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Thread: .357 Cylinder throat question/ cylinder reaming.

  1. #1
    Boolit Master wonderwolf's Avatar
    Join Date
    Apr 2007

    .357 Cylinder throat question/ cylinder reaming.

    Questions: What should Cylinder throats on a .357 Magnum gauge to with pin gauges? and is Cylinder reaming just correcting overly tight throats?

    The Story: I have a much loved GP-100 5" that I often pair up with a Ruger M77-357, one thing I've been working towards is a heavy load in 357 mag they both like. After several group buy molds disappointed me and I discovered things I wouldn't have if I hadn't tried to get a load to work in both a revolver and a bolt action (Driving bands and nose shapes as well as OAL for the shorter than needed M77 mags). The M77 proved obviously the harder customer to please due to the more dynamic feeding action. All the revolver needed was the bullets to fit. And that is where my problem is now.

    Trying different molds went on for a time till the MP C358-180 RF came out. This bullet is everything I've been searching for. HP options, good weight smooth feeding in the 77 but when sized to .358, loaded in .357 cases, seated and crimped at the crimp groove it will not fully chamber in my Gp-100, I have not tried it yet in any of my other .357 wheel guns though admittedly, I'm casting up another batch of them this weekend while I'm visiting my folks to check on that this week. The round will stick out just about 1/16"-1/8" from being fully seated. I have a set of pin gauges I'm going to use to check and see exactly where I'm at with my cylinder throats, but if I'm thinking about this correctly the pin gauges are only going to tell me the smallest constriction in the cylinder (at the mouth)....not necessarily the area the bullet is trying be seated into, right in front of the actual chamber which in theory is a bit of a taper?

    I could trim brass and crimp further down (hardly ideal)
    I could size the bullets down till they seat correctly (Far from ideal with leading)
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  2. #2
    Boolit Grand Master

    DougGuy's Avatar
    Join Date
    Apr 2013
    just above Raleigh North Carolina
    Sounds like your throats are tighter than the boolit you want to load. Very common. Send me a PM and we can get the fix started... I size 35 caliber cylinders to .3585" unless there is a request to go larger, this will generally solve any issues with chambering .358" boolits.

    You have a chamfer in there often called a ball seat, this eases transition from case mouth to throat, which the throats should be parallel, no taper at all in the throats. The interference comes just in front of the chamfer, where the boolit is larger than the throat so it won't fit into the throat. Incidentally, best accuracy is achieved when the boolit is chambered and the driving bands or the shoulder are seated into the throat. It holds the round concentric in the chamber.
    Last edited by DougGuy; 01-13-2019 at 10:24 AM.
    Got a .22 .30 .32 .357 .38 .40 .41 .44 .45 .480 or .500 S&W cylinder that needs throat reaming? 9mm, 10mm/40S&W, 45 ACP pistol barrel that won't "plunk" your handloads? Shoot me a PM! Also on Facebook:

  3. #3
    Boolit Master
    Lefty SRH's Avatar
    Join Date
    May 2011
    Athens, AL
    I have the same mold and same issue. Im not discounting what Doug said, but I ended up nose sizing that bullet the only the length needed, not much. The shank is still .358”. Not ideal being a two set up sizing but it worked.
    Last edited by Lefty SRH; 01-18-2019 at 11:39 AM.
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  4. #4
    Boolit Man
    Join Date
    Dec 2009
    Ball Ground, GA
    DougGuy has done a couple for me and he does GREAT work. I am a revolver nut and I would have Doug look at it and strongly consider his advice. I am a fan of cylinder reaming to the largest one then size boolits to that. Most times that is all you have to do assuming the timing and forcing cone are good. None of my revolvers needed to be more than .358. I do have one 38 special that all the chambers are about 1/1000 short. My easy fix on that one was short trim the brass. It only caused problems on wadcutters not seated flush, but that is my Bullseye competition gun which I only shoot wadcutters.

  5. #5
    Boolit Buddy wildcatter's Avatar
    Join Date
    Aug 2010
    N.W. Ohio
    He did mine, from 1 3/4" - 2" was about all I could do, then when Doug got done, I have one chamber that don;t like to plat with the other 5 but I don't think this is in the throating, always the same chamber. But never over 1" for all six, usually 5 are under 5/8" @ 25 yards w/ open sights, and my eyes are not the best, same load before and after.
    Click image for larger version. 

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  6. #6
    Boolit Buddy
    Join Date
    May 2011
    Reaming can also makes the throats a consistent diameter. There’s usually some slight variation among the chambers.

  7. #7
    Boolit Grand Master tazman's Avatar
    Join Date
    Jan 2014
    west central Illinois
    Tight throats can be a problem if the barrel slugs larger than the throat size. The throat would size the boolit down to throat size regardless of what you wanted it to be. Inaccuracy and leading would be the results.
    If your .358 sized bullets won't pass through the cylinder throats or chamber in the cylinder, they are getting sized down on the way through.
    DougGuy can fix the issue for you. I have used his services for barrel issues on my 45ACP and he did an excellent job.

  8. #8
    Boolit Grand Master

    Wayne Smith's Avatar
    Join Date
    Mar 2005
    Hampton Roads, Virginia
    Another vote for Doug's work.
    Wayne the Shrink

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