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Just Dicking About

I spent a few hours this morning playing around with my collection of gun software. I’ve got pretty much all of it: exterior ballistics, interior ballistics, trajectory calculators, cartridge design programs, you name it. It’s fun stuff to play with, and I’ve learned a lot from it.

Every once in a while I approach the old Poodle Shooter problem, and attempt to find a solution. The what? The who? I could write pages and pages, but to try and be succinct as possible, I’ll say that, in the post WWII period, a gunmaker named Roy Weatherby pushed the idea that really high velocity equated to tremendous killing power. He was both right and wrong. Another gunmaker named Eugene Stoner took that idea, and sold the US Military on the idea of a small caliber high velocity rifle for our combat troops. Thus was born the M16 rifle, which fires the 5.56 NATO cartridge. I won’t repeat the story that you probably already know, but for whatever other great benefits that weapon has, it lacks close range stopping power and long range killing power. 

If you don’t know that difference between stopping power and killing power, go read that post I did on the .600 Nitro Express pistol. Or look up “the caliber of any self defense pistol starts with at least a 4”. That’s stopping power. Delivering such a tremendous kinetic impact that it overcomes the target’s internal hydraulic flexibility and causes massive damage to the nervous system, physical support structure, and (as a secondary issue) massive damage to the major organs. In other words, it pretty much blows the target to shreds, at least internally. Bang. Drop. Dead. Generally you use the biggest bullets you can find to do this. Killing power is the ability of a cartridge to propel a bullet to and through it’s living target at some designated distance so that it leaves a hole at least half an inch across, with sufficient bullet mass and proper bullet and rifle design such that the bullet penetrates in a nearly straight line. Put a hole like that through any creature, and eventually it will die. Be a good hunter and make that hole in the vitals (heart, lung, upper spinal column) and “eventually” becomes “pretty quickly”. Controlled expansion bullets give you the best killing power. Too bad that such bullets are outlawed by the Geneva Conventions/Hague Accords.

Back to shooting poodles. I am not going to have the 6.8SPC vs 6.5 Grendel debate again. Both are great improvements over the 5.56 NATO at any range, and neither one is ever going to be adopted by the military. No, their decision is to go with that futuristic space gun micro-cannon thing that got mentioned here the other day, where computers and lasers in the weapon lob a small bomb out to the target and then set it off. It will cost billions, with the dual advantage that troops won’t need to learn to shoot all that well, and the military will never ever have to admit to the 47 year old mistake that is in our soldier’s hands right now.

Several tons of research by better folks than I have concluded that a mid-range assault rifle needs to accurately shoot a bullet with a sectional density of more than 0.2 and a ballistic coefficient greater than 0.35 at a velocity of 2500-2750fps. 1200 lb/ft of bullet energy at 300 yards/300 meters would be real nice. And to minimize shooter fatigue (ie recoil) this needs to be done with a caliber smaller than 30. But it also needs to be done with a caliber greater than .24.

I’ve designed a whole slew of cartridges on paper that can do the job, but it is just about impossible to do the task with the length and size constraints of the M16 rifle. The cartridge chambering area is simply too thin (in the bolt face) and too short (in the bolt length and cartridge well) unless the cartridge’s working pressure is cranked right up to the danger line. And really high pressures wear guns out really fast.

Hold a AA battery in your hand so that it lays across your fingers. The length of that battery is just 1/4” shorter than the 5.56 cartridge. The body of the cartridge is about the same diameter as the negative (-) terminal, that flat shiny disc on the bottom of the battery. See the little nipple on the positive (+) end of the battery? That’s the diameter of the bullet it fires. Pretty damn tiny, ain’t it? The .600 Nitro Express cartridge is about the diameter of a Churchill cigar, 3/4”, and nearly 4” long. [ or you could just click here and look at the picture. L-R: 7.62 NATO, 5.56 NATO, AA battery ]

Anyway, here’s today’s time waster. I call it the 6.5x39_25, since it’s based on the 7.62x39 Russian cartridge, blown out straighter and with a 25 degree shoulder, and necked down to 6.5mm.


image



Taking a good chunk of the body taper out of the 7.62x39 gives you more internal volume. More internal volume means more room to fit more gunpowder. It also means the cartridges will stack straighter in a magazine, which can mean easier feeding. The 25° shoulder angle gives more power room as well, without being “Ackley’d” out to a steep angle that makes for difficult feeding. Think of it as a stretched, softened, and necked up 6mm PPC. Taking the caliber down from euro-30 ( the 7.62x39 and the .303 British both use .311” diameter bullets, whereas the American designed .30-06 and the 7.62x51 NATO both use .308” diameter bullets ) to 6.5mm (.264") allows the same approximate weight bullet (120-125 grains) as the Red’s 7.62x39 to have a much greater sectional density and a much higher ballistic coefficient. And that’s nearly double the bullet weight of the 5.56, which will give you much better short range stopping power.

My calculations show that this cartridge can push a 120 grain FMJ bullet at over 2500fps, which would generate a very minor 8.15 lb/ft, 8.66fps worth of recoil in a light 7lb rifle.


image

The Norma 120gr FMJ has a sectional density of 0.246 and a ballistic coefficient of 0.428. That compares very well with the 7.62 NATO’s 0.226/0.411, the 5.56 SS109’s 0.179/0.397 and the commie 7.62x39’s 0.184/0.275. This means that this is a solid little bullet with great aerodynamic efficiency that should penetrate better than all of those others at similar velocity. It means that this one can be zeroed in at 255 yards and will stay +/- 4.5” out to over 300 yards, where it still carries more than 1000lbs/ft worth of energy. Which means point and shoot out past the point most folks are comfortable using regular sights. The standard 7.62 NATO ("way too much gun!!!") has 1400lb/ft of energy at that range, whereas the 5.56 NATO with it’s “heavy” 63gr SS109 bullet has only 690lb/ft of whack at that distance.  So my design splits the difference right down the middle. Another advantage is that my round will burn up all of it’s powder in a 17” barrel, which makes for less muzzle flash and a quieter bang. It’s very close to being optimized for the really short barrels that the current M4 carbines use. In a full length M16 barrel this one would give you a bit more velocity, a bit flatter trajectory, and a bit more whack (1109lb/ft) at that same 300 yard distance.

Is this the magic cure all? No. It’s a little down in terms of energy at distance compared to the 6.5 Grendel, but it eclipses the 6.8SPC. And it would require opening up the M16’s bolt face diameter by 0.069”, which is doable. Don’t forget that my numbers are all calculated; actually building one and testing it would result in real-world differences (ie lower performance). Too low? Try one and find out.

So I’ve built a solution in search of a problem no one official will admit exists. It would make a very mild 250 yard deer rifle though.

Ok, fun time is over. Let’s go see what Onitwit is up to today. I hear the far-left is PO’d at him now over the tax cut thing. Guy sure knows how to make himself popular, don’t he? LOL



Posted by Drew458    United States   on 12/08/2010 at 01:06 PM   
 
  1. HUH????????????????????????????????? gulp

    Posted by Rich K    United States   12/08/2010  at  04:06 PM  
  2. As I said, I think the British .303 is one of my favorite general purpose firearms.. I don’t know the specs of it, but I prefer it to my Bro in law’s 30 ought 6 for range or close up.

    Bill

    Posted by Doctor DETH    United States   12/08/2010  at  06:38 PM  
  3. Interesting.  Your proposed cartridge is barely a step behind the venerable 6.5x55 Swedish Mauser cartridge. Just for a giggle, check the SD of the 6.5mm 140 gr match slug.  At 2600 fps, it’s trajectory is basically the same as the 7.62 NATO and recoil is modest. Penetration is just plain awesome.

    Posted by Dr. Jeff    United States   12/09/2010  at  01:03 AM  
  4. Exactly my point Doc. Had Stoner made his design just a hair bigger, then moving to any standard cartridge would have been a snap. 2.5” OAL instead of 2.25”

    Yes, I know, he made that AR10 .308 variant. Nice gun, but like Goldilocks and the porridge, it was just a little too big.

    The 6.5 Swede is one of the best military cartridges ever made IMHO. It’s even better when you look at the internal specs - it runs at much lower pressure than modern ammo, which is far kinder to the rifle itself. Less stress and less heat mean longer parts life.

    The 140 and 160 RN 6.5mm bullets will shoot through anything. SD is way over .3, which is incredible. That’s one reason that this little cartridge is still used to hunt moose with.
    Unfortunately, to chamber those slugs you need a really long throat, which lowers the performance of more normal size bullets.

    PS - .303 Brit vs .30-06: performance is virtually identical. .303 has a slightly fatter bullet by .003”, .30-06 usually has more bullet sizes to choose from. Depending on how hot you load them (the SMLE is NOT a magnum strength rifle) velocities overlap 80%. Want to have fun? Dig up the specs on the .30-40 and compare to the .303. The two are so alike in size, shape, specs that it’s scary. The only real improvement that the .30-03 brought was the rimless design, and then the .30-06 with a lighter pointy bullet. Once horses left the battlefield there was no longer a need for the 220RN bullet, but for hunting big critters that’s the kind you want in any of these 3 cartridges.

    PPS - more fun: Read the story of the 6mm Lee Navy, the great-grandfather of assault rifle cartridges. 80 years ahead of it’s time. And here is some more, with pics. Yes, it’s the same Mr. Lee, the “L” in SMLE.

    Posted by Drew458    United States   12/09/2010  at  11:25 AM  
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