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Author Topic: 6mm/5K Gossamer; The cartridge story  (Read 199 times)

Offline Dutch-Hunter

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6mm/5K Gossamer; The cartridge story
« on: April 02, 2019, 04:57:35 PM »
PLEASE DON'T TRY THIS!! I'm crazy but I know how to be safe with these types of pressures. PLEASE DON'T TRY THIS!!

As with any custom build you must start by designing the cartridge first. Iím posting the cartridge story first if I added the rifle story it would be very long and get boring. I hope you enjoy.

Iíve been planning this project for about a decade. I wanted to develop a 243 cartridge that would propel a projectile at a muzzle velocity of 5000fps or greater. Did a bunch of research and data analyzing and came up with the following scenario.

The first obstacle was to find a bullet that could handle the geometric stress and pressures. I decided to try a 107 grain monolithic that I could reshape with a lower angle nose. I had to scrap the idea of reshaping and had to turn my own from round stock. I first considered a nose with an 8 degree straight taper. After analyzing in greater detail I decided to go with an exaggerated radius secant of 11x caliber radius. Though harder to accomplish in production over the straight taper the performance improvement made it the best choice. The same radius was used on the tail rather than a straight angle cut boatail. I successfully made 40 bullets (out of 150 attempts) with these dimensions; length= 1.102, nose= .617. tail= 1.83, bearing(body)= .312, final bullet weight 98.746 grains. The dimensional tolerances I held to were +/-.0015 and weight had to be +/- .003 grains. I estimated the BC of these bullets in the range of 390-405. Only important in determining trajectory if I would like to see what the accuracy and range distance could be.

The second obstacle was choosing a case with enough capacity. I was expecting to need about 105-107 grains of powder. I looked at many cases and first ruled out any belted ones. The inherent case blowout just above the belt I thought would be problematic. The pressures that will be generated are off the chart and dangerous. The case I chose was the 300 RUM who's parent case is the 404 Jeffery. The .015 taper of its body I thought was advantageous and it had the capacity I felt I needed. I decided to use Norma brass because of its inherent resiliency. The shoulder angle will be dropped to 50 degrees and maintain a neck length of .309. The result would be a case OAL of 2.835. With the bullet seated to ogive the cartridge OAL of 3.452 with a bullet penetration into case of .495 which should allow for a powder load of up to 98% of capacity. All of these dimensions may slightly change with the actual case once formed and measured. With the case and bullet dimensions in hand I ordered four reamers. I had the neck cavity made with an extra .015 length to give flow room for the case neck. The throat angle was 6 degrees to accommodate the exaggerated nose length of the bullet. This throat and should give me a jump space of around .020, but will be actually determined after sizing and seating.

Once the reamers arrived I could really get started on getting this project on the road. The first step was getting the dies made. The next step was to make a fire-forming chamber. Mark my smithy and I made a fire-forming chamber out of an old 243 shot out barrel cut off both ends and reamed and mounted it to a 700 Remington action. Just for fun we put a pistol grip on it. Now just imagine a 300RUM pistol with a 16Ē barrel. We got many funny looks and shaking heads. Donít think we ever told anybody it was only going to shoot paraffin wax.

The dies were ordered with one full length die having an oversized neck diameter of .249 and would initially shape the shoulder angle and form a slightly oversized neck. After using this oversized neck die the cases were initially length trimming to 2.845. Now itís time to fire-form the cases to a chamber. The .249 necks would push into the fire-forming chamber with some but not much persuading. The cases were now ready for final length trimming to 2.835. I had a neck sizing die made with an expander ball diameter of .2425. I was hoping to have a consistent ďtight slipĒ when the bullet is seated. The bullet was seated to the leading end of the bearing surface this depth gave an estimated .35 freebore. Having and assembled cartridge I can now measure the actual water capacity of the case with a seated bullet. The capacity of the case was 108.74 grains. With this data I could then refer to the computer to determine the load density possibilities with a few different powders. I found that US869 loaded to 92% of capacity or 106.565 grains should generate enough pressure to achieve the velocity with the barrel I had made (more about the barrel in the rifle story to follow). Iíll use my waste bullets to slowly work up the velocity. Thereís nothing wrong with these just a bit underweight. When and if I reach my targeted velocity Iíll load up 5 rounds to record a string. These rounds will have near identical tolerances as far as weight of; primer, case, bullet and powder. The finished cartridges will not deviate more than .001 grain.

Iím emphatically saying that this project was generated in between my crazy old ears as a speed priority project. PLEASE DO NOT TRY THIS!!! Iím only planning to shoot this bench missile enough to fulfill my speed dream. This will be safely done by vising onto the bench with ballistic blankets over it and using a remote trigger. All pressure signs will be carefully monitored.

The rifle story contains my two "idiot waivers". You'll just have to wait a bit more Mad, sorry.
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Offline Madgomer

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Re: 6mm/5K Gossamer; The cartridge story
« Reply #1 on: April 02, 2019, 07:55:39 PM »
Aw dang, I gotta wait even longer? 

Sounds like a fun project Dutch - I know it's not one to be taken lightly from a safety standpoint but it sounds like you're taking plenty of time to think through the details & do it right.  Would love to see a photo of one of these custom rounds next to something like a standard .243 cartridge if you are willing to share.
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Re: 6mm/5K Gossamer; The cartridge story
« Reply #2 on: April 03, 2019, 09:31:33 AM »
Thanks for the details Dutch can't wait to hear about the rifle.
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Offline Split toe

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Re: 6mm/5K Gossamer; The cartridge story
« Reply #3 on: April 03, 2019, 10:32:44 AM »
Great information Dutch...looking forward to your results!
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Offline Gutshot

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Re: 6mm/5K Gossamer; The cartridge story
« Reply #4 on: April 03, 2019, 10:44:45 AM »
Good stuff here, I'm waiting  ##$%#1119
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Offline gobihawk

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Re: 6mm/5K Gossamer; The cartridge story
« Reply #5 on: April 05, 2019, 06:25:43 PM »
Thanks,  DH. Great info. I don't think you're crazy. Sounds like a well thought out project.
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Offline BoBallistic

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Re: 6mm/5K Gossamer; The cartridge story
« Reply #6 on: April 06, 2019, 12:36:07 PM »
Dutch - If anyone can, you can do it!! Like Split says, cannot wait to follow this to the conclusion....and the results...Wow all base on a 243....cannot wrap my head around a 5K fps bullet?? Wow, image the pressure and the recoil and the very short barrel life that lies ahead of you...humm...maybe our military would be interested in such a round especially on a AR platform and with any AR you can change barrels out in a matter of minutes....

I'm sure you will "puff" a few bullets, wonder if any bullet manufacturers would back you? maybe a Burger or a Barnes might be your best bet for such a round....humm... 

Keep us posted.....thanks for sharing.....

But everyone here that is building a custom rifle and want to shoot long range, it all starts with the bullet. Once you build your rifle, you cannot change bullets weights without headaches that follow it....I learn that many years ago.....


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