Twist Rate

[Finney]

I am putting a new barrel on my 243. I shoot 70 & 85 grain bullets. A barrel maker has sugested 1 in 12 twist. How do you determine what twist is needed for a specfic weight bullet?

 

[jo191145]

There are twist calculators available on the web. I don't put much faith in them myself.
Last time I played with one my 6ppc would not stabilize a 68gn pill until it hit 3900fps.
My ppc totally disagrees ;D Maybe I did it wrong anyway.

FWIW Bullet length is the big factor. Weight has little to do with it.
A 12 should be just fine for anything in the 85 range. Personally I'd rather have more twist than not enough tho.

 

[Tomahawk608]

iv got a 1-10 twist and it shoots 75g vmax like a dream

 

[fdshuster]

Finney: Many years ago Remington tried a 1-12 twist when they introduced their otherwise excellent 6mm chambering, called the 244 Remington. it was a complete flop, while the very similar Winchester 243, with a 1-10 twist took off and never looked back. I've had several 243's, Douglas & Shilen barrels, very good choice, if you can accept their relatively short barrel life. If I did another it would be in the proven 1-10 twist.

 

[Finney]

Thanks guys, My old barrel is a 30in Federal 1-10, is was built in the early 70's. I think I am going to give the 1-12 a shot as I have used quite a few barrels from this company and have always had good luck with them.

 

[chambered221]

Here's a link to a ballistic calculator, I've recently started to use this in conjunction with twist rate charts.
http://www.jbmballistics.com/cgi-bin/jbmstab-5.0.cgi

The ideal range is 1.3-2.0, Anything under 1.3 the stability is compromised. Over 2.0 doesn't mean it's no good but accuracy may improve by getting it under 2.0

 

[Laurie]

There are twist calculators available on the web. I don't put much faith in them myself.
Last time I played with one my 6ppc would not stabilize a 68gn pill until it hit 3900fps.
My ppc totally disagrees ;D Maybe I did it wrong anyway.

Joe,

I use the Don Miller's Twist Rule spreadsheet regularly to calculate stability coefficients (Sg Values) for various twist rate and bullet length combinations. I'd imagine the JBM one is the same or similar.

What I've consistently found is that it understimates the Sg (or overestmates the rifling twist rate required) for plastic tipped Hornady V and A-Maxes and similar bullets to a considerable extent. Presumably the light plugged-in tip increases the bullet length measurement to a greater degree than any actual increase in the size of destabilising forces that must be countered by the bullet spin. For instance, it predicts the 0.224" 53gn Hornady A-Max won't stabilise at .223 Rem MVs in a 1-12" twist barrel, while we know that it works perfectly well at lower MVs from a .222 Rem 1-14".

The same applies to some HPBT match bullets with a lot of void space inside the bullet between the top of the lead core and the jacket tip, but to a lesser extent. While the usual advice is to go for a combination producing an Sg of no less than 1.4, I spent last season running 155gn 0.308" Lapua Scenars in a .308W F/TR rifle with a 1-13.5" twist barrel at a bit over 3,000 fps, very satisfactorily indeed despite a calculated Sg of only 1.07. The GB / European / World F/TR champion Russell Simmonds has used this bullet and in his championship winning loads, the 155.5gn Berger BT FULLBORE in a 1-14" twist barrel at similar Sg values. And ............. those with experience of the British weather will know we get more than our fair share of cool moisture laden air that decreases bullet stability for any given rate of spin!

I think this (plastic tip / void inside the bullet) issue may be why you found such a formula 'pessimistic', presuming your 68gn match bullet has some space under the tip. If (Heaven forbid!) you'd loaded true FMJ bullets in the PPC, they might not have stabilised.

Laurie,
York, England

 

[chambered221]

The JBM calculator is the Don Miller based formula.

Thanks for the added info, I will definitely keep it in mind.

 

[DCRYDER]

I love stability calculators, but this JBM is too much on the safe side. I mean a 62gr Berger that is .770" long and launched 3300fps is MARGINAL in a 14 twist???? Im not a veteran benchrester but to my knowledge this similar setup is used by 95+% of short range benchresters. I think this 1 is much better.

www.uslink.net/~tom1/twistrate.htm

 

[Laurie]

Thanks for the link.

The results with this calculator tie in with what I know from my own experience and that of others. For instance, it gives a twist rate of 14.3" for the 155gn Scenar at the 3,075 fps I was running it at, and I know people who have got this bullet to work in 1-14" twist barrels despite being told it won't!

Laurie

 

[chambered221]

The thing you must remember about these calculators is they are designed as a guideline not hard fact rules.
Miller took the Greenhill formula and added newer experimental bullet data that was obtained from the Ballistic Research Lab. By doing this he ended up with a formula that was more in line with modern technology.
One must also keep in mind that the variables that exist in today's bullets are more so than years past. Because of the different alloys being used today the specific gravity of said metal also needs to be taken into account.
And as Laurie has pointed out there is an obvious issue dealing with polymer tips and hollow cavities. Maybe a center of gravity shift correction needs to be implemented.
I do like the link that DCRYDER has posted because it gives the user the ability to make adjustments to correlate known and proven experimentation.

There are just way to many variables to make it a one size fit all !!!

 

[mikecr]

I doubt you'll find better than those using either McCoy's math, or Miller's rule of thumb.
Of the two, I trust McCoy.

 

[Finney]

Thanks for all the help.
I got the barrel mounted and the chamber cut today.
I have about 50 rounds of 85grGame King with 36.8 of 4064 to break the barrel in.
I am thinking about 45gr of 4350 behind a 70gr BlitzKing to start out.

 

[cliffy]

70 grain Speer T-N-T bullets shoot extremely accurately from a 9 1/8" twist 24" barrel as is STOCK in a Remington Model 700 CDL. Slowing the twist below 1-in 10" seems quite restrictive of bullet weights and accuracy way downrange. Cliffy