3rdgeargrndrr wrote: ... I am in the process of rebarreling my 30-06 to 308 now, but might change my mind or start a new project. ...
3rdgeargrndrr-
Is this a discussion of 308 caliber? Or 452? I'll admit readily to being an individual who is easily confused.
3rdgeargrndrr wrote: ... I'm leaning toward Mcgowen to build this barrel. ...
Is McGowen going to be building a 308 barrel or a 450B barrel or 450LM barrel for you ? ? ?
Let's assume you want a discussion of twist rate for a 450B or 450LM, with a caliber of 0.452. My understanding of the twist problem is limited, but I think you should be able to find an appropriate twist by knowing answers to these questions:
- What is the minimum temperature at which you plan to shoot?
- Do you plan to shoot where air pressures are higher than sea level, like on the shore of the Dead Sea? If so, what's the barometric pressure?
- What are the length and weight of the longest bullet you ever intend to shoot?
- What is the least muzzle velocity (fps) at which you ever plan to shoot this bullet?
With answers to these questions, it's possible to get a pretty good idea of the twist needed for adequate stability to enable accurate shooting.
One of the better on-line twist rate calculators is that at jbm:
jbm ballistics The articles from
Precision Shooting that are given in the bibliography for the Stability calculations are pretty informative on the subject of twist rate.
Using the jbm calculator, let's plug in some values for the
shortest 450B bullet I've got on hand: a Prvi Partizan 185-gr jacketed hollowpoint that is 0.525 inches long.
Let's use the default values for temperature (59F) and pressure (29.92mmHg). If we plan on a pretty slow muzzle velocity of 2000 fps, the calculator says we need a twist rate of 1 turn in 55.6 inches to obtain a spin needed for stability. More velocity well result in requiring less twist for stability.
For a worst-case trial, we can insert the values for the
longest 450B bullet I've got on hand, a 300-grain Hornady flex-tip at 1.075 inches length. Using the same 2000 fps, the calculator says we've got to have a twist rate of at least 1-in-29.4 inches for stability. A lesser rate (for example, 1-in-30) provides marginal stability. A rate of 1-in-24" provides plenty of stability.
The subject of defining "adequate", "marginal", etc., is complex, and will depend on multiple factors. See the articles referred to above on the jbm site.
3rdgeargrndrr wrote: ... With these twist rates, now barrel length comes into play. What are your guys thoughts on the correct length to fully stabilize a bullet? ...
Perhaps I'm not understanding your statement. Barrel length does not influence bullet spin and stability,
except as more length may result in higher velocity. If muzzle velocities are equal, stability and rate of spin will be the same, regardless of barrel length.
3rdgeargrndrr wrote: ... I would be under the impression that for me primarily shooting 300grainers @ max of 2400fps- 1:20 or 24 twist with a 24" bull barrel might be the best choice. ...
Let's plug your values into the jbm calculator: Assuming temperature and pressure defaults, and if we use the 300-gr Hornady FTX bullet, a 1:24 twist will certainly stabilize the bullet at 2400 fps. In fact, if the calculator is correct, that bullet will be stable at a subsonic 1000 fps.
If I've used the calculator correctly, you could swage a 500-grain jacketed SP .458 bullet to .452 at 1.35" length, and at 1400 fps MV still have stability with the 1-in-24" barrel.
So, unless I've messed up above (Quickload sharks should feel free to chew on me), select the McGowen 1-in-24" barrel with no qualms about bullet stability. If I've not addressed your question adequately, please try again.
--Bob
Footnote question for you: Do you have, or know of, any published and reliable 450B loading data what will produce 2400 fps with a 300-grain bullet in a 24" barrel? What are the pressures involved in producing this? Thanks.