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Old November 21, 2010, 09:08 AM   #1
10 Spot Terminator
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Best barrel twist rates for cast boolits ???


So I have been casting for a couple of years now starting out loading for my pistols and not that long ago started to cast for a couple of rifles and am getting some fair results for accuracy at less than stellar performance. My quest is taking me into improving ballistic performance without getting into such things as paper patching . I am aware of how key issues such as alloy and BHN along with proper bullet fit influence potential bullet performance and as recently found out pressure ( PSI and CUP ) are the controlling factor based on BHN as to what can be deemed the "upper limit" for any particular bullet and powder combination before gas cutting and iminent failure occur. So that being said I guess what I am interested in is how rifling twist rate may come into play insofar as velocity vs. BHN to get an idea as to if the softer BHN alloys are more tolerant of slower twist rates in a barrel when driven to say 2000 fps vs faster twist rates to avoid overdriving the riflings . How little twist rate might actually be needed to stabilize a cast boolit ? I would like to stay around 22 BHN on my rifle casts so as not to have the bullets become brittle and have other issues. Little things like this cause me to lose sleep sometimes if you can relate to that. Any personal insight or steerage to some articles on this subject ( if any ) would be greatly appreciated guys,,,

Thanks, 10 SPOT
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Old December 1, 2010, 04:51 PM   #2
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I just saw this has no responses in my way of thinking the longer twist will be better for cast .

Not torquing on the bullet so bad , but there is a balance of weight & twist!

This is not a cast bullet related incident but I had a Rem model 7 in 243 that would not shot a 100gr bullet , dropped back to 87gr & it would drill em all day long cold,hot ,dirty or clean bore !!
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Old December 1, 2010, 11:38 PM   #3
10 Spot Terminator
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Yeah GP100MAN ,,,

My gut feeling was the same about twist rates and was thinking about different rifling styles as well. One of the most accurate lead shooters I have seen to date was an old 96 swede . Have you ever looked down the bore of one of these ??? Looks like the bore on a Howitzer. Rough tough and viscious only begins to describe them. The reason I am mulling this around in my pea brain is I am thinking of putting together a dedicated lead shooter for target and varmit . I have seen what rifling can do for jacketed bullets when taken to the extremes such as those Krieger produces . After looking at the Swede maybe an aggessive slower twist would fit the bill ,,,
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Old December 2, 2010, 09:34 AM   #4
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I've never really had any problems with twist as I've been lucky and my guns have the twist I need (or close to).

The one exception being my Roberts. I just changed boolits for it thinking the longer, heavier boolit would be better and didnt check the twist.

At 50yds I'm getting 15" groups and the boolits hitting the target sideways. The gun has 1/14 twist and near as I can tell, I need 1/9 twist for the length of this boolit. Thats according to the twist formula at the LASC site.

I'm going to try a couple things and see what happens.
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Old December 2, 2010, 06:03 PM   #5
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Lead bullets obey the same laws of physics and air resistance that jacketed bullets do. Your Swede may have a 220mm twist, like mine does (a rebarrel job). That's one turn in 8.66 inches. It may also have a 190 mm twist (7.48"). Measure it.

After that, if you know the weight, length and velocity you expect to fire these bullets at, you can use the barrel twist estimator at the JBM ballistics site to see how stable they are likely to be. To minimize the amount of twist per the weight and velocity, you want to look at something like a gyroscopic stability factor of s=1.4 or so.

You may want to consider firelapping the bore to smooth it for lead shooting. Be aware that if you switch to jacketed bullets after using lead, at high power rifle pressures that can cause a pressure spike. Also, lead seems to like to accumulate on copper fouling. Whichever bullet type you normally use, you really want to clean it out thoroughly before you go on to shoot the other type.
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