Barrel twist vs boolit length

[Racing]

I´ve recently come to play around with paper patching a bit,and was recommended a given boolit length for a approx 40cal carbine of Wesson style.

That got me thinking a bit though,looking at the actual boolit. As for length vs rifling twist,wouldn´t the operative word here be the actual load/bearing surface of the boolit and not the actual total length?

Ie; a given twist will take a given boolit bearing surface length? When paper patching this of course comes out a tad different vs a grease groove setup seeing that the patch first of all "builds" diameter,and at that the patch often extends into the ogive.
With a GG boolit it´s just plain..what you see is what you get.

Just a thought..but wouldn´t this be a more correct n proper manner in which to advance this?

CG of a boolit of course comes into play,as does BC and what not,but in reality that´s a matter after the slug has left the muzzle. The actual spin we put on it though,just me thinking out loud here,would be imposed due bearing length right? Or more to the point,the limitations thereof.?

Yeees..i´m all familiar with the given formula et al,it´s just that i wonder if what i´m proposing here wouldn´t be a more correct way to regard it all?

[charlie b]

Twist does have a bearing on how much leading you might get from a given bullet. The faster the twist the more you might experience leading.

When dealing with lubed bullets the bearing length and amount of lube held in the grooves is important to consider, regardless of twist, but, a fast twist will cause leading before a slow twist given the same velocity and bullet. So, a long bearing surface that has a minimal amout of lube will show leading in a fast twist more than a slow twist.

BUT, none of this is applicable to whether or not the bullet will be stable once it leaves the muzzle. Once out of the muzzle aerodynamics are key and with most ML designs the simplified equations will work, as long as the bullet is supersonic. Just keep in mind the equations are designed around a muzzle velocity of 2700fps and a football shaped bullet. There are some correction factors for lower than 2000fps but they do not tell you what will happen at subsonic or during transition.

FWIW, I have found the equations to be fairly accurate for typical ML bullets. For example, with my Lyman 1:32 twist a 450gn PP bullet is marginal in stability while a 550gn is not. And, that was the case when I fired those bullets on the range. The 450gn bullet was stable until the temps dropped and the air density changed. If I had used the 'long form' of the equation I would have seen that.

[GregLaROCHE]

« The actual spin we put on it though,just me thinking out loud here,would be imposed due bearing length right? «

In my opinion and I could be wrong, the spin of a boolit shouldn’t be effected by the bearing length, unless there is some slipping tendance of the boolit not being engaged deep enough in the rifling.

What are these equations that are being talked about? Where can they be found ?

[charlie b]

http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi
Simplified version

The long version I use came from an Army technical publication on external ballistics. You can probably look up the miller stability equation and then find where the source equation is along with all of the assumptions that went into the simplified version.

[GregLaROCHE]

Thanks Charlie b.