Thursday - March 15, 2012

So where were we?

Peiper is taking a couple days off to deal with some issues. He will return when he can.

I’m a bit late posting this morning. I was playing guns with another blogger, designing a couple more mid-power military assault rifle cartridges. It’s easy to do based on .308/.30-06 cases, although scrunching them up to fit into the AR rifle platform really limits performance. It can be done though. One thing that I don’t like is the tendency to cram a “big” bullet deep into the cartridge case to try and make it fit into some arbitrary overall length (OAL), like the 2.25” OAL that the AR rifles are limited to. Here’s an example:





This is the 6.5 Grendel cartridge, an excellent lower-mid-power design that is no longer trademarked as of last month. It is shown with a large bullet seated way into the main body of the case. I don’t like this. In an optimum world, I don’t think any flat based bullet should be seated so that the heel is beyond the base of the neck, or a boat tail base bullet seated deeper than the boat tail being past the end of the neck. This is shown by the purple and aqua lines, respectively. Doing so makes it almost impossible to utilize a full charge of powder without compressing it or smashing some of the grains when you force the bullet in. A full charge - what shooters call 100% load density - not only keeps the bullet from shifting backwards under recoil, it also maximizes potential accuracy. In my opinion, if you’re forced to use such a large bullet to get the BC and SD you want (Ballistic Coefficient [aerodynamic drag] and Sectional Density [a measure of mass to diameter that is usually directly proportional to penetrating ability]) and have to seat it like this, then your cartridge design needs some work. Large for their caliber bullets are of course heavier than normal, and that heaviness gives you a bit more inertia at both very short and very long ranges, which increases killing power. In theory. Personally I think you’d be better off seating them to optimum depth and then working up the powder charge to get the velocity/pressure/powder burnt percentage/muzzle flash you want. Unfortunately that can’t be done when you’re trying to retrofit a bigger cartridge into an existing weapon platform, and that’s what this is all about: finding a more potent round for the AR rifles than the 5.56 NATO poodle shooter. The 6.5 Grendel is a great choice for this, in my opinion better than the 6.8 SPC from Remington. Both cartridges are significantly more potent than the 5.56 NATO cartridge, and both fit into the AR rifle with only a small loss in magazine capacity. Both generate more recoil, but I don’t see that as a real issue. The recoil they generate is quite light, less than half of the 7.62 NATO/M14 cartridge and rifle, but it’s still more than the finger tap level of recoil that the 5.56 makes in an M16/AR15 rifle.

My cartridge design is a bit more potent than either of these. I call it the .270 Savage Short because I based it on the old .300 Savage, which itself was a shortened .30-06 developed a few years after WWI. A .270 is the same caliber as a 6.8mm; both use bullets .277” in diameter. With a light 115 grain bullet my design makes a potent 400 yard military assault weapon with very little recoil, and with a heavier 130 grain bullet it becomes a 600 yard battle rifle with only a bit more recoil. Unfortunately it would need a rebated rim cut to fit the largest possible AR bolt face, though with a cartridge body just a couple hundredths bigger than the Grendel’s (.470” vs .445") magazine capacity will be the same or perhaps 1 or 2 rounds less. And my design produces much more velocity from a short barreled gun too. A calculated 2800fps with a 115gr bullet in a 20” barrel, all powder burnt, with reduced muzzle flash and lowered noise, and only 10lbs of recoil in a 7lb rifle. 2700fps with a 130gr bullet and 12lb of recoil. Certainly manageable.





Armchair cartridge design. It’s a fun way to play and you can talk about your creations for hours. All you need is the software. Actually building the cartridge would cost a few hundred. Add in another thousand for the custom barrel, and 10 or 20 thousand for professional pressure measurement, which you’d need if you wanted to bring the thing to market. So doing on the computer is much cheaper, even if it’s only 95% realistic.

Posted by Drew458     on 03/15/2012 at 10:12 AM   
Filed Under: • Daily Life • Guns and Gun Control • Military •  
• Comments (2) • Trackbacks(0)  Permalink •  

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