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Thread: Lets start over: how do you design a GPC?

  1. #1
    Trooper
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    Lets start over: how do you design a GPC?

    I think that we have been going about this from the wrong end. Just about any cartridge can out perform the 5.56x45 NATO. So our goal is to find the right caliber and small case that could replace the 7.62x51 NATO. From Tony Williams writings we know that the caliber is somewhere between 6.5 and 7mm.

    So how small a case can we make that still provides the same performance as the 7.62 NATO? This is the real design parameter.

    And if we find that caliber and case, how should the bullet be made in order to be compared to the M80 cartridge? We can always compare, easily, to the 5.56 NATO. it is the comparison to the 7.62 NATO that is the challenge.

  2. #2
    Chieftain
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    That all depends on how you define performance if you want a cartridge capable of killing effectively at 1k the Grendel can do it on paper it still has the muzzle energy of a 357 mag at 1k. Now if the question is to make a smaller cartridge that can match the muzzle energy of a 7.62 nato while maintaining down range performance the answer is simple, no not practicaly I mean you could have something like a 6.5x47mm the case I 4mm shorter though the overall cartridge case length is near as doesnt mater the same my point being with some of the newer high performance powders and strong brass you could bump the pressure up a little to match 7.62 NATO with a smaller case but the case wouldn't e so much smaller as to fit into say a ar15 chassis but maybe not need an ar10 but then there is that dirty L word that keeps things the same so long as the system is functioning in its current state
    Punctuation is for the weak....

  3. #3
    Didn't they do that back in the '20's and come up with the .276 Pedersen?

  4. #4
    JDub
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    I think we have the right cartridge maybe what we need are longer magazines/magwells/lowers to allow the grendel to fulfill it's potential. Possibly a little more case capacity but then we're talking more pressure/recoil etc. that we're trying to limit to make it controllable. I don't think we could ask for a more "efficient" design in the grendel. We just have to figure out which compromises are going to dominate. We're already at the BC sweetspot, yes?

  5. #5
    JASmith
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    Several ways to slice that cat---

    Once we start stretching magwells, strengthening bolts, and so on, where do we stop?

    If, for example, we allow COAL of 2.4" for the Grendel, would we not have to do the same for the 6.8 SPC? Or, for that matter, combine the two and go with a longer COAL 6.5 mm version of the SPC, say 2.5"?

    Of course, we wouldn't be putting that cartridge into AR15 frames.

    How do we know, for the purposes of discussion, when we have a design that works?

    7mm should not be a player because high BC bullets have weights so close to those of .308" dia bullets as to not offer an advantage.

  6. #6
    Warrior Michael's Avatar
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    Do you want to design a new GPC, or a new system?
    Is your only parameter a smaller case to outperform the 7.62 NATO? A cartridge that is essentially 60 years old? In what ways? Penetration? Range? BC? Felt recoil?

    I'm not trying to be a killjoy. I also understand from reading your posts you really know your stuff... BUT... if you are truly going after something to seriously change the DoD's mind about the 5.56, you need to come out with something a lot more significant than outperforming a cartridge that has been around almost as long as the television.

  7. #7
    Tony Williams
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    Designing a military cartridge, like any other piece of kit, is a matter of choosing the right compromises.

    I believe that to be of interest to any military, a new cartridge will have to do two things: at least match the long-range ballistics of the 7.62mm M80 while saving at least 25% of the ammo weight; and generate recoil about half-way between the 5.56mm and 7.62mm so that burst fire is controllable at short range. It will then be able to replace both calibres in the infantry squad, simplifying procurement, supply and training as well as providing an overall capability boost to the squad.

    I agree that the 7mm calibre is too close to the weight of 7.62mm to achieve this. Using lead-cored bullets, 6.5mm seems to offer the optimum balance between weight saving and effectiveness, although 6.8mm might be better with lead-free bullets - that remains to be determined. Around 2,500 J (1850 ft lbs) muzzle energy seems to be about right to deliver the performance required while keeping weight and recoil down.

    The 6.5mm Grendel is probably a bit too small to deliver what is wanted, especially when the use of lead-free and tracer bullets, plus the probability of polymer-cased ammo coming along, reduces the space for propellant. I think that there are basically two options: a Grendel case stretched to around 44-45mm, or a Rem SPC case stretched to 47-48mm (these are rough estimates). Whichever case is chosen, the overall length must be enough to allow the use of long, finely pointed bullets (which the 5.56mm, 7.62mm and SPC cannot do). Obviously, neither of the options could fit into an AR-15 action.

    The illustration below, from my web article on The Next Generation of small arms and ammunition (http://www.quarry.nildram.co.uk/The%...Generation.htm ), shows several things. I took the Russian 5.45mm 7N6 bullet as an example of a well-shaped military bullet and electronically scaled it up to 5.56mm, 6.5mm and 7.62mm calibres. As you can see, a bullet of this form fitted into the 5.56mm and 7.62mm cases would result in overall lengths too great to be usable in existing military weapons. The two rounds on the right show what I have been talking about - 6.5mms based on the Grendel and Rem SPC cases. They are not intended to be definitive, just to give a rough idea of what the end result could look like.


  8. #8
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    Quote Originally Posted by Tony Williams View Post
    Designing a military cartridge, like any other piece of kit, is a matter of choosing the right compromises.

    I believe that to be of interest to any military, a new cartridge will have to do two things: at least match the long-range ballistics of the 7.62mm M80 while saving at least 25% of the ammo weight; and generate recoil about half-way between the 5.56mm and 7.62mm so that burst fire is controllable at short range. It will then be able to replace both calibres in the infantry squad, simplifying procurement, supply and training as well as providing an overall capability boost to the squad.

    I agree that the 7mm calibre is too close to the weight of 7.62mm to achieve this. Using lead-cored bullets, 6.5mm seems to offer the optimum balance between weight saving and effectiveness, although 6.8mm might be better with lead-free bullets - that remains to be determined. Around 2,500 J (1850 ft lbs) muzzle energy seems to be about right to deliver the performance required while keeping weight and recoil down.

    The 6.5mm Grendel is probably a bit too small to deliver what is wanted, especially when the use of lead-free and tracer bullets, plus the probability of polymer-cased ammo coming along, reduces the space for propellant. I think that there are basically two options: a Grendel case stretched to around 44-45mm, or a Rem SPC case stretched to 47-48mm (these are rough estimates). Whichever case is chosen, the overall length must be enough to allow the use of long, finely pointed bullets (which the 5.56mm, 7.62mm and SPC cannot do). Obviously, neither of the options could fit into an AR-15 action.

    The illustration below, from my web article on The Next Generation of small arms and ammunition (http://www.quarry.nildram.co.uk/The%...Generation.htm ), shows several things. I took the Russian 5.45mm 7N6 bullet as an example of a well-shaped military bullet and electronically scaled it up to 5.56mm, 6.5mm and 7.62mm calibres. As you can see, a bullet of this form fitted into the 5.56mm and 7.62mm cases would result in overall lengths too great to be usable in existing military weapons. The two rounds on the right show what I have been talking about - 6.5mms based on the Grendel and Rem SPC cases. They are not intended to be definitive, just to give a rough idea of what the end result could look like.

    so what your wanting is to take a cartridge like the 308 and trim it down to say 47mm so that we could use long high bc bullets in a 7.62 NATO mag well, neck it down to 6.5mm such as to get a little more velocity or less recoil than 7mm or 7.62 bullets of similar high bc and put a very efficient shoulder on it for accuracy and then build an entirely new platform to shoot it out of...

    if the goal is weight savings and performance and you want to design a new metallic or polymer cartridge to do that imho your already doing it wrong you may be able to save a few grams with a conventional case but if your starting from scratch i would go caseless
    Punctuation is for the weak....

  9. #9
    LRRPF52
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    The military needs to define what it's looking for first, if anything. Since there doesn't seem to be any real push from the military, there's a vacuum of sorts, filled by various people who think they know what the military needs. I have yet to see a specific request from the upper levels of the Pentagon issuing a needs statement for a new small arms cartridge and systems that will shoot it.

    I disagree with the premise that everything outperforms 5.56 NATO, since there isn't another cartridge that can give a unit the same endurance for the same load and logistics footprint. 5.45x39 comes close, but still takes up a little more space, while providing an excellent form factor for exterior ballistics. Its terminal ballistics has been a point of debate for years, ever since the Soviet-Afghan War anecdotes about the "poison bullet".

    I also disagree with requirements to be controllable on burst or automatic fire, since professional infantrymen and combat arms personnel don't shoot on automatic from service rifles or carbines, with a few exceptions. However, the ability to be controllable on auto also transfers over to being controllable while firing rapid strings of fire on SEMI automatic. The point isn't to question controllability, rather to emphasize a realistic understanding of how infantry weapons are actually employed.

    Most would agree that 5.56 NATO isn't an ideal cartridge for ranges that it wasn't meant to engage targets with (200/300m and beyond), and that 7.62 NATO takes up too much space and weight on dismounted soldiers' loads, while exhibiting too much recoil that stresses weapons quickly, requiring heavier weapons, and exhibiting difficult tracking when firing for these reasons.

    Soldiers who have experienced the downrange performance of 7.62 value that type of energy and trajectory, but would welcome this capability in a smaller, lighter package. They're an easy sell. The logisticians and procurement types at the upper levels need to be sold, as do the porkulent politicians who want to know what's in it for their districts. They could care less what it is, as long as some Colonel or General declares it's the best thing since sliced bread, and defense contractors are willing to spend billions in their home State or Congressional district.

    That said, there isn't the amount of interest in small arms by porculicians, since the margins aren't usually in the billions. As to designing the cartridge itself, I would start with terminal performance requirements on personnel and material, then move backwards to exterior ballistics requirements (trajectory and wind drift), then to internal ballistics requirements, which will drive cartridge dimensions, powders, pressures, weapon size, ammunition feeding devices and mechanisms, etc.

    There have been several viable cartridges already developed and demonstrated since after The Great War (.276 Pedersen, .270 Enfield, .280 Enfield, 6mm SAW, and now the 6.5 Grendel). A cartridge that fits inside a detachable box magazine that will stagger for maximum capacity per space, and can still be called an intermediate cartridge will not really do much that the Grendel can't as far as practical performance is concerned. Anything larger in rim diameter is no longer an intermediate cartridge and will be too large for a lightweight/low-recoiling weapon, and within those constraints, it would be difficult to match the BC advantage of the 6.5mm bore diameter, unless some uniquely long projectiles were made for 6.35mm.

    The main improvements you could make would be getting Grendel performance in a narrower cartridge, which is going to require some pretty high pressure if even possible without exceeding Grendel COAL.

  10. #10
    LRRPF52
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    Quote Originally Posted by mongoosesnipe View Post
    so what your wanting is to take a cartridge like the 308 and trim it down to say 47mm so that we could use long high bc bullets in a 7.62 NATO mag well, neck it down to 6.5mm such as to get a little more velocity or less recoil than 7mm or 7.62 bullets of similar high bc and put a very efficient shoulder on it for accuracy and then build an entirely new platform to shoot it out of...

    if the goal is weight savings and performance and you want to design a new metallic or polymer cartridge to do that imho your already doing it wrong you may be able to save a few grams with a conventional case but if your starting from scratch i would go caseless
    That last image is simply there to illustrate what extrapolating the 5.45 7N6 form factor would look like when placed in existing cases. It demonstrates the importance of case length and diameter and how they affect COAL, which will determine action length of the weapon and magazine size. Every single one of those examples exceeds the current magazine dimensions for any of the intermediate cartridges, with the possible exception of the new Chicom 5.8x42mm.

  11. #11
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    Quote Originally Posted by LRRPF52 View Post
    That last image is simply there to illustrate what extrapolating the 5.45 7N6 form factor would look like when placed in existing cases. It demonstrates the importance of case length and diameter and how they affect COAL, which will determine action length of the weapon and magazine size. Every single one of those examples exceeds the current magazine dimensions for any of the intermediate cartridges, with the possible exception of the new Chicom 5.8x42mm.
    I know I was just poking fun since the listed specs were pretty much fit by an existing cartridge wich was made with a shorter case to accommodate longer bullet. That bing the 6.5x47 which if you want to get down to it isn't a shortened 308 but a reformed 250 savaged which spawned the 300 savage which was the parent case for the 308 but they wanted a little more power so they added 4mm worth of powder to the case and raised the pressure spec...

    The military isn't actively hunting for new cartridges and platforms but there are small number of what I lie I call door kickers that do often try and field new equipment that isn't field by the rest of the military
    Punctuation is for the weak....

  12. #12
    Trooper
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    Quote Originally Posted by mongoosesnipe View Post
    so what your wanting is to take a cartridge like the 308 and trim it down to say 47mm so that we could use long high bc bullets in a 7.62 NATO mag well, neck it down to 6.5mm such as to get a little more velocity or less recoil than 7mm or 7.62 bullets of similar high bc and put a very efficient shoulder on it for accuracy and then build an entirely new platform to shoot it out of...

    if the goal is weight savings and performance and you want to design a new metallic or polymer cartridge to do that imho your already doing it wrong you may be able to save a few grams with a conventional case but if your starting from scratch i would go caseless
    If we merely wanted to out perform the 7.62 NATO, the 260 Rem (6.5x51) does that now using the same case. The 260 Rem might be the starting point for comparisons. Then the process would be creating a smaller case that has similar performance. One might want to have a bullet that is similar to the M80. Maybe a FMJ lead core for comparison tests.

    Also there is a need to have something other than a brass case as it is just too expensive to leave on the battlefield. So part of the design is a plastic (polymer) case.

  13. #13
    Chieftain stanc's Avatar
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    Quote Originally Posted by Trooper View Post
    So how small a case can we make that still provides the same performance as the 7.62 NATO?
    That might best be answered by looking at cartridges that are actually being developed by private individuals for this purpose:

    - Cris Murray created the 7x46 UIAC and 6.86x46 UIAC, based on the 0.44" diameter 7.62x45 Czech case. Cartridge OAL is ~2.6" IIRC.

    - RRBud, a member of Tony's MG&A forum, has made the .270 Sidewinder (6.8x50), based on a strengthened .30 Remington (0.42" diameter) case. Cartridge OAL is 2.8" IIRC.

    - Harrison Beene, another MG&A forum member, plans to develop 6.5mm and 6.8mm rounds based on (0.45" diameter) 6.5 Carcano brass. Case length and cartridge OAL to be determined.

    Judging by these examples, it looks like cartridge OAL will probably be equal to that of 7.62 NATO, or somewhat shorter, depending on variables such as case diameter, bullet diameter, bullet weight, muzzle velocity, etc.
    And if we find that caliber and case, how should the bullet be made in order to be compared to the M80 cartridge?
    My opinion is that a comparison of relative performance requires that a lead core FMJ in the candidate caliber(s) be tested against the lead core 7.62 M80. Improved 6.5/6.8/7mm ball rounds could/should also be tested, if available, to give a more complete picture of performance capability.
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  14. #14
    Chieftain stanc's Avatar
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    Quote Originally Posted by Michael View Post
    Do you want to design a new GPC, or a new system?
    Because cartridge size would preclude use in 5.56 rifles and carbines, a new GPC would necessarily require development of a new individual weapon. Possibly, existing 7.62 rifles and machine guns could be converted to fire the GPC.
    Is your only parameter a smaller case to outperform the 7.62 NATO? A cartridge that is essentially 60 years old? In what ways? Penetration? Range? BC? Felt recoil?

    I'm not trying to be a killjoy. I also understand from reading your posts you really know your stuff... BUT... if you are truly going after something to seriously change the DoD's mind about the 5.56, you need to come out with something a lot more significant than outperforming a cartridge that has been around almost as long as the television.
    It appears you do not understand the premise for the GPC concept. The idea is not to outperform 7.62 NATO. It is to provide equal performance, but with a substantial reduction in weight.
    If at first you don't succeed, give up.
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  15. #15
    Warrior Michael's Avatar
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    Quote Originally Posted by stanc View Post
    Because cartridge size would preclude use in 5.56 rifles and carbines, a new GPC would necessarily require development of a new individual weapon. Possibly, existing 7.62 rifles and machine guns could be converted to fire the GPC.

    It appears you do not understand the premise for the GPC concept. The idea is not to outperform 7.62 NATO. It is to provide equal performance, but with a substantial reduction in weight.
    Altogether possible. My understanding is the desire as stated by Tony Williams is to "Replace both existing rounds (5.56 and 7.62) in the dismounted infantry section with one new general purpose cartridge (GPC) with good long-range performance."

    My stance is that if you want DOD support and backing, it has to be a very significant improvement over the current ammunition/weapons that we have in the inventory. An example would be what may have been the argument for going from the 7.62 NATO and M14 to the 5.56 and M16, as this would;
    -Maintain the max effective range of the individual weapon.
    -Decrease weapon weight by about 20%.
    -Decrease weapon length by 10%
    -Decreased the individual soldiers ammo weight by 50% and provide inexpensive, disposable magazines.
    -Decreased felt recoil by 40% (this is a guess from what I feel firing an AR15 and a M1A1).
    -Decrease cost per system by making use of high impact plastics and forged/milled aluminum.

    Above data are estimates of what I think the pitch was back in the late 50's/early 60's for the M16.

    BLUF - the current ammunition/weapons are not perfect. The problem will be how to have such a dramatic leap forward in performance parameters or technology to lure the DOD to invest in the current economic climate.
    Last edited by Michael; 06-30-2013 at 05:40 PM. Reason: typo

  16. #16
    LRRPF52
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    The .260 Remington is a niche long-range rifle cartridge, that lacks something the .308 has in strides: barrel life. While the .260 Rem has great barrel life compared to 6.5x284, .300 WM, and .338 LM, it is still too short for a military general issue weapon, and is still in the battle rifle size class, since it is a necked-down .308 Win.

    The performance of the .260 Rem is definitely advantageous for a sniper or DM, but way more than a rifleman needs. It is possible to get .260 Rem performance from an AR15 action, but you need a fat case in excess of the diameter of the .308 Win., which is a non-starter for military use. I also don't want a 140gr high sectional density projectile exiting a rifleman's muzzle at 2700fps within close quarters from a high pressure chamber. That is a bad thing from a practical perspective in my opinion, for a number of reasons:

    * disorienting effects of muzzle blast and overpressure

    * muzzle climb and recoil on a lightweight weapon

    * effects of high pressure on barrel, bolt, receiver, and reciprocating parts life

    For COAL, once you start getting to 2.500", you're pushing the limits of the intermediate cartridge concept towards something that is significantly heavier, and will occupy more space on the soldier's load.

    With what I've observed of the existing 6.5 Grendel's performance with 123gr OTM, I see no need for more velocity. The question then becomes how to mass-produce a military projectile with a BC of .510, while still being able to achieve 2350fps from carbines, and 2500fps from a Light Machine Gun.

    Even a 12.5" Grendel carbine can push a 123gr to 2300fps, if not 2350fps with canister powder and the right components. I think Variable is able to get 2179fps with the 123gr A-MAX from his 10.5" Grendel.

    The real area that needs improvement is the current status of our belt-fed, Squad Support weapons, and the decision has to be made whether or not a new caliber should be exclusive to it, or should it share a chambering with carbines/rifles. I personally think the performance requirements of the LMG are different enough to warrant a separate chambering, but one that could be common with Semi-Auto Sniper Systems (SASS), as well as DM carbines, but that goes against the General-Purpose Cartridge concept, which prioritizes logistics over performance.
    Last edited by LRRPF52; 06-30-2013 at 06:01 PM.

  17. #17
    Chieftain stanc's Avatar
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    Quote Originally Posted by LRRPF52 View Post
    The question then becomes how to mass-produce a military projectile with a BC of .510
    Worth repeating.
    If at first you don't succeed, give up.
    Anything that can't be done easily, probably isn't worth doing.

  18. #18
    Moderator bwaites's Avatar
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    Quote Originally Posted by stanc View Post
    Worth repeating.
    I really don't understand why this is a concern, it's harder to consistently make a polymer tipped bullet than an FMJ bullet, and several different companies make polymer tipped bullets in bulk without problems. On top of that, multiple piece bullets, i.e. steel 2 piece with copper jackets are harder, but those also are made in bulk.
    ”You seek escape from pain. We seek the achievement of happiness. You exist for the sake of avoiding punishment. We exist for the sake of earning rewards. Threats will not make us function; fear is not our incentive. It is not death that we wish to avoid, but life that we wish to live.” - John Galt

  19. #19
    Chieftain stanc's Avatar
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    Quote Originally Posted by bwaites View Post
    I really don't understand why this is a concern...
    Perhaps because a high BC is vital to the GPC concept, and no one has yet proven that the necessary BC can actually be achieved in a military ball projectile of the requisite diameter and weight.
    If at first you don't succeed, give up.
    Anything that can't be done easily, probably isn't worth doing.

  20. #20
    I don't see how it is rocket-surgery, you make a long pill with good aerodynamics, shove it hard, and you are made in the shade. Only way to get that work done is with a short fat case within the constraints of the lowers we have. I just don't see how there is more than one way to do it.

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