338 ARC - An Early Guess At Reloading Data

I have been playing around with the idea of a wildcat to improve on the 300 BLK for a while. Requirements were better subsonic performance, able to be run in a standard length AR15 mag (for howa mini compatibility) and to be made from easily available brass. The 8.6 BLK is very interesting, but is far too big for a howa mini.

Hornady have just released the 338 ARC based on a shortened Grendel case which ticks all these boxes, so my wildcat ideas are out the window and I am probably going to build a rifle on this new cartridge.

Information is very sparse at the moment, but there is enough out there to create a case and chamber profile in Gordons Reloading Tool and get some ball park figures on what powders might be suitable.

It looks like 2205 will be almost perfect for 300gr subsonic loads. Achieving 1050fps from a 12" barrel with a 95% case fill and 97% powder burn. Charge would be somewhere around 9.5gr

Blue dot and V3N38 are also good selections at around 7-7.5gr for around 85% case fill and 100% burn rate.

Supersonic info is more challenging to guess, but I think you could probably get around 2100-2200fps from a 150gr projectile. 12" barrel while still being under 45k psi.

Trueflite are making 1:7 twist .338 barrels which should be perfect for this cartridge and will be able to stabilize 300gr copper monolithics such as Lehigh or Maker. The 300gr monos are able to be seated deep enough to achieve mag length. I dont know if you could go much heavier in copper as the projectiles may be too long.


Attachment 264142

Could be fun. What would a 300 grain mono be good for at that velocity?

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Originally Posted by Gkp

Could be fun. What would a 300 grain mono be good for at that velocity?

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They are designed for subsonic, have petals that shear off

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Originally Posted by Gkp

Could be fun. What would a 300 grain mono be good for at that velocity?

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Killing stuff quietly. Have a look at the Lehigh maximum expansion or the Maker Rex projectiles. They are very spendy, but I have a CNC lathe and produce my own versions in small quantities for my own use.

This type of bullet really makes subsonic hunting viable.

That's interesting. Will look into it

Can you please tell me where you found the final dimensions of the 338 arc cartridge?
I looked up these ones: https://www.pacifictoolandgauge.com/..._p_119606.html
Which is a copy of their reamer print. And I also necked up a 6.5 grendel Case and fitted a 338 projectile into it.
The dimensions on the reamer print are quite close to my mockup round . But my mock-up round seems to have a way longer body than that of the hornady pictures.

Greetings,
The gaps between existing factory and wildcat cartridges must be becoming exceedingly small by now.
Regards Grandpamac.

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Originally Posted by SixtyTen

I have been playing around with the idea of a wildcat to improve on the 300 BLK for a while. Requirements were better subsonic performance, able to be run in a standard length AR15 mag (for howa mini compatibility) and to be made from easily available brass. The 8.6 BLK is very interesting, but is far too big for a howa mini.

Hornady have just released the 338 ARC based on a shortened Grendel case which ticks all these boxes, so my wildcat ideas are out the window and I am probably going to build a rifle on this new cartridge.

Information is very sparse at the moment, but there is enough out there to create a case and chamber profile in Gordons Reloading Tool and get some ball park figures on what powders might be suitable.

It looks like 2205 will be almost perfect for 300gr subsonic loads. Achieving 1050fps from a 12" barrel with a 95% case fill and 97% powder burn. Charge would be somewhere around 9.5gr

Blue dot and V3N38 are also good selections at around 7-7.5gr for around 85% case fill and 100% burn rate.

Supersonic info is more challenging to guess, but I think you could probably get around 2100-2200fps from a 150gr projectile. 12" barrel while still being under 45k psi.

Trueflite are making 1:7 twist .338 barrels which should be perfect for this cartridge and will be able to stabilize 300gr copper monolithics such as Lehigh or Maker. The 300gr monos are able to be seated deep enough to achieve mag length. I dont know if you could go much heavier in copper as the projectiles may be too long.


Attachment 264142

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Would you not go a faster twist (1:3) like the 8.6 BLK?

similar capacity to my old 338br ? i ran n110, and it threw 310s and 280 nice and quiet.

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Originally Posted by Friwi

Can you please tell me where you found the final dimensions of the 338 arc cartridge?
I looked up these ones: https://www.pacifictoolandgauge.com/..._p_119606.html
Which is a copy of their reamer print. And I also necked up a 6.5 grendel Case and fitted a 338 projectile into it.
The dimensions on the reamer print are quite close to my mockup round . But my mock-up round seems to have a way longer body than that of the hornady pictures.

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I used Mansons reamer print to get neck and throat dimensions and Hornady specify the case length and COAL in their release data. Its not just a 6.5 Grendel necked up, it is shortened also. My dimensions might not be perfect to the final thing, but it will be close enough for estimating performance.

It should be SAAMI approved by January, sounds like dies and brass are already making their way around the globe.

I am planning to order a reamer once dies are available.

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Originally Posted by Rich007

Would you not go a faster twist (1:3) like the 8.6 BLK?

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I have been doing a lot of reading and research on the whole 1:3 twist thing as I thought it was a very interesting idea when I first heard about it. The below is what I have determined

- It seems to be mostly a hype thing driven by Q to try and make the 8.6 BLK seem special.
- 1:3 twist improves performance of segmented mono's by around 10%, maybe a little more.
- it seems to reduce effectiveness of jacketed subs through over stabilization
- It seems to be causing accuracy issues with jacketed projectiles and is even not flash with mono's (this is even with non Faxon barrels)
- Cannot shoot jacketed supersonic (they will explode on leaving the muzzle)
- Almost certainly cant use any lead projectiles, subsonic or not

Basically it limits you to mono copper projectiles and also reduces accuracy for a small increase in terminal performance .
Hornady have determined that a 1:8 twist is ideal for the 338 ARC, and that will allow 300gr + jacketed to stabilize. I plan to use a 1:7 barrel to use 300gr mono's which are a lot longer than jacketed, so need a bit more twist.

An interesting note around barrel twist that I came across was comparing twist rates for different calibers by the calibers/rotation rather than rotations per inch. doing this shows that a 1:8 .338 barrel is the equivalent to a 1:5 .223 barrel. or a 1:7 .308. Gives you a little perspective on how fast a 1:3 would be.

Interesting thanks.

Watching with interest.....

Trueflite have a reamer on the way