The Ultimate Combat Round

[GunTech]

Just as an aside, I ran the number for the 144gn 6.5mm FMJBT against the M118LR special ball (the current sniper ammo) to see how it comapred.

The 6.5mm has similar energy past 500 yards, and passed the M118LR at 800 yards. The 6.5 has less drop at all ranges, still with about half the recoil.

SDDL-UP, I expected the 7mm to do slightly better, but I could not find any FMJ bullet in 7mm or in 6mm that came anywhere close to the Lapua. And of course sectional density is a big factor in barrier penetration, which is a major issue in MOUNT. I'd be interested in some real number comparisons with the 7mm version, but I doubt it could come close - due to a lack of suitable FMJ bullet.

The only point where the 7.62x51 beats the 6.5x45 Czech with the 144gn FMJBT is in energy at short range. In every other measure, the M80 ball is equal or inferior: Drop, sectional density and retained energy at range, recoil, weight of cartridge, chamber pressure.

The 6.5 at 52,000 psi beats the 7.62 at 60,000 psi, meaning you could build a lighter weapon with better performance. I was actually rather surprised, as i have always favored lighter bullets in the 110-125gn range driven at higher velocity.

finally, it should be noted that the 6.5 x 45 described to be easily built with existing components, requiring now new case or exotic bullet. The only downside for development is that the 2.500 COAL means it won't fit into any 5.56 platform. You'd have to use something designed for 7.62x51, so the weapon size and weight would be exaggerated.

 

[GunTech]

SDDL-UP

You don't even need a special sniper round. I was amazed to see that the 144gn 6.5 beats the M118LR ballisitically. The question would be bullet quality and accuracy. Should the 144gn FMJBT be found wanting, you could simply drop down to the 139gn Lapua Scenar, or 142gn Sierra Match king, both well known accuracy. The SMK has a BC of 0.595, and the LS is 0.615. Both are inferior to the 144gn FMJBT in terms of BC.

One round to rule them all.

In order for the 7mm to beat the 6.5 in BC, you have to go to 168gn or 180gn VLDs like the Berger. These heavier bullets mean your recoil goes up unless you drop velocity down into the 2300 fps range. They are also open tip match bullets, not FMJ.

 

[buzz_knox]

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.

The 7.62 was optimal for being relatively easy to adapt American machinery and weapons to after World War II. Given that the US was the key player in both NATO and the world economy, everyone bought into it (often modifying the round to make it more controllable). The round is great, but it was politics and necessity that made it world standard, not just its ballistics.

The reality is that the optimum rifle caliber was, at the time, considered to be something less poweful and more controllable than a full battle rifle cartridge, such as the 7.62. That was the conclusion reached after WWII, and which just happened to be the same conclusion reached before and after WWI.

 

[GunTech]

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

A moving shoulder is a disaster waiting to happen. Anyone who forms cases knows that you lose a few when fireforming, particularly at high pressure. In a semi auto, it's a recipe for case head separation or worse. You are basically creating an intentional excessive head space situation.

High pressure is also a problem. The SPIW operated at 70,000psi, and mechanical fatigue became a real problem. Even with modern metallurgy, you get into the law of diminishing returns. Consider the fact that there is no rifle in that pressure range. Even something like a high velocity 105mm tank gun still operates at 30-06 pressures.

It could be done, but is it cost effective?

The trend for longevity and reliability is t go the opposite direction. The AK with M43 operated at a very modest pressure around 45,000 psi, the 5.56 around 55,000. The 7.62x51 is actually the highest for operational service rounds at 62,000 psi (commercial - military loads are actually less). The most common high pressure round is the 22-250 at 65,000 psi, and several of the hot magnums also operate at this pressure.

In general, it's a good idea to design military weapons to operate in the middle range - rather than the extreme. You want a system that is highly fault tolerant, and running things at the outer edge is generally not a good way to achieve that.

You could achieve high velocities at modest pressures using exotic technology like elctrothermal, but you'd still have recoil issues. Drive a moderate bullet at really high velocities and you get more recoil Make the weapon lighter and you get more yet.

Keep the recoil at under 10J and you have a very manageable weapon. The lighter the weapon, the lighter and/or slower the bullet will have to be to maintain that 10J.

YMMV

 

[PercyShelley]

It's a pity that all the metals denser than lead are so expensive, otherwise that would be a quick BC fix.

But since it's been pretty well hashed out that if you allow the luxury of a new magazine length, but otherwise want to keep with existing components, something that looks suspiciously like a grendel magnum comes out. IMO, the .276 Pederson had a lower than desirable bullet BC, and probably a little more body taper than is ideal, a concession to the toggle blowback design of the original Pederson rifle. IIRC the .280 British used the .308 bolt face, so I suppose it was a sort of 7mm/08 kurz, and I don't think that too much has been done with the 8x57 case since the 7.92x33.

So, what if we take Nolo's idea about modifying pressures and case/chamber arrangements to their logical extrema and posit a completely new design, seeing as existing design paradigms are pretty well played out?

This article has a pretty eye-opening idea; blowback assault rifles.

The idea is pretty straightforward: if the case has a rebated or, as in the above, inverted rim, the bolt can follow it some distance into the barrel. In a blowback gun this means that the case must back out a much greater distance before it becomes unsupported, which means that it can survive a greater rearward bolt speed for a given pressure curve without danger of case head separation. This means that the bolt can be much lighter.

Combine this with the Advanced Primer Ignition trick; fire the primer just before the bolt gets into battery, and the bolt's forward momentum will have to be overcome before it will start moving rearwards. This can cut bolt wight down by 50%.

Not only is it possible to make a blowback assault rifle using these techniques, the entire family of Oerlikon 20mm autocannons worked along these lines (and also the fearsome German Mk 108 30mm), scaling it down requires only a new cartridge with the rebated or inverted rim.

The author of the above article also proposes bullets that are spun inside sabots and lack any contact with the rifling. They may thus be aerodynamically more optimized.

I am satisfied that this approach will work, however, current experience with sabots and open-bolt operation suggests that taking the sten-like assault rifle the article's author suggest verbatim will result in unacceptably low accuracy. Let us specify, therefore, that the bolt retreat at a safe rate when the weapon is fired from a closed bolt (e.g. by one of the many tried and true blowback retardation mechanisms), and that the API mechanism only be used in full auto as a way of smoothing out the ride. Open/closed bolt operation increases the coolness of any rifle by at least 200%, as proven by the FG-42.

Let us also mandate that some alternate means of spinning the bullet must be found. Using conventional rifling has proven sub-optimal with sabots, and conventional rifling greatly increases the mechanical wear on a rifle's barrel. Advances in lightweight barrel materials are coming along slowly, and no doubt the whole thing would be a lot easier if we weren't swaging down a piece of copper-jacketed lead at 50,000 PSI every time the trigger is pulled.

And while we're at it, let's change the priming system too. No point treading in the shallow waters of the kiddie pool of mechanical conservatism!

Since the first thing to go when a case is loaded at too high a pressure is often the primer, we should do away with the silly, unreliable percussion primers that require a thin piece of metal to be deformed and then survive high temperatures and pressures. Instead, let's use electrical ignition, as in aircraft cannons, and reinforce the primer until we can run the case pressures up to 100,000 PSI. Why? Because we're Americans and we hate bullpups, that's why!

By using the saboted bullets the diameter of the barrel can be increased, which is a much more efficient means of gaining barrel volume than increasing its length. By jacking the pressures up to some unholy level, we can increase the work done on the projectile to the point where we can countenance the loss of barrel length.

The use of electrical ignition also dovetails nicely into the alternate means of imparting spin upon the bullet. By arranging permanent magnets and wires on the bolt and receiver, or perhaps by putting piezoelectric lining in the blowback retardation scheme, a constant supply of electricity can be supplied to the weapon's onboard capacitors shot after shot, greatly reducing the likelyhood of running out of power for the electrical ignition.

A similar arrangement of permanent magnets or other inductiony things shall be used to impart spin upon the bullet. This will likely mandate a ferrous projectile, but this is no great matter as it will greatly improve through-barrier performance, and the reduced density will mean a larger wound channel for a given bullet weight, in much the same spirit as the experimental aluminum CETME bullets.

Finally, the gun should use a sheet metal exterior with wooden furniture to confuse and mislead our enemies into thinking that it is an old-fashioned weapon. Since all the technologies described were essentially mature by the 1930's, perhaps it is. A steampunk aesthetic may therefore be adopted at the discretion of the ordinance board.

BWUHAHAHAHA!

 

[Funderb]

the blowback mechanism is the ppsh41 applied to larger rounds.

oh, and rail guns use an aluminum projectile, not ferrous.

but it sounds cool!

A moving shoulder is a disaster waiting to happen. Anyone who forms cases knows that you lose a few when fireforming, particularly at high pressure. In a semi auto, it's a recipe for case head separation or worse. You are basically creating an intentional excessive head space situation.

That's what I said!

 

[Nolo]

I still can't help but think an advanced shoulder would work, especially in a rimmed cartridge (where it'd headspace on the rim instead of the shoulder, and yes, I know rimmed cartridges suck for semiautos), this is, of course, assuming you did it right. I can also see a billion ways you can do it wrong.

The reality is that the optimum rifle caliber was, at the time, considered to be something less poweful and more controllable than a full battle rifle cartridge, such as the 7.62. That was the conclusion reached after WWII, and which just happened to be the same conclusion reached before and after WWI.

I did not say the energy and recoil of the 7.62 NATO, just the ballistics. The same speed and BC, that's all. Not bullet weight, not muzzle energy, just ballistics. The 6.8 SPC mirrors .280 British ballistically, but their recoil and muzzle energy are very different.
I really think the 144-grainer is too heavy for an infantryman. I think, if you shaped the projectile more aerodynamically (it sure looks to me like it ain't shaped that way), you'd get the same BC for a better weight, maybe in the 130-grain range?

 

[Nolo]

The idea is pretty straightforward: if the case has a rebated or, as in the above, inverted rim, the bolt can follow it some distance into the barrel. In a blowback gun this means that the case must back out a much greater distance before it becomes unsupported, which means that it can survive a greater rearward bolt speed for a given pressure curve without danger of case head separation. This means that the bolt can be much lighter.

My God, PercyShelley!
Wow...
Back before I had the Internet, I was maybe 13 at the time, I designed a system an awful lot like that. This was before I knew much of anything about semiautomatics (for instance, I had no idea how gas-operation worked), but blowback is a fairly simple system to come up with on your own. Anyway, my design had it that the bold head would be sleeved by the barrel/chamber, like the above system. But it was not so that it could use an accelerator projectile or reduce bolt weight. It was so that it could use cartridges with different case lengths. If one used cases with no case taper and the same shoulder, one could fire cartridges in the realm of 7.62x25mm 7.62x39mm, 7.62x51mm and .300 Magnum [they were all .308 caliber] all in the same weapon (with different magazines, which resembled SKS detachable magazines, to compensate for shorter OAL of the some of the smaller cartridges, also so that one could identify what load one was shooting in his/her weapon), merely by having the bolt move into the chamber sleeve varying distances.
Now, I'm not proposing this system as a solution (it is a logistical nightmare, but a Spec Ops wet dream), I'm just amazed that I came up with a system so interesting when I was so young. I'll have to come up with the specs again, just for posterity.
I later tried to design the system for gas-operation, then abandoned it for conventional operation.

 

[Nolo]

Yeah, like I said before, I've learned so much from this thread. Probably more than all four years of high school.

 

[SDDL-UP]

GunTech,

I agree you don't "need" a special sniper round, but they would make them up special anyway - it would be cost prohibitive to load everything to match grade I think.

My thinking is that you could load a lighter bullet with lower BC for "general use" (normal combat ranges, controllability in three round burst, etc.) and then the heavier bullet for machine guns and the heavier match bullet in sniper rounds.

 

[GunTech]

Agreed. I was just working from available components, and the 144 FMJBT was the only high BC Hague compliant bullet I could find. I was shocked to see how well it theoretically performed in almost every role.

I actually have ordered some of the 144s to try in my 6.5 Grendel and 260. The grendel won't be able to push them at 2500 fps, but i should be able to report on accuracy

http://www.midwayusa.com/eproductpage.exe/showproduct?saleitemid=326284

Care in loading will undoubtably distinguish between standard issue and match/sniper grade, but it's interesting to consider that the same basic bullet could be for standard issue and sniper rifles. The only difference would probably be selected components (brass and bullets with higher tolerances), and perhaps better and more carefully weighed charges.

As noted, QL gives the best velocity for the 144 in the 6.5 with W760, but ball powders are note always noted for most consistent velocities. A stick power might be preferable in this application.

 

[Nolo]

GunTech, what do you think about that article Percy showed us?

 

[GunTech]

SDDL-UP, Initially, I was very interested in a 6.5 in the 108gn range, or even a 6mm around 100gns, but no suitable Hague compliant bullet could be located.

In particular, a 6mm 100+ bullet would have a very high BC and sectional density, and would still have modest recoil even when driven at M80 ball velocities. The trick is driving it fast enough to have any advantage over the 144gn 6.5. It's fairly difficult to gain much against the 6.5x45, although recoil is reduced.

 

[PercyShelley]

The Oerlikon autocannons were so sucessful that I'm frankly surprised that nobody's tried scaling them down before. It would require a new cartridge rim design, but then so did the first rimless cartridges.

EDIT:

And maybe not even that much. Existing rebated rim cartridges might work too.

 

[Nolo]

GunTech, I've been thinking about this for a while (can you tell? ).
In my observations, you need at least four different types of cartridges in a modern military.
Now, you can use the same caliber to achieve these four things, but you will have four distinct loads.
I'm going to assume we've already settled on a 6.5 (.264") bullet, since that's sure what it looks like.
You need a standard infantry round, what I'll call the MBR (Main Battle Rifle) round.
You need a dedicated sniper/DMR round, made to match tolerances.
You need an MG tracer round.
You need an MG armor-piercing round.
Now, how I see it, this gives us the opportunity to do a bit of specialization, and I'm gonna take it:
MBR Load: 108 grain, thin jacket, deep cannelure @ 2700 f/s
Sniper/DMR Load: 139 grain, thin jacket match @ 2650 f/s
MG Tracer Load: 144 grain tracer @ 2550 f/s
MG AP Load: 144 grain armor-piercing @ 2550 f/s
Now, I have my own pet cartridge, and so do you, but here I'm just showing basic weights, construction and speeds.
The MBR load looks very appropriate at the aforementioned 108 grains, but I honestly think that 2900 f/s is too much. The BC of the Lapua Scenar 108 is .478, which is a lot better than the 147-grain, so shooting that round at 2650-2750 f/s will give you equal or better range than an M14, which I think we can all agree is plenty for the basic infantryman. I think the SAW would also be able to use this round quite effectively.
The sniper already needs his weapon to fire match-grade ammunition, so why restrict him to the same load as the infantryman? Thus, the sniper/DMR load I found to be appropriate is a 139-grain Lapua Scenar match at 2650 f/s. I'll address pressure issues later. This load gives the DMR range that he hasn't been able to fully enjoy before, which I think is a great benefit in the major range extremes we face today (either CQC or open, flat desert).
For the machine gun, I think the bigger the BC, the better. So I went with the effective maximum weight for the 6.5, the 144 grain. The MG could very well get away with this at 2500-2600 f/s, so I just went with the median of that. It parallels your 144 load very closely, GunTech. This would come in both Tracer and AP versions, that would be matched together with both BC and speed as closely as possible, so that the gunners know where their non-tracer rounds are actually going.
Now, as to the question of pressure. The DMR load exceeds the pressure limit for the 6.5x45mm GT by a decent margin. I have come up with a few plans for dealing with that.
The first is to merely reduce the velocity of the 6.5 DMR load. That would work, but you'd sacrifice some of that beloved range that you may need.
The second is to keep all the loads the same and increase the case head of the 6.5x45mm GT by about 1mm for the sniper load only. That way, you can make your sniper/DMR rifles built heavier to withstand the greater pressure (which should still not exceed 65KPSI), and then they can use standard infantry rounds but the infantry rifles are in no danger of accidentally loading the high-pressure DMR loads.
The third is to just increase the size of the case to deal with the sniper load, and thus have lower pressure MBR and MG rounds, which increases weapon service life at the expense of added weight.
In addition, you could adjust slightly both your case and the velocity of all the rounds except the DMR load to try and get as small a case head as possible (I'm shooting for a 10.7mm case head, like 6.5 SPC), and then extend the neck, etc. for the sniper/DMR load like you would with the second option. This would have th advantage of reducing weight (as most of the weight outside the bullet is contained in the case head) in addition to increasing magazine capacity and/or decreasing magazine size.
I also think that the OAL for all of these cartridges should be 65mm, to compensate for the longer bullet and case of the DMR and MG rounds.

What do you think?

 

[JShirley]

Your requirements are off. For the assault rifle, you say
"The round must have recoil no greater than 2 times that of the 5.56x45 NATO."

That is a bogus requirement. It should be -The round must have recoil no greater than 1.3 times that of the 5.56x45 NATO. If it has double the recoil, you are no longer in assault rifle territory.

John

 

[GunTech]

The 6.5x45 is theory no more. Shown below is the 6.5x45 with a several other rounds for comparison. We've already seen the numbers, but I hope to be able to check performance in RL soon.

Left to right:

7.62x51, 6.5x45, 6.5 Grendel, 6.8 SPC and 5.56x45

Here's 6.5x45 compared to the Nato standards (7.62x51 and 5.56x45). You can see it's literally right between the two, but actually outperforms the 7.62x51 NATO at longer range.

 

[GunTech]

JShirley,

The number I selected for target recoil was 10J. The M16 is approximately 5J, the AK 7.72 is approximately 9J and the M14 is 18. Please note this is the maximum recoil and represents the 144gn projectile at 2500 fps.

The original concept was for a 108gn projectile, but there is no Hague compliant bullet at that weight range. As noted, the Lapua Scenar at this weight has a BC of 0.478, so it's not probably not unrealistic to posit a FMKBT with a BC around 0.400 - still far better than the 5.56x45 M855 and M193 and about equal to M80 ball.

This loading would be almost ballistically identical to the M80 ball in terms of trajectory and have 73% of the energy of the M80 at all ranges, while the 5.56x45 quickly loses energy.

In this loading, recoil would be less than the AK at only only 8J, still higher than the 5 of the M16, but few would call the recoil of the AK unreasonable or unmanagable. Basically, the AK has double the recoil energy of the M16, so that doesn't seem like an unreasonable target given what you gain in performance.

 

[BrianB]

I'm not 100% sure, but I think some prop guys in Hollywood have achieved performance even beyond your specs!

 

[Nolo]

Neat stuff, GunTech. I'm happy to see you're taking this somewhere.
Kinda feels like I actually did something...
But, anyway, what do you think of my load plan?

 

[Kosh75287]

I haven't had time to read all 20 pages on this topic. I fully expect that I'll touch on some things already covered, but here goes....

My first thought was "what would be wrong with the 6mm PPC?" but it appears that performance specs require 70,000+ psi chamber pressures. THEN, I wondered about knecking down the Czech 7.62x45 round which, also, was discussed. The only thing I haven't found a reference for is a weird piece of priming technology that Elmer Keith played with during WW II. It consisted of a short tube, extending from where the primer pocket meets the interior of the case to some point very close to where the cartridfe's shoulder and neck meet. The idea was to direct the initiating energy of the primer to the TOP of the powder charge, engendering said charge to burn from the front to the back. It's been a while since I've read the book, but it seems that the advantages were higher velocities, lower chamber pressures, and cooler operating temperatures. I don't know what quashed the idea 60+ years ago, but I think it involved the expense and engineering difficulties associated with getting the tube properly placed within the cartridge on a mass production scale.

Any thoughts on if this might help?

 

[Nolo]

I think it'd be a quite viable answer if one used polymer cases, as those are extremely easy to make in whatever shape one desires.
It's a similar proposition to the "Shuttle SRB" concept which is described on page 9 of this thread, however that concept merely describes solid powder with a channel in it to control the pressure curve. It's a similar concept, but differently executed.

 

[GunTech]

Priming tubes don't buy you much in cases with small volumed. The complexity of manufacture isn't worth it.

Nolo, I'm inclined to a lighter bullets as well, but there isn't a Hague compliant one in the weight range. Besides, having multiple loads defeats the point of a single universal cartridge. If you are going to have multiple loading, you might as well have multiple cartrdges

Ideally, you'd just have a FMJBT with a penetrator insert - something like the Barnes MRX. That might even up the BC if you use something like a tungsten alloy.

You'd only need a couple of loadings: a ball with penetrator and and a tracer - plus match grade ammo for snipers and DMRs, but any ammo could be used in a pinch by any user. Imagine only having one round for everything except heavy MGs. Logistics becomes simplified.

In an ideal world, a ball round in the 120-125 gn range would probably cover all bases nicely, if you can keep the BC up. That gives you your combination of flatter trajectory, reasonable recoil and penetration.

In the mean time, I am going to order a barrel and pursue the 6.5x45 with 144 FMJBT since that's what I have to work with, but I'll look at other bullets too and report back.

I have spec'd out final dies and will have them and a chamber reamer soon. I'm going to mod a Remington 799 for the cartridge as a test bed.

More to follow.

BTW - I am also going to investigate a 6mm and 5.7 version of the same cartridge. The latter should fit in an AR mag and give superior velocity compared to the 223. Might make a dandy little varmint round.

 

[Dr. Tad Hussein Winslow]

Tangential question: Is the Russkie 5.45x39mm "poison pill" bullet with the hollow nose which makes destabilized more quickly and violently, a "Hague-compliant" bullet? If so, that could open up some possibilities of making a lighter bullet with better BC in 6.5 or other sizes as may be needed. It could make a 115 grain 6.5 bullet have the BC of a 139-144 grainer, AND go faster, AND destabilize and tumble/fragment more quickly. But does it successfully make and end-run around Hague? I'm not sure where to go with that from here, not being an expert - I'm just trying to spur and channel to greater heights the genius that is Nolo. (and Guntech, and Percy.....)

 

[Tony Williams]

I've just spent some time talking to people on the AAI stand at Eurosatory. They are very interested in developing an intermediate calibre version of the LSAT LMG (c. 6.5mm), and a carbine using the same ammo. If they can meet requirements, the caseless version will prevail (the ammo is much more compact as well as lighter), but if that meets insuperable problems then the plastic-cased telescoped version will be used (that's already been through a successful testing regime). Earliest in-service date is 2014.

Naturally I gave them every encouragement to go for a 6.5mm which could replace the 5.56mm and 7.62mm. Watch this space....