Saturday - March 23, 2013
On The Cusp Of Greatness
I’ve posted a couple times over the past year or so on my digital pal John Jay’s work on his little wildcat cartridge, the JJ’s Brit (trademark asserted). He brought the early 1950’s .30/.280 British cartridge back to life, modified it a bit for more modern specifications, and built a test rifle for it on a shoestring budget. I helped him work out some of the specs and with load development. Finding himself quite satisfied with the performance of the little round, he then went parts shopping, and with the help of a gunsmith or two, has made the cartridge fit and function in the AR rifle platform. This is quite an accomplishment, because that rifle was not built for a cartridge with the “standard” head diameter of 0.473”. It’s too fat to fit the original boltface. Good thing that the AR rifle has thousands of aftermarket parts to fit it, and one of them was a larger diameter bolt that he made work. The result is that he has created a perfectly useful deer hunting cartridge for this modern sporting rifle, one that also just happens to give better long range performance than the 6.8SPC, the 6.5 Grendel, the .30 Remington AR, and scads better long range whack than the original 5.56 NATO. In other words, he has found a viable solution to the old “poodle shooter” problem. Outstanding.
In a fairly short 18.5” barrel, the JJ’s Brit (trademark asserted) with either the 120gr or the 130gr bullet outperforms the 123gr 7.62x39 Soviet cartridge, pushing both at least 200fps faster than the Russian round, and since this is a 7mm bore, both bullets have better sectional density (SD) and better aerodynamics (BC or Ballistic Coefficient). The 130gr bullet rivals the SD of the 7.62 NATO’s 168gr bullet, which means it will penetrate targets nearly as well at equal velocities. But with far less cartridge weight, and only about half the recoil. This means he has found a solution to the MBR (Modern Battle Rifle or Medium Battle Rifle) challenge, creating an accurate little round that should be combat effective to 500 meters or a bit further. Nice. Darn shame no one in authority wants to hear such things.
Ballistics calculations show that the 130gr bullet retains 1000 lb/ft of energy to 355 yards, and keeps 750 lb/ft out to 490 yards. Put plainly, it’s a 400 yard deer hunting cartridge and a 500 yard military cartridge. The 120gr bullet is similar. The 63gr SS109 military load for the 5.56 NATO round carries 1000 lb/ft to only 142 yards, and 750 lb/ft to 270 yards. The current issue M885a1 “Enhanced Performance” load might give another 30 yards advantage over the SS109 but that’s about it. The JJ’s Brit (trademark asserted) [hey, that’s what he calls the damn thing, parenthesis and all! I suggested just calling it the .280 JJ, but he likes his naming better] is far superior to that.
And this is the short, low pressure version of the cartridge. Seating the bullets further out and using a 2.55” OAL gives lots more room for powder, and when mated to a 22” or longer barrel and loaded to the same pressure point can give another 150fps, which translates into another 50 yards at equal retained energy levels. Taken up to magnum pressure levels (63K psi) you can glean another 100fps. Either of these approaches makes it a bit much for a military MBR round, but gives you a very nice 450-500 yard deer rifle cartridge, which is quite a bit further than most hunters can accurately shoot. So the power is there if wanted, but it isn’t really needed. And if you don’t want it, the 2.55” version makes the same velocity as the 2.25” version at considerably lower pressure, which means less wear on the rifle barrel. Not that that matters in this spendy day and age.
We always knew the cartridge had power to spare. The real challenge of this project was making it fit in an AR, and now he’s done that part too.
Nice work John. Now if we can just convince Keltec to build a downsized version of
ooh, I want one of these!
Posted by Drew458
on 03/23/2013 at 11:12 AM
Filed Under: • Guns and Gun Control •
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