Twist rates v powder charge

[Tinker2]

Sorry about that.

William

 

[Tinker2]

I have CVA Mountain Stalker in .54 with a 1:38 twist
I have not shot it much but with 50 grains FFg and a patched round ball it’s deadly.




William Alexander

 

[Col. Batguano]

Some interesting math observations;

We've got a couple of other elements in play here. We have acceleration rate, and ball obduration. The faster the ball accelerates (and more maleable the metal) the more it will deform, and larger surface area of the rifling it will engage However, that same obduration will take it out of being a perfect sphere.

If we can agree that a major league 4-seam fast ball is stable, the average rpm's on such a pitch is around 2250 rpm's. With a circumference of 9.0", that gives us a rotational surface speed of approximately 50' per second. It's not a perfectly smooth sphere, but it travels as one.

Now let's look at a round ball. Assuming a 36 caliber ball (.350 diameter / 1.1" circumference) exit velocity of 1700 fps we have a rotation rate (48" twist) of 425 revolutions per second, or about 39' per second, which is remarkably close surface rotational speed.

Because a round ball has a relatively small surface contact area, the faster the acceleration, the more likely it will be to skip or jump the rifling. Square bottom rifling will have a better "bite" on a ball to engage it than round bottom rifling.

I'm not entirely sure what all these computations mean, but it would seem to say that what we're really after, is a surface rotational speed. This would seem to imply that low exit velocities are going to need a faster rate of twist than faster ones.

What I haven't factored in here is the drag coefficient of the air resistance, as that resistance is not a straight line based on velocity.

Somebody else please take this from here regarding RB and bullet stability.

 

[BrownBear]

I'm not entirely sure what all these computations mean, but it would seem to say that what we're really after, is a surface rotational speed. This would seem to imply that low exit velocities are going to need a faster rate of twist than faster ones.

Don't forget the assumptions that go with the computations. They're as debatable as the numbers.

 

[Eterry]

I have CVA Mountain Stalker in .54 with a 1:38 twist
I have not shot it much but with 50 grains FFg and a patched round ball it’s deadly.


Me too, but I didn't wanna drive 12 hours and use it on elk. I haven't chrono'ed mine, but Lyman indicates velocity in a 43"barrel at 1100fps. If it hit a rib or shoulder I don't think it would penetrate enough. A lung shot deer wouldn't go far, but I didn't want to chance it.

ET

 

[Tinker2]

I like it, plastic stock and all.
When I got it I decided what load I wanted to shoot in it (50 gr.) and never tried anything else.
Does what I need it to
I don’t hunt anything bigger than mule deer any more, lately anyhow
And then I mostly hunt deer with a .62 flintlock smoothbore with 55 gr. FFg


William Alexander

 

[Tom Compton]

That's interesting, but (intuitively thinking) it doesn't seem like the same twist rate would be "best" for all those various calibers does it? I mean, the suppository rifle crowd uses VERY different twists based upon projectile weight and velocity. For example, a .22 l.r. uses a 1:16 twist, but a .223 uses a 1:9 twist. That cf round is shooting a bullet 50% heavier, but 3x faster. The actual rpm's these things spin at (15 million rpm's) is absolutely mind boggling.

I suspect the bigger companies settled on their twist rate as "good enough" rather than actually researching which one would actually be the ideal twist rate for the individual given caliber / ball weight / diameter / rpm range. thus, the decision on twist rate became one of economics more so than one of what is absolutely the "best". Since most barrels are sold to be dual purpose (field and target range), they settled on the twist so as to satisfy as many of their customers as they could, rather than truly custom build it for 1 purpose and velocity range. Of course, I have absolutely NO data to back that up. However, it makes sense, since they ARE a business, and business decisions get made for business purposes.

The other thing that makes me think a faster twist might be more appropriate, is that in a given caliber, the twist rate for rifle barrels is much much slower than the twist rate in the same caliber for pistol barrels.

Gets you to thinking doesn't it? I was going to have Charles at FCI build it for me, and he'll put in any twist I want, so that's why the inquiry. It's easier to do the research ahead of time than it is to find out you guessed wrong afterwards.

“Normal” pistol powder charges are lower and combined w shorter barrels result in much lower muzzle velocities than a rifle of the same caliber and “normal” charges. To spin the same diameter ball at an optimal revolutions per second, pistols must have much faster twists.

Similarly a rifle w faster twist rifling will reach optimum RPS with lower powder charges. If a shooter is only shooting paper w a .50 caliber at 25 yards in no wind conditions a very fast twist can allow say a 30 gr charge. If hunting a faster MV will flatten trajectory and be less susceptible to cross wind.

So the same twist CAN be appropriate for different calibers depending on powder charge.
TC

 

[azmntman]

Id shoot 75 gr min at an elk, 50 ok for deer. :idunno:

 

[Eterry]

After a few range sessions and prbs all over the place I never hunted with it. It sits beside the safe, I needed the room. I'm not sure I'd use 50gr ffg on deer. My buddy did, longest tracking job of his life. Buck had a perfect 54 cal hole through its heart, no one told it to die.

 

[azmntman]

Had that same thing with a lil cow elk once using a 30-30. I shot, knew I hit heart, turned n went to the truck for ropes n such and went back....she got up and ran and a 2nd shot did it. Perfect hole in heart. Guess bullet didnt mushroom?

50 min for deer BUT just always shoot yer most accurate load (mine r all near 70-80) and no worries. Ya some animules have an incredible will to live, I know I would :idunno:

 

[hanshi]

It's amazing how much damage some higher organisms - deer/elk/humans/etc - can sustain and still function for a length of time. I remember the story of an off duty woman cop who was attacked and shot through the heart with a .357mag. She pulled her 9mm and chased the maggot down and killed him. She, unbelievably survived.

So, failure of game to drop quickly is not necessarily an indication of a bad shot or underpowered rifle. The critter just doesn't realize it's been killed.

 

[1100 Remington Man]

I'm getting good accuracy in my .54 Rengade 1-48 twist and 90 grains Pyrodex and .530 patched round ball .018 patch.
I don't even think it kicks very hard.

 

[hanshi]

That load will kill any whitetail around.

 

[RedFeather]

One thing about the Greenhill formula, since it was brought up. The constant (K) is not fixed at 150. It varies up or down depending upon velocity. For example, high velocity varmint rifles use a higher constant. I've found that 125 yields muzzle loading twists that match pretty well with what are considered "standard".

 

[M. De Land]

I think I read somewhere that the Hawken Bros. rifled most of their rifles regardless of caliber with a 1 in 48 twist which seems fast to me for any round ball rifle. A ball just does not need much spin to be accurate as attested to by some smooth bore guns.
Generally the tighter the twist the greater the fouling build up.
I've had 1 in 48 through 1 in 66 and could not tell any difference in accuracy potential.
I think it has a lot more to do with straightness of bore and evenness of diameters than it does with twist rate when it comes to accuracy.

 

[mooman76]

I think I read somewhere that the Hawken Bros. rifled most of their rifles regardless of caliber with a 1 in 48 twist which seems fast to me for any round ball rifle. A ball just does not need much spin to be accurate as attested to by some smooth bore guns.
Generally the tighter the twist the greater the fouling build up.
I've had 1 in 48 through 1 in 66 and could not tell any difference in accuracy potential.
I think it has a lot more to do with straightness of bore and evenness of diameters than it does with twist rate when it comes to accuracy.

I heard that too. I also heard the reason( or thought) they rifled them with 1/48 was because that is all they were set up to do.

 

[Artificer]

Yes, but if the Hawken Shop wanted to make other barrel twists, they could have made the rifling guides and heads of rifling machines for as many types of rifle twists that they wanted.

Gus

 

[Rifleman1776]

In museums that post information about their ml rifles, the 1:48" twist constitutes 99% of their collections. Slower twists require larger charges to attain optimum accuracy. I have often bemoaned here the trials I have had with my Jaeger and it's 1:72" twist barrel. Optimum sweet spot is north of 120 grain charges. More than I care to shoot.

 

[hanshi]

As long as the rifling is deep enough you'll likely find little difference in the accuracy of 1-48" vs 1-66" regardless of powder charge.

 

[M. De Land]

No bullet spins 15 million RPM's. Rotational velocity is figured by converting turns per foot x velocity in feet per second x 60 for revolutions per minute(RPM's)
If a .223 has 1in9 twist and muzzle velocity of 3200 fps then the rotational velocity of the bullet is 144,000 rpm's.
The bullet makes 3/4 of a foot of turn in one rotation x 3200 fps of muzzle velocity x60 seconds in a minute which equals RPM.
Rotational velocity decays along with forward velocity but not as quickly.