Rate of twist

[Andy Hawkins]

I'm planning on making a half scale or there abouts rifle for my daughter which means I'm going to be using a barrel of about 20 inches, give or take a couple. I plan on this rifle being a 40 calibre. It seems most 40 barrels are 1-48. Is this fast enough twist to take a longer barrel and cut it down or do I need to look for a faster twisted barrel.

 

[musketman]

The .40 caliber barrel will be a great starting size, as will the barrel length...

The rate of twist will somewhat dictate what she will shoot out of it, I will assume its a patch round ball.

My Lyman book has loads for the .40 caliber round ball (92 grain .395 diameter and a .028 patch) in a 44 inch, 1-in-66 inch twist barrel.
If the barrel is cut to 20 inches and she is still using round balls, the 1-in-66 inct twist will still be needed...

I thought a 1-48 twist is kind of like a "middle of the road" twist to cover both RB and Bullets somewhat well, but not favoring any one projectile.

So, cut your barrel at 20 inches, (if you're working in centimeters, make sure there's enough centimeters to make 20 inches) this should balance well for her and she can grow into the muzzleloader.

Like clothes and shoes, (kids outgrow them quickly) she may outgrow the rifle soon, you might want to make it just slightly bigger in size than she needs today, so that tomorrow she can still shoot.

 

[roundball]

The 1:48" twist in a .40cal is the optimum twist rate for that caliber round ball....ie: .40cal competition match rifles are made with 1:48" twist for shooting round balls so it should be as perfect as it can be for your purposes

 

[Birddog6]

The 1-48 twist is not a middle of the road twist for a .40 cal round ball. IMHO it is the best twist for a 40 caliber. I have built several 40 cal. long rifles & all of them had that twist and all ove them would shoot easily shoot one hole groups at 50 yards.

The basic common calculation for figuring the twist for a round ball is caliber x 120 = optimum twist.

.32 x 120 = 38 (1-38 Std.Twist)
.40 x 120 = 48 (1-48 Std.Twist)
.45 x 120 = 55 (1-56 Std.Twist)
.50 x 120 = 60 (1-60 Std.Twist)
.54 x 120 = 65 (1-66 Std.Twist)
.58 x 120 = 70 (1-70 Std.Twist)

Because most of the barrel makers only do one or two dif twists, you will usually find small bores up to .40 cal usually 1-38 to 1-48, and then the .50 caliber barrels & larger at 1-56 to 1-70. And then you have some of the barrel makers doing a 1-104 twist & etc. for those shoulder breaking loads ! ha ha !

These are not set in stone figures but just a calculation allot of fellers use. Personally, anything over 45 cal. I want a 1-56 twist or slower. For a 40 cal & under I like a 1-38 to a 1-48 twist & they shoot real good with the faster twist & smaller ball.

To me the most important thing in a RB barrel is the depth of the rifling, then the twist. I don't want any barrel with shallow grooves as they are harder to load a tight patch/ball combo & the rifling will not hold the patch as well as a deep groove rifled barrel. This is just my experience, your mileage may vary.

 

[tg]

The short barrel may be a factor but might not be enough of one to seriously effect accuracy as a rule when barrels get closer to pistol length the twists get faster in any cal.

 

[Zonie]

IMO the reason pistols have fast twists is because in the relatively short barrel and the minimal powder charge involved the velocity is low. Usually under 900 feet per second.
The whole idea with twist is to get the bullet up to a minimum RPM to stabilize the bullet. Because the velocity is low, the twist must be increased to get this minimum RPM.
In the case of a rifle with a 20 inch barrel I am sure the velocity will be over 1200 feet per second so the necessary spin or RPM will be more than adaquate to stabilize the bullet, especially a round ball which doesn't require high rotational velocity to stabilize it.