Throat Length

BrettT/C
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Joined: Sun Aug 23, 2009 8:56 pm

Throat Length

Postby BrettT/C » Sun Aug 23, 2009 9:25 pm

Hello Dan
I am designing a bullet for my Freedom Arms revolver in .475 Linebaugh. I am trying to figure out the how long the front driving band can be and still fit the cylinder throats.

I would like to know how the Rifle Throat Length is calculated? Is the Rifle Throat Length front driving band length + leade length?

Thanks Brett

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mtngun
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Re: Throat Length

Postby mtngun » Mon Aug 24, 2009 7:58 am

I am glad you asked because a lot of folks get confused about how to use the program's "rifle throat length."

First off, the "rifle throat length" number is just that -- for RIFLES.

Rifle throats, in the perfect world, start at slighlty over groove diameter and taper to bore diameter.

Revolver throats, in this crazy, screwed up world that we live in, start at cartridge OD diameter, and taper to groove diameter. This results in a grossly oversize funnel in front of the cartridge. I personally question whether this is a good idea, but nonetheless, that is how revolver throats have been made for a century or more.

To further confuse the issue, lever actions and single shots chambered for revolver cartridge are usually cut with a so-called throat that starts at cartridge OD and tapers to bore diameter, again resulting in a grossly oversize funnel, sometimes lovingly referred to as a "toilet bowl throat."

The program's "rifle throat length" is based on where the ogive tapers to bore diameter. It is just a guesstimate, so it is not very accurate for rifles, and it does not apply at all to revolvers.

There are two schools of though on how to set up a bullet for revolvers, depending on whether you believe a bullet should be a slip fit in the throat, or if you believe the bullet should be at least as big as the throat, in which case it will not be a slip fit.

The "slip fit" school says use a long front band and/or full diameter ogive and size the bullet so that it can be pushed through the cylinder with finger pressure. This means that the bullet is smaller than the throat, and it does not make allowances for fouling. A bullet that is a slip fit in a clean cylinder may not chamber in a fouled cylinder. Nonetheless, many people use the "slip fit" method with good results.

My approach is to size the bullet at least throat diameter or 0.001" larger. To chamber reliably, the front band or full diameter ogive must terminate at or just before the end of the throat's funnel.

The first order of business is to measure the length of this funnel.

Then you set about to design a bullet that will fit in this funnel. There are several easy ways to do this -- use the bore riding option to put a step on the nose, use the TC option, or use the Keith option. Then just select a front band length that is slightly shorter than the funnel, making allowances for the crimp groove and for normal tolerances like case length variation and seating/crimping variation.

There is a hard way to do it, and that is to use a full diameter ogive and try to guess how it will fit in the funnel.


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