Correct twist rate on my 375 H&H

[WestCo]

I had a Rem 721 that was originally chambered in 300 H&H, had a gunsmith rebarrel it to 375 H&H. He mentioned that he installed a barrel with a 1:10.5 twist on it, and best I can determine doing the tight fitting patch and tape on the rod measuring, it checks out to be correct. I thought that he said it was a Douglas barrel, however now that I've put 50+ factory premium rounds down it and can't get better than 1.5" - 1.75" group at 100 yards makes me concerned there is something wrong with the barrel or it isn't a quality one? I've picked up some 300g TSX to start the reloading process, just don't want to waste my time if it isn't going to do any better. Any suggestions or ideas for any other smith's on here?

 

[BRICKBURN]

hmmm. Hope someone has some ideas for you.

 

[375 Ruger Fan]

WestCo: How did you break in the new barrel? Clean, shoot 1 round, clean, etc?

Did you talk with the gunsmith that did the rebarrel about headspace and COL measurements?

Regarding the twist rate, I can't see that being a big issue. You might try a different weight bullet or a different flavor of ammo and see if your results change.

Berger Bullets has a interesting calculator on their website: http://www.bergerbullets.com/twist-rate-calculator/

I made an Excel spreadsheet and plugged in some representative bullet weights, muzzle velocities and compared a 1:10.5 twist rate with a 1:12. The 1:10.5 actually looks better. Makes you wonder why the standard seems to be 1:12

Supposedly the ideal RPM is around 190,000
rpm = MV x (12/twist) x 60
This simplifies (as math teachers would say) to RPM= MV x 720/twist

GR is bullet weight in grains
Muzzle Velocity in feet per second
Twist
60 seconds per minute

 

[WestCo]

I cleaned it, shot 1 and cleaned after each shot for 10 shots, then repeated the process for 2 shot groups for 10 shots, then repeated the cleaning process after 2 three shot groups, and again after a 5 shot group. Hoping to find a powder combo that will work better with the 300g TSX. I guess on the positive side I don't really need to be so concerned with ES of the velocities nor with the seating depth to much due to the max length allowed by the magazine box.

 

[CAustin]

Interesting process that you have to go through to get a new rifle working the way you want it to.

 

[sestoppelman]

That's a pretty fast twist for a .375, never heard of anyone using such. However it may not be an issue. It would tend to shoot the heaviest bullets best so try 300 and up. If I was having a rifle built in that caliber I would have gone with the std twist, its been working for about a thousand years just fine!

 

[375 Ruger Fan]

Here's a link to an article on a CZ 550 in 450 Rigby with a 1:10 twist. Bullet RPM = 172,000
http://www.realguns.com/articles/340.htm

 

[Areaonereal]

Bore scope the barrel, my experience with Douglas barrels is there are better barrels if you are accuracy inclined...actually your groups are all not that out of line for a 375 for 100 yds. Not what I would expect, but could be a lot worse. Hope you get the problem solved, remember those Barnes need to be seated off the lands .050 or more. Check for coppers get after you shoot the. Was your Douglas air gauged and lapped..xx designation. My experience with rebarrelling is to not be concerned with the price of the barrel, and to have the best smith and reamer do the job. I have had at least 12 barrels done and all shoot very good, my Douglas have been acceptable, but not as good as the others, Bartlein, Kreiger, Rock Creek, Shilen, Brux, and a new company...X Caliber is an up and comer with their barrels. Good luck.

 

[Pheroze]

Good luck and I think definitely talk it through with the gunsmith. Seems to be a bit of a puzzle

For what it's worth mine has a shorter magazine too and the Woodleighs shot very well at the listed max COL of the cartridge.

 

[375 Ruger Fan]

That's a pretty fast twist for a .375, never heard of anyone using such. However it may not be an issue. It would tend to shoot the heaviest bullets best so try 300 and up. If I was having a rifle built in that caliber I would have gone with the std twist, its been working for about a thousand years just fine!

Twist rates have remained the same, but the same can't be said for bullets. The Greenhill formula is a rule of thumb or empirical equation that most rifle makers base the twist rate on. This formula was based on lead, relatively flat nosed bullets.

From Wikipedia:
In 1879, Greenhill developed a rule of thumb for calculating the optimal twist rate for lead-core bullets. This shortcut uses the bullet's length, needing no allowances for weight or nose shape.[4] Greenhill applied this theory to account for the steadiness of flight conferred upon an elongated projectile by rifling. The eponymous Greenhill Formula, still used today, is:

where:

• C = 150 (use 180 for muzzle velocities higher than 2,800 f/s)
• D = bullet's diameter in inches
• L = bullet's length in inches
• SG = bullet's specific gravity (10.9 for lead-core bullets, which cancels out the second half of the equation)

The original value of C was 150, which yields a twist rate in inches per turn, when given the diameter D and the length L of the bullet in inches. This works to velocities of about 840 m/s (2800 ft/s); above those velocities, a C of 180 should be used. For instance, with a velocity of 600 m/s (2000 ft/s), a diameter of 0.5 inches (13 mm) and a length of 1.5 inches (38 mm), the Greenhill formula would give a value of 25, which means 1 turn in 25 inches (640 mm).

 

[sestoppelman]

Fancy formulas aside, a 10.5 twist for a .375 is still an oddity. May work just fine, especially with heavier bullets. I would almost always prefer more twist than less as in my experience a steeper twist will shoot a broader range of bullets well.

 

[rookhawk]

Definitely 300gr Bullets or better yet 300+ grain copper Bullets (longer yet and that twist rate should favor them)

I'd play a lot with bullet seating depth on the lands. You'd be amazed how much that can have an effect.

 

[WestCo]

Only issue with the seating depth is the magazine length. Can't seat it out much further than the specs in the books and for sure can't get it very close to the lands for this reason.

 

[rookhawk]

I think you may need to have the barrel set back a thread turn or two to overcome this situation. I'd let a really competent barrel man have a look.

I don't think the barrel or the rifling is the culprit. I'd the focusing on the aforementioned set back issue and the crown as potential causes. Something about finish work and assembly is more likely to be a problem than the 20+" middle section of the douglas barrel.

 

[BWH]

I have a 24" barrel in my 375 h&h.... It's a 1:12 twist.

 

[rookhawk]

One more thought. Ignore the "need heavy bullet" statements. More precisely, you need long Bullets. Nothing will be likely to perform as well as a copper or bronze bullet of heavy weight because it's volume will be much greater than lead.

Did you try 300 grain copper or bronze?

 

[WestCo]

Think I might have found a load it likes. Shooting 300g TSX using Reloader 15. Getting sub 1" groups. Happy with that, much more confidence, which to me is half the battle.

 

[greyfox]

Westco,
I doubt it's the barrel, it's the BULLETS. Barnes tend to be LONGER than other brands because they're solid copper and copper is lighter than lead, THUS the same weight copper bullet, MUST be same diameter, HAS to be longer. Think of it this way: 1 Inch SOLID PVC pipe and 1 inch solid gold, and you need 1 Pound, (2.2 kg) The PVC would be about 5 feet long, the gold would be maybe 4 inches long, yet BOTH are 1 Inch, 1 Pound projectiles.

Save a BUNCH of money and headache, 2 choices: try Sierra, Hornady, or Nosler 300 gr with your loads. OR
Try some 270 gr Barnes, se if the groups tighten up.

Good Luck, Have fun, Don't overthink it!!