Thanks!
 
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Horsesoldier       3/26/2007 3:23:39 PM

Interesting posts doggtag and Horsesoldier  

 

So the next caliber change will only likely happen when we transition to a different type of ammunition, such as caseless or telescopic. This makes me wonder what is the reason that the transition to telescopic ammunition has not started already --> unlike with caseless I was under the impression that there are not any inherent problems with caseless ammunition outstanding.

 

BTW: Does anybody have any info on the exact dimensions and weights of the various components (projectile, case, propellant) of ammunition? I would like to calculate how much weight and space (both in terms of width and length) you could save with telescoped ammunition of the same caliber as current rounds and need the mass and dimension data for that. Thanks!



 
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Horsesoldier       3/26/2007 3:29:06 PM




Interesting posts doggtag and Horsesoldier  



 



So the next caliber change will only likely happen when we transition to a different type of ammunition, such as caseless or telescopic. This makes me wonder what is the reason that the transition to telescopic ammunition has not started already --> unlike with caseless I was under the impression that there are not any inherent problems with caseless ammunition outstanding.



 



BTW: Does anybody have any info on the exact dimensions and weights of the various components (projectile, case, propellant) of ammunition? I would like to calculate how much weight and space (both in terms of width and length) you could save with telescoped ammunition of the same caliber as current rounds and need the mass and dimension data for that. Thanks!








Ooops.  Let me try that again.
 
For dimensions and precise information on ammunition, I'd recommend this guys webpage:  http://www.inetres.com/gp/military/infantry/index.html#pistol
 
It only deals with US military issue ammunition (including the older WW2 era stuff), but does provide the level of detail you're looking for, I think.
 
There are some similar sights concerning Russian/Warsaw Pact ammunition that I've seen, but that was before losing my bookmarsk on this computer.  Googling things like "7.62 M43" and the like may turn up what you're looking for.
 
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Roman       3/27/2007 8:01:55 AM
Thanks Horsesoldier - that is a great site with excellent info. Unfortunately, I am still lacking separate data on the mass of the case and the propellant and the width of the case that would be needed to make my calculation with a good degree of accurracy.

Are there any major unresolved problems with telescoped ammunition given the fact that the switch to such ammunition is not yet underway?
 
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Horsesoldier       3/27/2007 9:12:43 AM

Thanks Horsesoldier - that is a great site with excellent info. Unfortunately, I am still lacking separate data on the mass of the case and the propellant and the width of the case that would be needed to make my calculation with a good degree of accurracy.


Are there any major unresolved problems with telescoped ammunition given the fact that the switch to such ammunition is not yet underway?


Hmmmm, I'm not sure if I know any website that includes the specific case and propellant weights.  If you can, pick up a copy of Cartridges of the World by Frank Barnes (available at places like Amazon, as well as most of the big chain bookstores).  Among other excellent information on most or all of the civilian small arms cartridges, it contains an appendix of US military ammunition that does include powder weight, case weight, bullet weight, and performance requirements, etc., for US military ammunition.
 
As for the use of CTA ammo, I think it is ready to use for larger weapons and either in the process of being fielded or going to be soon.  Concerning it's use in small arms, I think the main issue is that it has never been pursued vigorously (with lots of funding) -- there have been some experiemental efforts that I guess would be considered CTA, but none has gone very far.  Now it is being pursued to reduce the weight of ammo since everyone has pretty much accepted that soldiers are going to be carrying body armor, night vision gear, etc., that is not going away, etc.

Tony Willams website has some good history on assault rifles and other weapons systems if you haven't already seen it:
 
 
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