Rate of Twist Question

[Owen49]

Can anyone tell me the standard rate of twist for two similar black powder cartridges, the .45-120 Sharps and the .450 Black Powder Express? I've been all over the internet trying to find out and am having no success. I did find the rate (1 in 15) for the .450 Nitro Express but don't know if they kept the same twist as its .450 BPE parent. Thank you.

[Don McDowell]

If you're talking about original rifles, there never was a 45-120 sharps, the longest was a 2 7/8. Sharps used either a 1-20 or 1-18 twist in their 45 caliber rifles, just depending on whether it was a sporting rifle or a target rifle.

[Lead pot]

Use a good cleaning rod with a good swivel handle that will turn when pushing a tight patch down the bore. Wrap long strip of masking tape on the rod about 2" from the jag on up past 24" from start to finish and draw a straight line the length of the strip of tape. Put some tape on the muzzle with a reference mark then insert the rod with a tight patch and put a reference mark on the rod tape that matches the muzzle mark and draw a line around the rod tap. Push the rod down the bore till the line on the rod matches the muzzle reference mark put a second line on the rod tape at the muzzle. Now you can measure a complete rot between the two lines start/finish. but it takes a rod with a good free turning handle.

[AntiqueSledMan]

Greenhill formula

T=150 x D
_____
R

T is the twist required (number of inches for one revolution),
D is the bullet diameter (in inches)
R is the bullet length to diameter ratio, (length divided by diameter)

Conversely, to find out what length bullet will be stabilized in a given twist, use:




L=150 x D x D
___________
T (that is, 150 x D squared divided by T)

L is the bullet length
The number 150 is a constant used by Greenhill and works well at velocities in the vicinity of 1500 f.p.s. or greater. At 2800 f.p.s. the constant can be changed to 180 with good results.

Note that it is bullet LENGTH, not weight that is important. Greenhill works well with all lead/lead-alloys commonly used for bullets.

Hope this helps, AntiqueSledMan.

[martinibelgian]

Anything 'express' used a light for calibre bullet and had a slower twist. Ratio powder / bullet weight could be at around 3:1. And there were lots of .45 bpe....

[BrentD]

antique sled man, for BPCR bullets, it is generally agreed upon that the Greenhill constant of 150 is too big. A number closer to 125-130 is what seems most compatible with observed performance.

There are other formulas as well since most of this has been empirical curve fitting and not theory derived from first principles. They tend to agree fairly well with the 125 constant in the GH formula. Some of these alternatives use velocity as well as twist rate, which makes some sense.

Anyway, you might try 125 instead of 150 if you are planning your next barrel or your next mould.

[AntiqueSledMan]

Hello Brent,

Thanks for the information. I have used this formula for determining proper bullet & twist for muzzle loaders & it seamed to work fairly well.

AntiqueSledMan.

[Good Cheer]

Anyone here used the RCBS 378-312-BP?
Thinking on giving it a try with this rascal.