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Sturgeon

The 6.8 SPC Haters Club

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I just did a study for a small saboted rifle cartridge. I reason that cup-type sabots can be made reasonable small, light, and accurate for a weapon of this type (SLAP rounds are in service, after all), and that the gains for micro-caliber rounds would be enormous. Not only would swept volume be drastically improved (reducing the necessary length of barrel, as well as shortening the cartridge substantially), but larger-caliber rifling could be made tighter, accommodating generously long projectiles with high ballistic coefficients. An .18" caliber projectile could be made of about 35 grains weight that had about the same BC as the projectile for the 5.45x39, with leadless construction. To fire this projectile from a .28" cal sabot to a velocity comparable to the 5.45x39 would require a case only 26mm long, and a cartridge of almost identical OAL to the 5.7x28, if the .30 Carbine's case base is used.

If the carbine-as-submachine-gun-PDW theory is to be implemented, this configuration or something similar would seem to be a low-ambition solution to the problem of designing effective micro-caliber rounds. I don't expect you could quite squeeze the accuracy from it that you're getting with rounds like M855A1 or Mk. 318, but it would be broadly comparable to M855 and 7N6. For 200/400m combat, that makes lots of sense.

Oh, and because I am fucking tired of having to explain this all the time to people (not anyone on this site), I've decided just now to create a shorthand for weapon range requirements. It's two numbers separated by a slash. Both are ranges. The first is the primary design criterion range. So a round designed for 200m shooting has a whole gaggle of requirements (lethality, penetration, etc) that it must meet, but it may also be designed around certain requirements for more distant ranges, for example retaining penetration with potentially lethal energy at 400m, as well. So I'm shorthanding this to "200/400m." One might have a sniper rifle cartridge designed primarily for 800m, but still designed with longer ranges in mind, so perhaps that would be an 800/1200m round. I'd say 9mm is a 25/150m round, or 5.7x28 is a 100/200m round, just to give you an idea. 5.56 is a 300m/600m round, one could say. You can get weirder with it, too. For example, I'd characterize the 6.5 Grendel with 123gr bullet loading as a 800/200m round. It has too steep a trajectory to really match 5.56 in the intermediate ranges, but retains velocity well enough to actually be really impressive at the longer distances in terms of energy retained and trajectory.

It's not some hard-and-fast metric, it's just a shorthand, mind you.

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I continue my experimentation with the 6.8 SPC case. It's pretty obvious that shortening the case and necking it down is the way to go to improve performance. 6.8 SPC proponents primarily tout its higher energy levels than M855, and downplay its obviously higher weight. So, can we reduce weight without obviously ruining the 6.8's energy characteristics? Yep (6.8 left, modified version on right):

8Y3sQdX.pngfQUWKA3.png
 

After 60 meters, the modified round has 90% or more of the retained energy of 6.8 SPC. Between 430 and 810 meters, it has more energy. It has 130 meters more supersonic range, and shoots as flat at 530 meters as 6.8 SPC does at 480 meters.

But most importantly, versus the 6.8's 17.6 grams per shot, it weighs only 14.7 grams per shot. That's 23% heavier than M855, versus 47% heavier for the 6.8 SPC. The cherry on top is that the .25 cal version was designed to use low-density (i.e., lead-free or low-lead) bullets from the outset, something that can substantially reduce the performance of the 6.8 SPC. 

This cartridge is very similar to Dedicated Technology's .25 DTI, being just a .25 caliber 6.8 SPC variant. The point of all this, besides me just fooling around, isn't to propose a new military round, or to talk about how the 6.8 garbage is for hunting hogs, or any of that, it's just to make it acutely obvious why the 6.8 SPC is a mediocre design. It's not bad, but it's not very good for what it's intended to do, either. It's just... OK. Its singular virtue is that it's basically the only moderate-energy AR round that anyone actually makes rifles for (the .300 Blackout doesn't count). Which is all some people need, but that still begs the question of what Chris Murray was thinking when he designed it as a DMR round... Though I reckon we have a pretty good idea of that, too.

 

I got performance figures by plugging in the relevant data to Powley, and then doing the same for 6.8 SPC. I used the same nominal velocity for the Hornady 110gr OTM that I used in the post linked in the OP of this thread, which was 5.9% higher than the calculated Powley value. I adjusted the Powley value for the .25 cal variant by the same amount to get the working velocity.

Anyway, there you go. The 6.8 SPC is fat and bloated for what it does. A much better round was possible, but too bad, I guess.

 

r5aTqhz.png

 

There it is. By the way, the bullet is an overgrown, modified version of the .224" 68gr Sierra from the 1950s, with a secant ogive and gentler boattail. As with a lot of this stuff, it's hard to get accurate figures without empirical data, but I think a safe bet for i7 FF for this bullet is somewhat less than 0.95. I went with 0.93, which I got from some back-of-the-envelope math. Regardless, the round has enough room for a reasonably slick bullet with a decent form factor.

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My hiatus from Tony's forum may or may not be unending.

I've just... Reached the end of what I can do there.

The only good decision Sisyphus could make was to simply stop pushing the fucking rock. 

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The thing is, i dont even have a problem with it in therms of hunting. If some people think they need it (price is questionable for its performance), they can go on. Its just an individual choice that doesnt matters for anyone else.

But... for me everything ends when people promote, heavy, slow, TOTALLY shitty ogive, poor aerodynamic, brass bottleneck cartridges for the "new military replacement". Thats literally my worst nemesis topic in existence.

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The ability to effect a large quantity of fires is important to demoralize and disperse an enemy (or so I believe).

 

I do find it puzzling that the recommended response to insurgents firing mortars at our troops it to get a more powerful rifle, particularly as airburst munitions are being developed.

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