twist rate for 6.5-300 Ultra

jtoews80

jtoews80

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I ordered a prechambered barrel in 6.5-300 RUM for a LR coyote rifle. Now I'm wondering if I have the twist right. It is a 1-12 on a 30 inch barrel. Intended bullet weight is 130 grn, but if that won't work a 123 SMK should. Of course, if it handles 140's great too, however that is asking a lot.

The build is just finishing the aquiring phase. Donor is a 700 sps 300 rum, stock is an A5 Bell and Carlson, barrel is a SS Pacnor, dies are Redding customs, reamer is from PTG and topped with a Mark 4 leupold 30mm in Badger rings. Still need a 30 moa rail, tact bolt knob and some better bottom metal, but the factory stuff will work for now.

Barrel and stock should be here monday.
 
You need a 1:10 or faster twist for 140's. A 1:12 would work for up to about 120 grs.

I have the exact same rifle on the go. :D My barrel is a 26" fluted 1:9 and it's going on a Rem 700 that is presently in 300 RUM. I have the reamer, just need to get it put together after hunting season.

Be prepared for the advice you didn't ask for:

- it's a barrel burner!
- it's inefficient!
- many other 6.5mm's are better!

Ignore and enjoy.
 
Andy said:
You need a 1:10 or faster twist for 140's. A 1:12 would work for up to about 120 grs.

I have the exact same rifle on the go. :D I have the barrel is a 1:9 and it's going on a Rem 700.

Be prepared for the:

- it's a barrel burner!
- it's inefficient!
- many other 6.5mm's are better!

Ignore and enjoy.
Click to expand...

LOL, no kidding. As much as I enjoy shooting, I haven't got enough time to wear out that barrel in less than 5 years. The 700 VTR 223 AI that Guntech built/trued for me has 2 rnds fired to date, so I need more time.
 
I am expecting about 450-500fps over a 264 win, 37-3800fps. I'll be using 130 grn Nosler Accubonds and WC 860 or US869 powder. If the bore is really good, and we can run 140's over 3500 they might stabilize, but that has yet to be seen. I certainly am not counting on it.
 
jtoews80 said:
I am expecting about 450-500fps over a 264 win, 37-3800fps. I'll be using 130 grn Nosler Accubonds and WC 860 or US869 powder. If the bore is really good, and we can run 140's over 3500 they might stabilize, but that has yet to be seen. I certainly am not counting on it.
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Seems like your going to end up with what amounts to a 7mm stw with a different name. Oh well have fun doing it.:rockOn:
 
I realize this. I've owned a 7mm STW and a couple 30-378's. I will try to be nice and not ruin that barrel before it has been broken in. The surplus powder should help keep things cool, and I am considering trying that gunjuice from microlon. Kinda like Slick 50 for the barrel, but not sure if it is hype or for real.
 
jtoews80 said:
I am expecting about 450-500fps over a 264 win, 37-3800fps. I'll be using 130 grn Nosler Accubonds and WC 860 or US869 powder. If the bore is really good, and we can run 140's over 3500 they might stabilize, but that has yet to be seen. I certainly am not counting on it.
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i tough you where better using barnes bullet (because it's all copper) at thoses velocity ??
 
With the velocities you'l be getting, I don't think you'll have a problem with any of the heavier bullets.

You DO realize that that cartridge has an overbore index of 2011!!!! That will give it a barrel life of only 5-700 rounds if you're lucky.
 
I had a similar setup 30 yr. ago, put together by Al Pedersen. Used a 35/404 imp case necked to 6.5. Never could get it to shoot to expected vel. Using 867 and 869 with some 700x over the primer proved to be the best, however, most of the experts figured too much unburned powder was plugging the case mouth, causing eratic pressures. We cut it down and rechambered to 6.5/284 years later. Good luck, Mark
 
Obtunded said:
With the velocities you'l be getting, I don't think you'll have a problem with any of the heavier bullets.

You DO realize that that cartridge has an overbore index of 2011!!!! That will give it a barrel life of only 5-700 rounds if you're lucky.
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2011?? How did you come to that exact figure?
I'm not sure if there is a formula to get an index off of, but I'm certain that this is a sinfuly ineffecient waste of barrel, powder, time and money.
It's great, ain't it!!!
And then, later, it might become something more reasonable.
 
Dividing the case capacity in grains of H20 by the surface area of the case opening reveals an index number called the overbor index. The indeces from such a calculation must be used within context.

A 308 has a useful barrel life of approximately 5-8000 rounds with an OBI of 751

a 6.5-284 has a MAXIMUM life of about 1200 rounds (I got 750 out of my last one) with an OBI of 1205.

At over 2000, that cartridge strikes me as being so overbore as to be very difficult to find a powder slow enough. Also a barrel long enough!

.......I would love to shoot it! :D

Here is a chart with a few cartridges and their OBI (Case volume to bore area ratio)

overborechart4a.png
 
Barrel is 30 inch. I want one 1000m coyote to hang on the wall, if I get more great, if not. So what. If I need a second barrel, I may settle for a WT deer so long as I can take it cleanly and humanely.
 
At the outset of this thread I knew that the obvious things that the OP already knew would be trotted out - as usual. I have the exact same chambering in the works and am fully aware of its strengths (high MV) and shortcomings (limited suitable powders, high powder use and short barrel life). After 100 or so rounds of load development, I'll likely shoot the gun 20-30 rounds a year, so I guess I can only expect 20-30 years of accurate barrel life. Oh my - I must reconsider!

As for being "Overbore", if that means that no suitably slow powders exist for it, then it's not. For the heaviest bullets you're restricted to the five or six slowest available powders such as US869, and Reloder 50 (when it hits the stores), but for the lighter bullets, the list expands quite a bit, to include Re25, H1000, Retumbo and the like.

If all that mattered to a shooter was barrel life and efficiency, all we'd have would be the 22LR, and for special uses, like to hunt or target shoot, at most a 308. Everything else is inefficient and a barrel burner by comparison.
 
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Andy, what you say is absolutely true, but then there are some people out there that appraoch this sort of an ambition from a completely naive perspective. As someone who sells barrels, intuitively one might think it would be in my own self interest to refrain from discouraging such experimentation. I want people to achieve success; achieving success will keep them in the sport and having fun.

Like I mentioned, I would actually love to try shooting his rifle when it was done.
 
Obtunded said:
Andy, what you say is absolutely true, but then there are some people out there that appraoch this sort of an ambition from a completely naive perspective. As someone who sells barrels, intuitively one might think it would be in my own self interest to refrain from discouraging such experimentation. I want people to achieve success; achieving success will keep them in the sport and having fun.

Like I mentioned, I would actually love to try shooting his rifle when it was done.
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Good points - :agree:
 
I've already built a sane 6.5mm. It's for sale on the EE. It is a 264 win tight neck, spec'd to shoot 140 smk's and if we can ever get them some of the heavy for caliber Wildcat bullets that used to be made in Alberta. It is very accurate, but velocity is limited by the twist rate. I guess more precisely, it's performance is limited to that of a 6.5-284 because of pressure caused by the rate of twist(1-8.5).
 
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