Twist rate, boolit weight, speed

[Rickk]

Not sure if the is the correct area for this post but I am working with a 30-30 so perhaps it is.

Can someone help me with the relationships between barrel twist, boolit weight and speed? I understand (or think I do) that long for caliber, heavy boolits require a faster twist than short or light boolits.

I guess this is my main question: If the barrel has a faster twist than needed or desirable for a certain weight boolit, can I speed the light boolit up or slow it down to partially compensate?

Specifically, the little "soup can" 117 grain Lee boolits work wonderful in my old prewar Mod 94 Winchester 30-30. Seem to be more accurate than I can aim with the open sights. Settled on a load around 1400fps which would do a half-inch at 50 yards pretty consistent.

I just got a new barrel for my TC Encore in 30-30. I was pretty sure that if the old '94 would do as well as it did, the Encore should put them all in one hole using a 4x scope.

Not so. While it did better with my 180 grain boolits, groups with the little soup can were around 2".

My understanding is that the Winchester twist is one-in-twelve while the TC is one-in-ten. Is one-in-ten just too fast for the little boolits? Or do I just need to do some load development?

Thanks in advance. I appreciate this forum very much, have learned a lot in the past months here.

RickK

[mooman76]

Look for the greenhill formula. It is actually the bullet length rather than weight but of coarse a longer bullet will be almost always heavier. Its is only an estimate though and nothing absolute. A shorter bullet should be ok in a fast twist but it doesn't always stablize properly or as well and when pushed to hard it will jump the lands so it doesn't get as much spin to stablize. You really have to try and see or ask others with similar guns what they use that works.

[Casting Timmy]

I think a Paul Mathews book said something about using the Greenhill formula with 125 instead of a 150 for cast bullets.

Every gun is a little different, so loads in one might not be the same in another.

The twist rate will tell you how heavy or long a bullet you can stabilize. (Longer bullets will always be heavier for a caliber) A twist rate that is just ideal for you bullet will give the best accuracy, but gun makers usually give the gun a twist to stabilize the heaviest bullets. That way the gun can shot anything you can buy for it accuracately.

I think there was a factory gun that came out as a slower twist rate so it would do great for variminting with light bullets. But the gun faded out when people tried to use it for heavier bullets (because it would never shot right). It didn't have the correct twist to stabilize the heavier bullets.

[Larry Gibson]

Rickk

Yes there is a definate reason for the difference. One is the twist. The 12" twist will not spin the bullet as fast as the 10" twist. The second is the longer barrel of the Encore gives additional velocity which worsens the situation. Spinning a cast bullet to fast can lead to poor accuracy or at least not as good accuracy as you've found. Shorter bullets of a given caliber require a slower twist for best accuracy. Probably why the slower twist of 18 -20" for the 30 Carbine shooting 107 gr bullets and probably the 32-20 also.

You can shoot a 115 gr GC'd cast bullet at 40,000 psi out of a 18" twist .30 Carbine at 1900 fps with excellent accuracy. Shoot the same bullet out of a 10" twist 30'06 at 40,000 psi and the velocity will be about 2800 fps and accuracy will be minute of target frame. Drop the velocity down to 1800 fps and accuracy can be very good. Has to do with rotational speed or RPM. I will note here that one individual (we will probably hear from him shortly) no doubt has shot said cast bullets at great speed with exceptional accuracy as he does with everything. However for us who don't we must understand what twist and velocity can do to accuracy as you've found.

BTW; are the 117 gr soup can bullets GC'd?

Larry Gibson

[Rickk]

Larry,
Yes, the bullets have GCs. From what you say it sounds like the first thing to do is start lowering the velocity. I wasn't sure which way to go, up or down but down sounds like the plan. I want to eventually shorten the barrel to use as a backpack rig but wanted to make sure everything was working before doing so. Now I realize I may also have to tailor the loads to bbl length!

When I ordered the barrel I had hoped the same loads would perform in both the 94 and the TC. I didn't even think to see if the twist rate was the same until after shooting a few groups. So much to learn. So little time. It is great to pull on the wisdom of those who have been there.

Thanks

[StarMetal]

Rick,

First the TC is an entirely different bore and chamber throat then the old Winchester 30-30. I would say there's not enough difference in twist between those two barrels especially at that low of a velocity. I say it has all to do with, in your case, load development and specifically throat fit of the bullet.

I'm the individual Mr Gibson speaks of. I and other's have blown his rpm theory or threshold theory out of the water. The first rifle was a 7mm-08. Larry said that I just had an exceptional rifle (there's proof of that statement from him too), then I did it with a 6.5 Grendel on an AR15, then I did it with a 6.5x54 Mannlicher Schoenaur on a Savage 110, and most recently with the 6.5x55 Swede, which by the way Larry even with 3 loads now you haven't been able to duplicate. Sorry fellows for going off subject here, but Larry hung the shingle for the invite. Larry seems you dug a hole for yourself in the thread about "beware of the man with one rifle". Bad day huh?

While we're on twist Mr Gibson please explain why such firearms like the German Luger in 7.65 caliber (known as the 30 Luger) shooting a short bullet under 100 grains, has a 9 1/2 twist.

Rick tinker with you your load. I just recently kept my 136 grain 6.5 cast bullets on the target paper out of my Swede at over 2800 fps. They did not go off the targer frame such as Mr Gibson would have us believe.

[Pirate69]

Has anyone compared the Don Miller Twist Rule to the Greenhill formula in this case. The Greenhill Formula seems to be independant of velocity where the Miller Rule is not. The Miller Rule gives a stability factor at a selected velocity.

[Doc Highwall]

I think for most guns and bullets the twist is not as important as bullet fit first and load for the alloy second.

[Wayne S]

Has anyone compared the Don Miller Twist Rule to the Greenhill formula in this case. The Greenhill Formula seems to be independant of velocity where the Miller Rule is not. The Miller Rule gives a stability factor at a selected velocity.
I think I remember reading some where that the "greenhill Formula" is based on 1800 FPS
Two to play with
http://www.jbmballistics.com/cgi-bin/jbmstab-5.0.cgi
http://www.uslink.net/~tom1/twistrate.htm

:bigsmyl2:

[Rickk]

Update on Encore: Yesterday I ran about 20 J-words through it just to smooth things out a bit.
Today I cut the barrel to 20", squared the cut and crowned the muzzle.
Same little boolits that grouped 2" now group 1".
The "hunting load" I developed for my Win 94 (Lee 180 grain with 3031 powder at 1850) that was shooting into 3/4" two days ago (in the Encore) now goes into 3/8' (three-shot groups at 50 yards)
The 180 grain boolits show light engraving from the lands when chambered then ejected. I guess that is good?
Any other Encore 30-30 shooters out there?