Thinking this twist rate is too slow for optimum results with the heavier bullets for long range work. What might be the heaviest, most aerodynamic mold choice to stretch the capabilities to the maximum?

Thanks Vall. Back when Marlin was making the Cowboy in .38-55 with the .379 bore, I took a suggestion from the late Ken Waters and had Dave Corbin build me a set of swaging dies to make .379 jacketed bullets, which I briefly sold under the name of the Boer Bullet Company. Ken even gave these a brief writeup in Handloader, bless his heart. These were 220-grain jacketed softpoints and they shot brilliantly out of the Marlin barrel. I did not pay attention to the twist at the time, because I was thinking more about smokeless hunting loads than lobbing heaver greasers way out yonder with black powder. I can imagine a 1:16 or faster barrel would be better suited for the heavies. Hope all is well with you, friend.

Hi Bill,
When did you move to NM?

The 18 twist has been used for many years in SS competition with 300 - 330 gr, Pope/Hudson style bullets. 18.5 gr of 4759 was a old (1970-80's) match load with those bullets.

In 1987, I bought the only 38 cal spitzer mold that I could find. It was a NEI, no long available but, you could have it made.

I also designed a 38 cal that will stabilize in a 18 twist and be competitive at 200 yards, with the 214 - 220 gr 32 cal spitzers.

Recoil is the main issue with the 38 but, it's a excellent caliber.

Frank

I won a .38 18:1 twist Atkinson and Marquette barrel.

It shot extremely well with a 310 grain Boomer Money bullet - just a little less pointy than Franks bullet.

Harvey Trace was pushing 360 to 370 grain bullets in 14:1 twists with very good effect on BPCR Silhouette.

We were using the .38-50 which is very close to the .38-55.

How far you're pushing the bullet also has a fair amount to do with whether a particular weight and twist rate will work well or not. At 200 yds. or less you might find the 300 gr. bullet  is great in an 18" twist. But if you start taking it out to longer distances it might begin to destabilize and might not be as accurate.

My NEI( Walt made) will stabilize to the ram line with no problem in an 18 twist. Its a round nose bullet that weighs 290 grains.

For my 310 grain Boomer Money bullet the gyroscopic spin factor is:

Caliber      0.378      Inches            
Bullet Weight      310      Grains            
Bullet Length      1.18      Inches            
Barrel Twist      18      Inches/turn            
muzzle velocity      1300      fps            
Temperature      50      degrees Fahrenheit (59 is standard)            
Pressure      29.92      inches of mercury (29.92 is standard)            
                       
Sg =      1.72                  

But there is a difference in the bullet's capabilities depending on the nose shape between Frank's Spitzer 310 grain bullet and my rounded nose 310 grain bullet.

The Spitzer is excellent at being less susceptible to the wind out to 200 yards but the dynamic balance means it becomes less stable out at about 300 yards and does poorly out to 500 yards. 

That is where the blunter nosed 310 grain bullet performs; as is it will stay more stable out to the Rams at 550 yards.

So for the Schuetzen game the Spitzers are a better bullet but for BPCR Silhouette it is better to use a rounded nose like the Creedmore, Postell, Money or Elliptical because dynamic stability of a bullet becomes more important at longer distances shot.