The Ultimate Combat Round

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The 6mm optimum is nothing more than a regurgitation of ideals that have been well know since WWII, although I want to see a Hague compliant 6mm bullet with a BC of over 500. Frankly, I don't think it can be done in a canneleured ball round with a conventional core. The very best heavy match bullets like the 105gn Lapua Scenar have BCs in the low 500s - but some have argued that these figures are highly opimistic in real world conditions, and such bullets are not cheap and lack penetration. You can get higher BC with VLD bullets like those from Berger, but they are long for caliber and are very fragile.

A realistic number for a Hague compliant bullets suitable for military use is probably closer to 400. The Sierra GameKing 90 gn FMJBT is right around 0.387, IIRC.

Another criticism of the 6mm Optimum is no actual proposed cases. For amusement, I decided to look at some simple possibilities for testing. We already have the 6mmAR, derived from necking the 6.5 Grendel down to 6mm

http://www.6mmar.com/

This requires a 0.445 case head (same as 7.72x39) and fits into a 6.5 Grendel magazine even with long bullets.

Another alternative is to neck the 6.8 SPC down to 6mm. With a 90gn FMJ and a 20 inch barrel, 2800 fps is fairly easy to obtain, and this round will best the 5.56x45 at any range with only a moderate increase in recoil. This round will fit into 6.8 SPC magazines, but bullet weight is limited to about 90 grains without intruding into the powder space.

Finally, I looked at necking down the Czech 7.62x45mm to 6mm. This case used the same case head size as the Russian 7.62x39 but is obviously longer and with an increased powder capacity. Due to case length, it is also limited to lighter bullets. It will also weight more than the 6mm SPC. Performance 2869 fps with the same 20 inch bbl and 90gn SGK FMJBT, only about 100 fps slower than a 243 with the same bullet.
 
I didn't think that cartridge dimensions could have nearly that magnitude of change on efficiency!
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It really depends on the parent case. Propellants, case design and other factors have made it possible to decrease cartridge size without losing anything, or build reduced size cartridges that come very close to related full sized cases.

Compare 6.5x55 to 260 Rem, 7x57 to 7mm-08, or look at how close 6.5 Grendel comes to 6.5x55 (original loadings).

The Swede will launch a 120gn projectile at 2800 fps, which the Grendel will do 2700 with the same bullet. That a 3.6% difference in velocity. But the Swede case is 55mm long versus 39mm, and the Swede has a 0.473 case head versus 0.455 case head for the Grendel.

Granted, you can load the Swede much hotter in a modern action, but that misses the point.
 
Which comes to something more like an 8% difference in KE.

But really, how applicable is high efficiency cartridge geometry when it generally requires sharp shoulders, short, fat cases and higher pressures?

Or, put another way, what could you wring out of the old 6.5x55 if the case head were thicker?
 
I am going to have to respectfully disagree with you here. A question is asked, and yes people on THR can respond to the original post. Not everyone is going to sit on the edge of their seat for 17 or 18 pages and read through the thread... and follow every development and twist.
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Fair enough. I just kinda dislike the guys who pop in and contribute little. It's not that they contribute nothing, it's just that they don't find the topic that's being discussed at the time. Sure they have no obligation to follow 18 pages of largely arcane discussion, but it's kinda nice.
Having said that, the real pet peeve of mine is the people that don't read the OP.
 
What if you did 6.5 MPC, increased the thickness of the rim by .2mm, loaded it with a 108-grain Scenar/SMK to 2750 f/s out of a 20-inch barrel, then had a high pressure (red-banded or some other marker, or with a slightly longer neck so that it wouldn't fully chamber) load with a 120 grain bullet at 2750 f/s (out of a 20") with a longer OAL for a sniper/MG load? You could also have a 90-grainer at 2900 f/s load, which would be pretty sweet before 400 yards.
I think that the MPC cartridge will do the most for the smallest effort, but I'm also running off of the parent company's data. Granted, they have marked the length of barrel they are using, so it doesn't seem to be as much of a deception as, say, the SPC and Grendel data.
 
Percy, there's not doubt the 6.5x55 can do much better. The point was only to illustrate what 100 years of chemistry and metallurgy have accomplished I doubt you'd want to shoot hot 6.5x55 loads in an original small ring 96.

Grendel is not the ideal shape for a combat round, which I pointed out previously. But you can build a fairly compact case that will give 90% or better of the performance of a much larger, but older case.

For modern combat rifles, going back to a full sized case is not a satisfactory argument, and one that flies in the face of everything learned since WWI. It means a bigger stronger and necessarily heavier weapon. It means less ammunition for the same combat load, and it really gains you very little in terms of actual combat effectiveness. The goal is to render an enemy hors de combat, not get a clean kill like you want in a game animal. And you are limited to Hague compliant bullets (unless you want to throw all that away), in which case many of the full power rounds actually perform worse on targets than there SCHV counterparts.

One of the main reasons for the decline of the effectiveness of the 5.56x45 has been the fiddling in order to gain more range and more accuracy, at the expense of lethality. It is worth keeping in mind that the Swedes and other 'forward thinking' Europeans countries originally pitches the SS109 and a less lethal, less cruel alternative to the then issues US M193, and the M855 is a direct descendant of the SS109.

There is no doubting that barrier penetration and short barrels work against the 5.56x45, and that a heavier bullet might improve lethality at the outer edge of the combat envelope, but ultimately, the argument for a true intermediate round is as a replacement for both the 5.56x45 and 7.62x51. Such a round requires elements of both rounds it would replace.

Right now the problem is that everyone is fixated on the platform, which happens to be the M16. Every new combat round proposed seems to need to either fit in the M16 magazine, or into one of the previous 7.62x51 battle rifles. If you are going to do that, why even bother with a new caliber.

My suggestion would be to create a true intermediate round, with a casehead, length and internal volume somewhere between 7.62x51 and 5.56x45, recognizing that it is going to be a compromise, and excel at no mission, but be fairly good at all of them.

The Grendel has decent performance figures, particularly in the 6mmAR iteration. But As Percy notes, it's sharp shoulder and modest taper aren't ideal for a combat round. Pressure isn't really all that high, easily within the operating pressure of military 5.56.

I would propose addressing the case design issue by having a longer magazine and cartridge OAL. Let's say 2.500, as a compromise between 5.56x45 (2.260) and 7.62x51 (2.800)

I've mentioned this before, but if you really use the 'in between' numbers - averaging 5.56x45 and 7.62x51 you get.

Velocity of around 2850
Caliber 6.5
bullet weight 106
Case head about 0.430
OAL 2.530

You can get that round with a modest body taper and shoulder angle by necking down the Czech 7.62x45 to 6.5 and loading to to safe M16 pressures, but it takes a longer OAL than the 2.260 of the 5.56x45 - in the case of my calcs, 2.500 is adequate.

If you are stuck with the M16 OAL, necking the 6.8 SPC down to 6mm gives you superior performance to the 5.56x45 at all ranges, and has a flatter trajectory and better long range performance than the parent cartridge. But you are limited to modest weight bullets around 90 gns.

As noted, none of these proposed rounds really offer a huge advantage over the current issue - merely modest improvements. Certainly not enough to justify million of dollars spent.

The advantage these cartridges have over clean-sheet-of-paper designs is that that can be formed fairly easily, and experimented with in commercially available platforms (6x45 cannot be easily adapted to semi-automatic rifles due to COAL, but could be chambered into properly modified bolt guns like the Remington 799 for the purposes of further testing.
 
I think there's something to say for a standard-issue carbine. Something light and small (and reliable) makes the troops like it, which may actually be more important than the caliber in the first place. :D
 
I was gonna start a new thread for this, but it fits fine in here.
I was wondering what you guys thought of a caliber requirement like this:
Cartridge M[X]
Velocity: Must equal or exceed the 7.62x51 NATO M80 (~2750 fps).
Ballistic Coefficient: Must equal or exceed the 7.62x51 NATO M80 (~.425).
Weight: Must not exceed 7.62x39mm M43 (.033 lbs).
Muzzle Energy: Must equal or exceed 1500 ft/lbs.
Pressure: Must not exceed 55,000 PSI

I'd like some comments on this, especially from GunTech, PercyShelley and TonyWilliams.
 
FWIW - I like the 7x46. Load it light for infantry rifles and load it full power for intermediate sniper use and machine guns. From the first page the 7mm Firebrand looks to offer similar performance. More capability than the 6.8SPC and not limited by the AR magazine requirements.

Of course the infantry rifles would be built to handle the full power round, but 3 round burst and full auto modes would be less controllable.

Just my two cents!
 
gun hose = ppsh? I think yes.
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I cannot deny that had a lot to do with the concept.
But it's not exactly a submachine gun. Nor is it exactly a PDW.
Anyway, it doesn't have much worth outside of fiction.
FWIW - I like the 7x46. Load it light for infantry rifles and load it full power for intermediate sniper use and machine guns. From the first page the 7mm Firebrand looks to offer similar performance. More capability than the 6.8SPC and not limited by the AR magazine requirements.
Of course the infantry rifles would be built to handle the full power round, but 3 round burst and full auto modes would be less controllable.
Just my two cents!
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7mm Firebrand would have to use slow powders to keep pressure down. And, thus, it has been thrown in the "scraps" bin on my computer. I've learned a lot since the first post. :D
But you're basically telling me you like .280 British?
 
I thought of something a few days ago:
What if you specced your cartridge and chamber dimensions to look like this:
65DiamondbackHigh-VelocityChamber.jpg
Basically, the shoulder of the round would be as normal, but the chamber's shoulder would be moved forward a little. I think that'd pretty much give you case capacity as if you had the shoulder further along, but allow your cartridge's OAL to be shorter and to use less brass. Note that the advancement of the shoulder is exaggerated in the picture.
 
well, what exactly are you trying to achieve by making the shoulder shorter than the chamber?


by the way, what I see happening is the firing pin hitting the standard hard military primer, dislodging the round, and sending it into the chamber farther and not going off, you'll have play in the brass and that would make you have to seal the camber with the bolt, headspace issues as you know. you have all this weird expansion, which may not be uniform and well... it's weird, but you just need to explain the purpose some more.
 
Another thought, metallurgy is coming along, and other parts of a rifle can be reduced in weight (look at the Carbon-15 rifles-4 pounds!), so I was wondering if one could just go with a higher pressure round?
What about 60K PSI? 70K PSI? What do you think? Use a built-up receiver in your weapon to handle the high pressure and lighten up the rest of the gun, same weight, more capability, lower ammo weight for performance.
 
well, what exactly are you trying to achieve by making the shoulder shorter than the chamber?


by the way, what I see happening is the firing pin hitting the standard hard military primer, dislodging the round, and sending it into the chamber farther and not going off, you'll have play in the brass and that would make you have to seal the camber with the bolt, headspace issues as you know. you have all this weird expansion, which may not be uniform and well... it's weird, but you just need to explain the purpose some more.
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I'm trying to reduce cartridge weight while retaining performance.
Yeah, you'd have issues to work out, and no one said the concept would even work, but I think it's worth looking at, if only to make a boltgun like this and test it out.
Behind armored glass, of course.
The round would still headspace, just on the neck, not the shoulder. Like I said, the advancement of the neck is a bit exaggerated for clarity.
 
Well, when you do try it, my vote is for possible shredded brass on the neck and shoulder. I don't know if you will get that much more performance out of the round. You may be better off with a rimmed cartridge too, brass may be too soft to be able to headspace on the neck.
You'll end up with weird play. I'm not being overly critical, and I think it's cool! I'm very pro-experiment. I wish I had the funds to do so. I just end up drawing a lot of schematics and using them as floor decoration.
 
Looking at the velocity, pressure, etc it seems that you are still captivated by full power ammunition - which is probably more than necessary in 95% of situations and of course any rifle is going to be a compromise and playing the probabilities. You just can't have it all.

BTW, BC on M80 ball is 0.393 at over 2546 fps and drops.

In terms of cartridge design, I think I would look more at the rifle/soldier interface. Things like KE are overemphasized. Keep in mind that with ball ammo, most of the energy won't be transferred to the target. While the 7.62x51 has far more energy than the 5.56x45, the latter was 11% more lethal in Vietnam - primarily because most combat took place within the fragmentation range of the M193 round. By contrast, the 7.62x51 tended to go right through the target. The Soviet 7.62x39 had an ever poorer performance, also tending to shoot through, but dumping even less energy on target.

The biggest problem with SCHV projectiles is that they obtain almost all of their lethality from velocity, and one velocity drops below a certain range, they rapidly lose that lethality. Larger projectiles rely more on mechanical destruction than rapid energy transfer, and past a certain point, pass the SCHV projectiles in lethality.

And just to put this in context, we are speaking only of non-expanding FMJ ammunition. One expanding ammo comes into play, total KE becomes as dominant factor and bigger will win.

To begin with, we have to make a number of assumptions so that we are all working from the same sheet of music. I am going to assume, under the mantle of 'Ultimate Combat Round' that the cartridge in question needs to fill the role of rifle, carbine, DMR, LMG and GPMG. This puts a set of requirements on the cartridge that aren't present when we are dealing with just a rifle.

Let's look at our requirements:

We know from the Hitchman and others that 90% of all infantry rifle occurs at 300 yards or less. However, our requirement for MGs and DMR expands that range, so our range envelope is now probably between 600-800 meters. With high BC bullets, that's actually not difficult even with relatively modest velocites.

With certain calibers, BC in the 500 or 600s are not hard, but these bullets are typically not Hague compatible. We know that 6mm, 6.5mm and 7mm bullets tend to have better BCs than 30 caliber, 270, etc. Without designing a bullet from scratch, let's look at available hague compliant bullets in these calibers.

A quick perusal of components shows only a few suitable bullets. In this case, and to narrow it down, we are going to try and keep bullet weights modest to keep recoil manageable. For this exercise, I am going to the maximum acceptable recoil at a modest 10J, slightly more than an AK firing M43 service ammo. This will keep things manageable under full auto. Given that the M16 is about 5 J and the M15 about 18, this keeps things reasonable, even without resorting to a muzzle brake. The less the better, of course, but we are dealing with a compromise weapon.

I am going to arbitrarily set an upper limit on bullet weight of 147gn, the same weight as M80 ball, since that makes it easy to estimate velocity to give the acceptable recoil numbers.

Doing a quick check, the number of high BC FMJ is very small, but one bullet stood out: The 6.5 Lapua 144gn FMJBT with a BC of 0.636

If we assume a rifle weighing about 9 pounds (lighter than than AK, but fairly representative of a 'fully loaded' M16, we can get an idea of how fast we can launch out 144 gn bullet and still keep recoil in the manageable range. With a 9 pound rifle, 2500 fps puts us at the max recoil and 2700J of KE. That's well over the 1600J or so for the 5.56x45 M855.

This is going to be a long bullet, so sectional density and hence penetration against barriers should be excellent. It's starting slower than 5.56 and 7.62. Bullet construction will determine if there will be fragmentation, but a lightly constructed bullet won't be a good penetrator, so it's a trade off.

Comparison with M80 ball should prove interesting, and determine how our round performs in the DMR and MG role.

M80 ball is 147 gns at 2750 fps and with a nominal BC of 0.393
Our round is 144gn at 2500 and a BC of 0.636.

The M80 wins the KE contest at the muzzle, but by 300 yards, the 6.5 is just about to pass the M80 (M80, 1439 ft-lbs vs 6.5, 1422 ft-lbs). Drop is very close, and by 800 yards the 6.5 has dropped 23 MOA vs 25 MOA for the M80 ball. The 6.5 has more energy at 800 yards too: 765 ft-lbs vs. 526 for the M80.

What about he cartridge itself? Since I am lazy, I put the 144 Lapua FMJBT into the already described 6.5 x 45 case (based on a necked down 7.62x45 Czech). It's the same case head as the 7.62x39 M43 - 0.445. I made the overall length of the cartridge 2.500 inches - between 5.56x45 and 7.62x51.

Running the number through quickload, the case will get the job done. 34gn of W760 will make a bit under 2500 from at 20 inch barrel at 52,000 psi.

This is merely one possibility, but constructed with an existing Hague compliant FMJ bullet and a modified existing case. This round delivers more energy than the M80 ball at all ranges past 300 yards, and has almost half the recoil and 2/3 weight (est) per cartridge.
 
Looking at the velocity, pressure, etc it seems that you are still captivated by full power ammunition - which is probably more than necessary in 95% of situations and of course any rifle is going to be a compromise and playing the probabilities. You just can't have it all.
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Oh, contrare.
I just think that the 7.62 NATO got something right: proportions. I think the 7.62 NATO's ballistics are optimal or near-optimal for a combat rifle.
I just don't know about trying to stretch a caliber across all four platforms.
Hmmm...
There are many propositions, but I'm mostly concerned about two things:
Weight reduction and performance retention (from 7.62 NATO ballistics, improving is good too).
The flatter trajectory, the better, but the more grains you put into your bullet, making the BC better, the heavier your cartridges will be.
You argue that no caliber will be adopted without solving the two caliber system.
But there is another way: make the ammo really fricken' light! If your ammo is so above and beyond M855 and M80 in regards to weight... well now that's something to look at, now isn't it?

What do you think about the other two suggestions, GunTech? High pressure and advanced shoulder?
 
Nolo,

I like the 7.62x45 Czech round necked down to 7mm, I've seen this designated as the 7x46 other places. As GunTech noted, a similar round using a 6.5mm bullet would also be an excellent choice.

I wouldn't go any smaller than 6.5mm and I wouldn't go any larger than 7mm, both would be very, very good. GunTech has a good handle on the situation IMO. Bullets in these calibers would allow a lot of flexibility, and as I stated before you could use a general purpose round, a machine gun round, and of course a specially loaded sniper round (no big deal since they are all specially loaded anyway).

Given the constrains of the AR platform I really do like the 6.8 SPC, but starting from scratch it's hard not to end up at the same place as Guntech and myself.
 
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