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Discussion Starter #1
Hi.. I asked this in the gunsmithing section with no results, so I'll try again here.

Does anyone know the nominal specs for chamber mouths and bore on a GP100?

I was experiencing an occasional tumbling bullet, amd when I checked the chamber throats they were .355. If the bore diameter is nominally .357, that might explain it, but I have no way to measure the bore diameter.

Thanks.
 

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I don't know what the "nominal specs" are, but my GP-100 has .358" chamber throats and a .357" bore.

If you can measure the throats, why can't you measure the bore?

Drive a soft slug through the oiled bore and measure the results.

If these bullets that are tumbling are cast or swaged and the throats really are that small, you should be getting a lot of leading.
 

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Discussion Starter #3
Never drove a slug through the bore before, just nervous, I guess. As for the leading, I don't get much, if any.

Thanks for your dimensions. I looked in Brownells catalog for a throating reamer, and only found chambering reamers, and no mention of the throat size.
 

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If you're not able to, or don't want to undertake slugging, take your gun to your local gunsmith. Shouldn't be too expensive, it's a fairly easy job. I've done all my firearms and it really isn't a difficult task.
 

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Discussion Starter #5
Thanks, I may try it myself. The first time I shot this revolver, it was accurate as you could hope for, and the next time I noticed some slight off center impacts. I clean my guns religiously after shooting, so I'm pretty sure it isn't leading. maybe some bad bullets, but I'll slug the bore to be sure.
 

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Never drove a slug through the bore before, just nervous, I guess.
That's the thing: If your throat is seemingly undersized you wouldn't want to pass the bullet through the bore, just into the bore and right back out the way it came. Running a bullet through the bore only tells you one thing, and that is the tightest you barrel gets throughout its entire length. May be at the muzzle, may be at the breech, or somewhere inbetween. Since you know what the throat is, get a soft lead round nose type of bullet and drive it into the barrel a ways but not all the way through. Then with increasing lengths of rod, push the bullet back out by pushing the revolver down on a narrow piece of steel clamped into you vise. Once you go a ways, you'll have to switch out to a longer rod and do the same all over again until the bullet clears the muzzle. That will be closer to a true bore measurement.

If the bore's fine then you know that the throat needs to be opened up a bit. If the bore is proportionally smaller than the throat, then you need to start thinking of a different diameter bullet. Smithy.
 

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Discussion Starter #7
Well, I did it. It wasn't as hard or as scary as I thought. Now I know that my bore is .357, I'll have to find a gunsmith to ream the cylinder throats to .358, and I'll be good to go. Thanks for all the replies.
 

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Discussion Starter #9
I wouldn't doubt that for a minute. Maybe it's a rare piece now.
 

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Discussion Starter #11
You may be on to something. Patent the idea, quick.
 
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