Barking Moonbat Early Warning System

Sarah Palin knows how old the Chinese gymnasts are.

Thursday - December 09, 2010

Still Dicking About

In the last Dicking About post, my response to comments from both Docs got me thinking about the great-grandfather of all assault rifle cartridges, the 6mm Lee Navy. That round did the job just fine in the Spanish American War, and was good enough for the Marines. It was about 100 years ahead of it’s time though, way ahead of the gun powder and steel technology of the day, so it got dropped.

Anyway, I took a look at the cartridge, cut it down, blew it out, and thoroughly modernized it to run at modern pressures and to fit in an AR platform. I then optimized the case dimensions to use the 6mm Sierra 100gr BTSP bullet, which was the closest thing I could find to a FMJBT design in that caliber at that weight. A 100gr .243 bullet has a sectional density of .242, which is more than good enough. The Sierra BTSP has a BC of .446, which is great. Heck, they have a 107gr HPBT target bullet with a SD of .259 and a BC of .518, which is trippingly fantastic. Unfortunately that slightly longer bullet tips the scales too far, and shoots much slower in this cartridge at the same pressures, which means it comes up short at longer ranges. Crank the psi up into the top magnum range and it still comes up a tiny bit shy. See? It’s tough working under that itsy bitsy 2.25” OAL constraint!

Running the calculations shows that with the 100gr bullet it will shoot +/- 4” out to 300 yards in an AR platform (sights 2” above bore). The long barrel loading holds onto 1100lb/ft of energy out past 400 yards, while the short barrel loading is 12% less but is powder optimized for even a 14” barrel. That means lots less muzzle flash and less velocity loss from those short barrels.

Either loading carries nearly double the whack of the SS109 5.56 round at extended ranges, yet recoil is an easy 8lbs/ft in a 7lb rifle. It would make a nice 400 yard deer rifle.

It won’t have quite the arm’s length stopping power of the 6.5mm 120gr round I designed yesterday, but it matches that ones performance past 100 yards. And this is exactly what the 6.8SPC vs. 6.5 Grendel debate was all about, except that the 6.8SPC fell flatter at longer ranges. Big heavy bullets do the job at close range, which is why the Army made the .45-70-500 130 years ago. Conversely, a 20 round magazine of this 6mm cartridge will weigh an ounce less than a 20 round magazine of the 6.5mm cartridge, and every ounce counts when you have to lug the stuff around.

I called it the 6mm AWB just to frighten the hoplophobes

Short barrel loading. The vertical pink line shows all the powder is burned up once the bullet has gone a foot down the barrel.

Long barrel loading. Slower burning powder gives you more velocity in longer barrels. So can a bit more pressure.

All of this just goes to show that the AR platform rifle was a bit too optimized for anything more than the 5.56 cartridge. Had the bolt been 0.10” bigger in diameter, and had the mechanism and magazine well allowed for a quarter inch longer 2.5” OAL, the military could have swapped in much more powerful cartridges long ago, even ones that run at markedly lower pressures. Alas.

UPDATE: I’ve created a monster!!

See More Below The Fold

Well, I mean to say that I’ve accidentally cloned one.

Grendel was the monster in Beowulf, right?

Honestly, I did not know it at the time. I really was just dicking about yesterday, playing with the gun software. It turns out that my 6.5x39_25 design of yesterday is within a hair of the specs for the 6.5 Grendel cartridge. If you think of the drawing as being elastic below the case neck, when you squish my cartridge down it gets a little shorter, the neck gets a tiny bit longer, the shoulder angle increases from 25 to 30 degrees, and the case body gets a couple thousandths of an inch bigger in diameter at the shoulder. At that point it is the 6.5 Grendel.

Ignore variations in measurement that are in the ten thousandths of an inch range. These are the same size in actual production. The chamfer angles on the rim and on the extractor groove are going to vary at the factory too. The only real spec difference between his design and mine once “squished” is that he specified a thicker case neck. I didn’t, and went with the evenly tapered brass tube that the design program gives you. In the real world, you want the case neck just as thick at the top as it is at the bottom. Which would have come out in pre-production discussions, as would have standardized 45° chamfer angles.

So mine is a hair longer and a hair thinner, with a more gentle shoulder. Internal volume should be nearly identical, though his design might have a tiny tiny bit more inherent accuracy and a small velocity advantage with the proper powders (short fat powder columns burn very consistently, which leads to the best accuracy), but that difference will be very very small. And mine will feed more smoothly, guaranteed.

Wiki says the 6.5 Grendel can push the exact same 120gr bullet I specified by chance! (there is not much choice in 6.5mm FMJ bullets) to 2700fps in a 24” barrel. So I’d expect my design to come very close to that, which proves once again that the ballistics software is set on the conservative side for safety reasons. That puts it right up there with the 6.5x55 Swede and gives it another 25 yards point blank range.

BUT ... one reason that the 6.5 Grendel is dead in the water is because the guy who diddled his software and came up with the design and the name wants a piece of the action. The name is trademarked. Dick. You trademark the brand name of your company. You don’t trademark the name of your cartridges. [ Look what happened to those bespoke English gunsmiths, ie Mr. Rigby - yes, he is remembered for 1 cartridge, but the others that once bore the Rigby name were borrowed directly from Mauser (aka the 7x57). I think Ross did the same thing, and now both are long out of business? So if you’re reading this Uncle Sam, you can have the design, free and clear. ] I just happened into it by sitting at the keyboard and saying “OK, we’ll stretch this here, fatten that part there, cut the neck down to about there ... Ok, that looks right” and wa la!

GET ME OUT OF HERE ...

${lang:my_avatar}$

Posted by Drew458

on 12/09/2010 at 02:10 PM
Filed Under: • Guns and Gun Control •
• Comments (6)

• Trackbacks(0) Permalink •

Page 1 of 1 pages

Five Most Recent Trackbacks:

Once Again, The One And Only Post
(4 total trackbacks)
Tracked at iHaan.org
Thｅ advantage to having a guide with you is thɑt an expert will haѵe very first hand experience dealing and navigating the river with гegional wildlife. Tһomas, there are great…
On: 07/28/23 10:37

The Brownshirts: Partie Deux; These aare the Muscle We've Been Waiting For
(3 total trackbacks)
Tracked at head to the Momarms site
The Brownshirts: Partie Deux; These aare the Muscle We’ve Been Waiting For
On: 03/14/23 11:20

Vietnam Homecoming
(1 total trackbacks)
Tracked at 广告专题配音专业从事中文配音跟外文配音制造，北京名传天下配音公司
　　专业从事中文配音和外文配音制作，北京名传天下配音公司　　北京名传天下专业配音公司成破于2006年12月，是专业从事中中文配音文配音跟外文配音的音频制造公司，幻想飞腾配音网领配音制作有海内外优良专业配音职员已达500多位，可供给一流的外语配音，长年服务于国内中心级各大媒体、各省市电台电视台，能满意不同客户的各种需要。电话：010-83265555 　　北京名传天下专业配音公司…
On: 03/20/21 07:00

meaningless marching orders for a thousand travellers ... strife ahead ..
(1 total trackbacks)
Tracked at Casual Blog
[...] RTS. IF ANYTHING ON THIS WEBSITE IS CONSTRUED AS BEING CONTRARY TO THE LAWS APPL [...]
On: 07/17/17 04:28

a small explanation
(1 total trackbacks)
Tracked at yerba mate gourd
Find here top quality how to prepare yerba mate without a gourd that's available in addition at the best price. Get it now!
On: 07/09/17 03:07

DISCLAIMER

THE SERVICES AND MATERIALS ON THIS WEBSITE ARE PROVIDED "AS IS" AND THE HOSTS OF THIS SITE EXPRESSLY DISCLAIMS ANY AND ALL WARRANTIES, EXPRESS OR IMPLIED, TO THE EXTENT PERMITTED BY LAW INCLUDING BUT NOT LIMITED TO WARRANTIES OF SATISFACTORY QUALITY, MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE, WITH RESPECT TO THE SERVICE OR ANY MATERIALS.

Not that very many people ever read this far down, but this blog was the creation of Allan Kelly and his friend Vilmar. Vilmar moved on to his own blog some time ago, and Allan ran this place alone until his sudden and unexpected death partway through 2006. We all miss him. A lot. Even though he is gone this site will always still be more than a little bit his. We who are left to carry on the BMEWS tradition owe him a great debt of gratitude, and we hope to be able to pay that back by following his last advice to us all:

Keep a firm grasp of Right and Wrong

Stay involved with government on every level and don't let those bastards get away with a thing

Use every legal means to defend yourself in the event of real internal trouble, and, most importantly:

Keep talking to each other, whether here or elsewhere

It's been a long strange trip without you Skipper, but thanks for pointing us in the right direction and giving us a swift kick in the behind to get us going. Keep lookin' down on us, will ya? Thanks.

THE INFORMATION AND OTHER CONTENTS OF THIS WEBSITE ARE DESIGNED TO COMPLY WITH THE LAWS OF THE UNITED STATES OF AMERICA. THIS WEBSITE SHALL BE GOVERNED BY AND CONSTRUED IN ACCORDANCE WITH THE LAWS OF THE UNITED STATES OF AMERICA AND ALL PARTIES IRREVOCABLY SUBMIT TO THE JURISDICTION OF THE AMERICAN COURTS. IF ANYTHING ON THIS WEBSITE IS CONSTRUED AS BEING CONTRARY TO THE LAWS APPLICABLE IN ANY OTHER COUNTRY, THEN THIS WEBSITE IS NOT INTENDED TO BE ACCESSED BY PERSONS FROM THAT COUNTRY AND ANY PERSONS WHO ARE SUBJECT TO SUCH LAWS SHALL NOT BE ENTITLED TO USE OUR SERVICES UNLESS THEY CAN SATISFY US THAT SUCH USE WOULD BE LAWFUL.

GNU Terry Pratchett

Oh, and here's some kind of visitor flag counter thingy. Hey, all the cool blogs have one, so I should too. The Visitors Online thingy up at the top doesn't count anything, but it looks neat. It had better, since I paid actual money for it.