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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)
    My firearms project blog

  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: https://www.facebook.com/Cylinderhone-756429174391912/

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Abbreviations used in Reloading

BP Bronze Point IMR Improved Military Rifle PTD Pointed
BR Bench Rest M Magnum RN Round Nose
BT Boat Tail PL Power-Lokt SP Soft Point
C Compressed Charge PR Primer SPCL Soft Point "Core-Lokt"
HP Hollow Point PSPCL Pointed Soft Point "Core Lokt" C.O.L. Cartridge Overall Length
PSP Pointed Soft Point Spz Spitzer Point SBT Spitzer Boat Tail
LRN Lead Round Nose LWC Lead Wad Cutter LSWC Lead Semi Wad Cutter
GC Gas Check