How to choose custom .357 maximum reamer throat dimension?

[Spokerider]

I have a question about choosing a throat length for my Uberti rolling block carbine in .357 mag. http://www.ubertireplicas.com/lever-babyrollingcar.php

I wish to re-chamber this rifle to .357 Rem maximum with a custom ground reamer from Pacific Tool and Gauge http://www.pacifictoolandgauge.com/reamers2.htm#remtab

I have decided to have the throat diamter at .358, as per here http://www.bellmtcs.com/store/index.php?cid=120

I reload, and have choosen this bullet http://www.castperformance.com/Detail.bok?no=5
as the main bullet for my purposes.

Wondering......just how long should I request the throat to be?
I understand that the SAAMI throat dimensions for .357 mag and .357 maximum are for a long .400, tapered throat that does not support the bullet base.

I have NO experince with choosing such dimensions, and am going on the advise and findings of others.

I realize that I have ONE chance to get this right!!

Thanks.

[leftiye]

There is a little taper to these straight walled cases. The reamer maker has industry standard info about the chamber dimensions. He will be able to accomodate you somewhat (maybe) with alterations that you want - that is before he decides that your alterations qualify for "custom" status, and the price soars.

If you want to design your own, then measure the rear of your chamber and use that dimension (as it already exists and can't be made smaller without setting the barrel back - a real mess on a lever gun). your front case neck dimension can be as small as a standard bullet plus twice the case neck thickness and as little as a couple of thou. clearance (use more). Use case length data from a reloading book, and add .010 to .020" for clearance and step down to groove diameter plus .001". Freebore can be built into the reamer, or cut to any desired lenght later using a throating reamer.

P.s. The throat (freebore) only has to be as long as the cylindrical part (groove diameter) of the boolit that sticks out in front of the case as you want it loaded.

[Spokerider]

leftiye,
Thanks, that helps.
Since I`m not a gunsmith and I don`t have a lathe, and I`m hoping to do the reaming myself, I`m trying to achieve a balance between using a SAAMI standard .357 maximum reamer and correcting the the long and too-large forcing cone resulting in excess freebore etc.

I have not made a bore cast of the chamber, and so do not know the exact measurements of the chamber at case head, case neck, throat and groove diameter of the bore. I don`t know that the industry standards are for these measurements either. So, to keep it simple, I`m thinking that the only alteration needed to the reamer would be the throat design, as per my dimensions. The other dimensions of the reamer should be *good enough* I suppose. Don`t want to get into setting the barrel back for a total re-chamber etc.

You`re thinking that the throat diameter should be equal to the groove diameter PLUS .001. Why not the same diameter as the bore??

How do I determine the exact bore size on my rifle......tap a lead bullet down it from chamber and out the muzzle? Or, tap the bullet part way down the chamber and bore...say and inch, then back out the chamber? I don`t have cerrosafe compound.

As for a throating reamer, does it ride on a pilot bushing in the bore just like a finishing reamer, no lathe work required?

Thanks.

[475BH]

Why don't you load one in a sized case and send it to them to measure?
That way they will have the case dimensions and all w/o having any wires crossed between you both.
Good Luck.

[Spokerider]

Why don't you load one in a sized case and send it to them to measure?
That way they will have the case dimensions and all w/o having any wires crossed between you both.
Good Luck.

It ain`t that simple shipping even a dummy round across the border........

[leftiye]

I wouldn't say to forget about industry standards, but it's your rifle, and regardless, a little variation won't be a big deal. You KNOW that you need to cast that chamber and a couple of inches of barrel - right? Put a light oil on the portion to be cerro cast or you'll never get it out!

This might freak you, but you could just find the diameter at the front of the old chamber and lengthen the case part of the chamber with a chucking reamer of that diameter to .357 max. length. Then see what your freebore looks like and use a throating reamer to make it like you want it to be.

You want the freebore to be big enough to accept your boolit, and your boolit should be at least a thou. over groove diameter. Yes, pound a pure lead oversized (just a little over groove diameter) slug into the bore from the rear a couple of inches, then put a (brass) rod down the bore from the muzzle, and drive it back out.

Yes, throating reamers have pilots. A chucking reamer of the right diameter can be made into a throating reamer by grinding the flutes off of the front part of the reamer (toolpost grinder with flute end on tailstock center!), and pressing a collar of the right diameter over the ground down portion (must be concentric). Front edges (cutting edges) of the remaining flutes then need to be reground and sharpened.

[DiamondC]

I have a 357 max barrell for my Contender. I have never cast or measured the chamber. I shoot 35 cal rifle bullets as they seat out father and give better case capacity. I use a cast 200 gr. gas checked bullet with good result. There is no problem chambering even with 200gr. jacketed rifle bullets. Also consider that 357max pressures are much higher than 357mag. Be sure your action is up to the job.