twist rate question?

[airaddict]

Extra velocity means less drop....but on the flip side.....smaller grain pills mean more effect the wind has at those longer ranges.

Brian

[Sundo]

A faster twist does dump rotational energy into the bullet, but the energy difference between two twist rates will be miniscule. The effect of twist rate on velocity would be my least concern related to twist rate.

A given bullet has a minimum twist rate required for stabilization. To the first order, the accuracy of a perfectly uniform bullet will not be hurt by faster twist. A real bullet is not perfectly uniform. A faster spin rate will increase the wobble resulting from these imperfections, and hurt accuracy. For best accuracy, the minimum twist required for stabilization is best. High quality (more uniform) bullets are less affected by excess spin rate than cheap (less uniform) bullets.

A good example is 55 gr .224 bullets in 1:7 twist. 1:7 twist is overstabilized for 55 gr. Cheap .223 ammo generally gets 2-3 moa while match ammo can get 1 moa or better. The better uniformity of the match bullet makes it less affected by excess spin rate than the cheap bullet.

There are consequences to excess stabilization. With the appropriate minimum twist, the bullet will "nose over" in flight as the trajectory curves downward toward the ground. With excess twist, the bullet will be more resistant to nosing over. This increases the angle of attack and increases the downrange drag, reducing downrange velocity (not muzzle velocity). Within the suitable range of twist rates, this difference should be very small.

Of course, an extremely fast twist can rip apart the jacket of the bullet with excess centrifugal force, so there is a limit to the tolerance to overspin.

[308law]

I hadn't thought about the possible loss in accuracy, but that is an interesting thought.

[bootsmcguire]

Great point on the accuracy. I am really thinking the CBI in a 14" twist is going to be the way to go for me shooting 55 and 58 grain bullets.

Funny part is I ordered some of the blemished 105 A-Max's from Midway to use in the 6BR and the 243AI I have now, but the new barrel will be for the 55's and 58's. Now to procure some funds ::)

[scope eye]

All my barrels are what you would consider slow twist my 223 AI is a 12tw sends 40grn's 4000 fps, and my 22-250 is a 14tw those 40grn's are good for 4400fps, and my sweetheart is my 220 swift with a 16tw yes that's right 16tw are good for 4800fps, I have hit 5000fps + with 35 + 36gr bullets, the swift is good for almost 500yds with out having to dope the scope,

Tanks Dean

[scope eye]

And for those of wondering about throat erosion, "I get that all the time" I never let the barrel get hot and when I go to the range, I always have at least three rifles with me and cycle them so they have plenty of time to cool off, and all of them continue to group in a tight and orderly manner.

Tanks Dean

[308law]

I would think the slower twist would also help with the erosion problem, but that is just an assumption. I would think it would be easier for the bullets to engage the rifling and the gas cutting in the throat would be kept to a minimum.

[nsaqam]

Throat erosion is solely a function of gas temperature and abrasiveness and the effect the bullet has is inconsequential.

I still say twist it fast and give yourself more options now and down the road if your needs change. Punch it out to .22-06AI and you're gonna want to throw 105's and 115's. And you ain't gonna blow apart a Barnes TTSX, Varmint Grenade or a Hornady GMX no matter how fast you spin them.

A whole lot easier selling a fast twist if you should ever decide to do that too.

[bootsmcguire]

nsaqam you are right, a 8" twist would sell better. I have never had luck with Barnes Varmint Grenades, and to use GMX's (and TTSX's IIRC) I'd have to jump up to 80gr pills. I want to shoot 70gr and less and mostly the 58gr V-Max. My current barrel is going to get punched up to something bigger (wondering about 6-06AI) once the throat gets too bad, and when that happens I'll be all set to run 105's (which I just laid into a supply thanks to Midways Blem Sale). I guess for me it comes down to, what is the best choice to shoot 55-70gr bullets as accurately as possible (and hopefully as fast as possible)? So far it seems most of the data points to a slower twist for that purpose.

[scope eye]

I know my way of doing things is not the norm and I'm fine with that, but I have found something that works for me and my style of shooting, and anyone who has shot my rifles and my loads all say the same thing, they say nothing, they all have that look, then after a while they utter I had no idea, I just though you were always full of it and your obsession with velocity, I did not realize a bullet could travel that far before dropping, sell me this rifle, No build your own, It cracks me up every time, it also vindicates me from all the krap I put up with about approach to firearms and ballistics, I have never built a dud so far so good, my next project will consist of a 300 RUM with a Shillen 17 twist that should be interesting

Tanks Dean

[bootsmcguire]

300RUM on a 17 twist. Now that should be interesting. Planning on launching some 110's at the speed of light are we?

[nsaqam]

Don't know where I got to .22 caliber in this thread. ??? ???

[bootsmcguire]

Don't know where I got to .22 caliber in this thread. ??? ???

I think we all knew you meant 6mm, especially with mention of 105s and 115s in the same sentence. Unless they started making super heavy 22cal bullets that I'm unaware of. :D

[scope eye]

Hey Boots how did you know that was my intention ;D and Lapua makes a nice 100gr HP pill, some one said in an earlier post, I might have trouble selling a slow twist barrel, maybe so, but If some one shot it I wound not, anyways I am right where I want to be, so no more selling hardware that's what KIDNEYS are for you sell them instead

Tanks Dean

[scope eye]

I heard CBI is coming out with a 14 twist in 6mm, then all I have to do is make up my mind between a 243AI and a 240 Weatherby :-\

Tanks Dean

[keeki]

just go ahead and do a 6wsm with a 14twist. You get to keep the short action and plenty of powder ;D

[scope eye]

Hey Keith I can't even tell you why, but I have never been a fan of any of they WSM based cartridges, and there's no rhyme or reason I'm just not ???

Tanks Dean

[keeki]

I cant tell you why, but I havnt either. Ive got a 270wsm now for my son, so Im gonna try and work up something with it. who knows, I may fall in love

[bootsmcguire]

I heard CBI is coming out with a 14 twist in 6mm, then all I have to do is make up my mind between a 243AI and a 240 Weatherby :-\

Tanks Dean

Yup talked to Jim Briggs this week, and thats what he informed me. He indicated that they should be available by the end of summer.