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Hot 6mm Rounds

 
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  #15  
Old 01-27-2009, 12:50 PM
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Re: Hot 6mm Rounds

With 6mm 100 grain bullets the velocity threshold (brick wall) is right around 3300-3400fps.

Lets do some math.

3400 feet per second comes out to 2,376,000 inches per minute. You need a 1-8 twist barrel to stabilize this weight class of bullet properly and this results in a bullet RPM of 297,000. The copper jacket (J-4) on target bullets is right around .005" thick; just slightly more than the thickness of a sheet of note paper.

Imagine the centrifugal forces at play here. Increases in velocity results in an increase in RPM. Some try to trick this by slowing the twist down so that the RPM stays in the proper threshold but this can result in a rifle that is a bit fussy and temperamental when fiddling with different bullets as now you must run the powder charges to the high end of the loading tables. The other side of this is the acceleration forces. The bullet goes from standstill to 1 rotation every 8 inches the moment the lands get a bite on the bearing surface. A bullet accelerates to its maximum velocity in the length of the barrel so imagine the incredible forces at work here as a little bullet accelerates from dead still to 3400 feet per second simultaneous to spooling up to 297,000 rotations a minute. It's amazing the thing even makes it out the barrel in one piece.

There exists a phenomenon where a bullets jacket can out accelerate (talking rotation here) the core while its "spooling up" down the barrel. This separation is almost surely a great contributor to bullets spontaneously "poofing" shortly after leaving the barrel.


If you want a great 6mm cartridge that is kind of a novelty, the original 6mm X 47 Swiss Match is still pretty bad azz. I have the distinct honor of being the FIRST bonehead in the US to ever chamber a rifle in this cartridge. You can buy the reamer from David Kiff (I have one floating around at home as well) and the dies can be made from Newlon blanks or custom ordered through Redding.

It's commercially available by RUAG in Finland but good luck getting any shipped here. The cartridge is tailor made around the 105 grain Hornady A max. It's velocity is right at 2800. You can go a bit faster. The Swiss team used it for 300meter CISM and free rifle stuff.

I built one in an AR-10 for a guy back in 2001/02 and he shot a 12X clean at 1000 yards down in Raton at the Whittington center the first time he took it out.

Neat little cartridge. The newer lapua 6.5X47 is a spin off from it. Biggest difference is the ND and the small rifle primer. (Cool upgrade in my book)

Good luck and have fun!

Chad

Last edited by NesikaChad; 01-27-2009 at 01:04 PM.
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  #16  
Old 01-27-2009, 01:35 PM
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Re: Hot 6mm Rounds

Nesika, I am trying to understand what you are saying. I have a degree in applied math along with the engineering so I don't mind you throwing the boring math at me to help me understand more. Are you saying the loads I quoted are not suppose to be shooting 1/2" to 1" groups at the 300 yard target like they do so well. Do you have your 30-338 lapua twisted with a 1-14 or 16 to shoot the 125 accurately at over 4300. You said it also shot the heavy bullets well so it must be a 1-10 or so. Talk to me and tell me what you are doing. I am here to learn like everybody else so let me know what you are saying. Those loads I mentioned will rarely miss a prarie dog at 500 yards if the wind is right so I am trying to figure out this brick wall you are talking about. Evidently I haven't hit it yet pushing 3600 fps with the 107's and 4300 with the 55's in the same twist barrel. I know my 30-378 blew up the 125 BT when I took it over 4300 with a 1-10 twist so I understand there is a brick wall there. I guess a guy just works up the top load for his rifle and sticks with it. Some shoot and some don't. Your big 30 shoots it but mine didn't. I was just wondering if you had a 1-14 twist for the light bullets. Why does the 55 BT shoot at 4300 fps with a 1-9 twist and the 125BT blew up at 4300 with a 1-10? My engineering tells me it is probably the larger diameter of the soft, heavy lead in relation to the jacket giving all that heavy lead more momentum to outspin the jacket. But I don't know.
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  #17  
Old 01-27-2009, 02:15 PM
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Re: Hot 6mm Rounds

Quote:
Originally Posted by Long Time Long Ranger View Post
Nesika, I am trying to understand what you are saying. I have a degree in applied math along with the engineering so I don't mind you throwing the boring math at me to help me understand more. Are you saying the loads I quoted are not suppose to be shooting 1/2" to 1" groups at the 300 yard target like they do so well. Do you have your 30-338 lapua twisted with a 1-14 or 16 to shoot the 125 accurately at over 4300. You said it also shot the heavy bullets well so it must be a 1-10 or so. Talk to me and tell me what you are doing. I am here to learn like everybody else so let me know what you are saying. Those loads I mentioned will rarely miss a prarie dog at 500 yards if the wind is right so I am trying to figure out this brick wall you are talking about. Evidently I haven't hit it yet pushing 3600 fps with the 107's and 4300 with the 55's in the same twist barrel. I know my 30-378 blew up the 125 BT when I took it over 4300 with a 1-10 twist so I understand there is a brick wall there. I guess a guy just works up the top load for his rifle and sticks with it. Some shoot and some don't. Your big 30 shoots it but mine didn't. I was just wondering if you had a 1-14 twist for the light bullets. Why does the 55 BT shoot at 4300 fps with a 1-9 twist and the 125BT blew up at 4300 with a 1-10? My engineering tells me it is probably the larger diameter of the soft, heavy lead in relation to the jacket giving all that heavy lead more momentum to outspin the jacket. But I don't know.


I have a 1-12 in my 300-338. It's 34" long and I use somewhere between 95-105 grains of powder. I have fiddled with RL22 and some others but RL25 seems to work best. (I'm not in front of my computer at home and I don't have my load data here and I have not shot this rifle since May of 2006) I use a GM215 Federal Primer and my cases are Lapua. With the extremely heavy loads I have to resort to my "vibromatic drop tube" method of hand loading.

vibrate the case as I dump powder down a 29" long drop tube made from an old Easton arrow shaft that screws into my reloading press. (trick set up) Then you "stand" on the press to get the bullet in there. I swear I can almost see the cases swell.

The chrono work was done in the test tunnel at COR BON ammunition. Pete Pi Jr. is a personal friend and when I worked for Nesika we leased their tunnel for our accuracy testing and load development. I believe they have a Oehler brand chrono. Tests were done on several different occassions over the span of a couple weeks. Mainly because I was having a very difficult time accepting the velocity being that fast. It always checked in that range though so I have to assume it's accurate.

With a thin jacketed (J-4) bullet in 6mm the "brick wall" (for me and for quite a few others) has been right about 3300-3400. This is in a 1-8 barrel. Obviously there are probably bullets out there that will tolerate more. The Barnes stuff would be a good candidate since they are made from one solid piece of material.

Yes, slowing the twist and cranking up the velocity will allow you to go a bit further, but as I've mentioned in other posts/threads this can be dicey and potentially harmful.

If your getting 3600 out of 107's DON'T CHANGE A THING!! It's workin and that's awesome.

In no way should anything I said be taken as a personal crack towards anyone or anything. It's just my experiences after doing this (building guns) for close to a decade.

That's it.

Hope this helped.
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  #18  
Old 01-27-2009, 03:43 PM
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Re: Hot 6mm Rounds

LTLR

How long is the barrel on your 240 Gibbs?
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  #19  
Old 01-27-2009, 03:58 PM
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Re: Hot 6mm Rounds

Chad, no offense taken, I know you work with this stuff so I was just trying to learn something and compare notes. I blew up several bullets in my 30-378 during testing many years ago.

Tyler, I have both 26" and 28" barrels in the 240 gibbs and did not look at which one those velocities came out of but I am certain it was the 26" because that is my ltwt varmint rifle I shoot most. The 28" is a straight bull target tube and doesn't get much use and since the loads were there for the light bullets which the target gun is not designed for and will not shoot my guess it was the 26". If you want to give me a few days I will load up some for it and shoot it to make sure for you. It's 24 below zero and a foot of fresh snow so a coyote day would be good anyway. The 240 wby is a 27" Hart barrel and it will be a gibbs after the last of the brass is shot. I refuse to pay the new prices for wby brass. The old prices were to much to begin with!
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  #20  
Old 01-27-2009, 04:30 PM
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Re: Hot 6mm Rounds

Don't worry about shooting any, I plan on a 30" barrel so I can load it milder for better case and barrel life. 115s at 3300 still fly super flat!
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  #21  
Old 01-27-2009, 04:35 PM
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Re: Hot 6mm Rounds

Tyler

If your interested there is a book on Gibbs cartridges, it's fairly inexpensive.

Gibbs' Cartridges and Front Ignition Loading Technique

ISBN# 0-935632-99-9
Wolfe Publishing
Published 1991

Southpaw
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