Info on Gain Twist Rifleing

[SHohen]

I read somewhere that the cal 50 BMG uses gain twist rifleing. I assume
that this means that the barrel twist rate changes as a function of
distance from the chamber.
Question 1. Is this true?
2. What does gain twist rifleing do for the cal 50 BMG?
3. Has gain twist rifleing ever been used on any smaller
calibers?

[Rock McMillan]

# I read somewhere that the cal 50 BMG uses gain twist rifleing. I
# assume that this means that the barrel twist rate changes as a
# function of distance from the chamber.
# Question 1. Is this true?
# 2. What does gain twist rifleing do for the cal 50 BMG?
# 3. Has gain twist rifleing ever been used on any smaller calibers?

You are correct in assuming that in gain twist barrels the the rifling
twist rate changes over the length of the barrel, starting shallow and
ending up at the normal twist rate. I think the idea behind this is to
achieve a higher muzzle velocity because it reduces the energy to
accelerate the bullet up to the final RPM. I have seen 20mm cannon
barrels with gain twist and have seen some .30 caliber barrels made by
Sam May at Apex Rifle Company that were gain twist barrels, if I
remember right Sam's barrels were M1A barrels. I have not seen any .50
caliber barrels done this way.

I can understand why a cannon shell might benifit from this, you have a
multi part projectile and the abrupt acceleration of a standard barrel
might upset some of the internal components. Plus you have a short
contact area on the driving band on a cannon shell.

For regular bullets the main dissadvantage I can see is that as the
twist rate increases the helix angle of the rifling changes. This
creates a condition where the engraved rifling is changing its track on
the bullet as the bullet moves down the barrel. Think of placing a file
lengthways on the outside of a bullet and then filing a shallow groove,
drop the file into the grove and then twist it to the side, this is what
the rifling is doing on the bullet and it just doesn't seem like a good
idea.

This process has been around a long time and if its advantages
outweighed its dissadvantages you would see lots of rifles made with
gain twist barrels, you don't, so I assume it's just not worth the
trouble.

Anyone with any different ideas or experiances please post them, I'm
never afraid to learn something new or look at something from a
different direction.

[Bartbob]

Gain twist rifling has been around for over 100 years. The famous
barrel maker Harry Pope put Stevens-Pope gain twist barrels on
the Krag-Jorgenson (yes, the .30-40) rifles used by the US long
range rifle team in 1902. And over the years, it has been proven
by good testing that gain twist barrels distort the bullet too much
for best accuracy.

BB