Ruger gp100 oversized throats.

[J-H-Pointman]

Howdy, I’m new here to the forum and joined to soak up as much info on reloading/casting as one possibly could. While not being new to the reloading scene in general I’m no veteran to the art. I bought a Gp100 .357mag a week ago and planning to reload for it, slugged the barrel and cylinder throats. The bore came out to .357 and the throats came out to .361-.362 I realize this piece isn’t cut for cast bullets due to the blatant gross over sizing of cylinder throats so what in the heck do I do? One fellow said to send it off to Ruger and another said to “fire lap” the bore? The bullets I have are a diameter of .358 and a hardness of 12bhn. I can’t wrap my head around the gun being inaccurate if the cylinder dimensions are off but the bore dia. Are perfect? Any help?

[DougGuy]

Sounds like BUBBA got hold of the cylinder! Did you buy it used?

Call Ruger customer service they rock!

NO to firelapping, don't "fix" the part that ain't broke. And no you cannot size to those throats, the rounds wouldn't even go in the cylinder all the way.

[J-H-Pointman]

Nope bought it brand new.

[ShooterAZ]

First, Welcome to the forum. Have you shot it yet? I'd recommend loading up some of your .358 boolits and give them a try before going to any drastic measures. That's what I'd do anyway...Please keep us posted.

[Outpost75]

Just use cast bullets which "fit" the throat, 0.001" under the pinned diameter is ideal. Use alloy of moderate hardness, 10-12 BHN, book loads, and move on.

Gun will shoot fine.

My 1939 Colt New Service .357 has .359" cylinder throats and .354" barrel groove diameter and is the most accurate .357 I own. I use 50-50 COWW and plumber's lead with 1% tin added, about 10 BHN. No leading even with full-charge loads. Hard alloy is neither necessary nor recommended.

[J-H-Pointman]

Ok sure will.

[rkrcpa]

What did you use to measure the cylinder? .361 seems awfully big from the factory.

[J-H-Pointman]

My midwayusa calipers.

[J-H-Pointman]

That gives me some hope. I’ll report back from the range as soon as possible.

[edp2k]

Calipers can be off by a few thousandths.
Try a pin gage.

[tazman]

Try pushing a jacketed bullet through the throats to see how loose they are. If a .357 slug drops through easily, you are probably close on your measurement. Check some unsized cast boolits for diameter and try sliding them through the throats and see what you get.
If this is your only 357/38 special, you may get by using boolits as cast without sizing them. Just tumble lube and go.
As someone else said, try what you already have. It may shoot just fine without anything special.

[DougGuy]

Just use cast bullets which "fit" the throat, 0.001" under the pinned diameter is ideal. Use alloy of moderate hardness, 10-12 BHN, book loads, and move on.

If throats really are .361" and the SAAMI spec for the chamber itself is .381" and the OP loads a .360" boolit into brass that's .011" wall thickness, .360" + .011" + .011" = .382" at the case mouth, it won't fit in the chambers.

However, an alloy of BHN12 would easily obturate to fill the throats provided the loads aren't light target loads. More than one way to fit to the throats..

[megasupermagnum]

I don't see what's so wrong. I universally load .359" bullets in 38 caliber. The only gun I loaded for that needs a .358" is a S&W J frame. I was recently loading for a Dan Wesson 357 maximum that had .361" throats. I loaded bullet sized .3605", it shot great, and had no problem chambering.

Try .360" bullets in that GP100, and you should be just fine.

[Outpost75]

If throats really are .361" and the SAAMI spec for the chamber itself is .381" and the OP loads a .360" boolit into brass that's .011" wall thickness, .360" + .011" + .011" = .382" at the case mouth, it won't fit in the chambers.

However, an alloy of BHN12 would easily obturate to fill the throats provided the loads aren't light target loads. More than one way to fit to the throats..

The .381 is min. chamber spec. Most run a bit larger with up to +0.005" being fairly common.

[Petrol & Powder]

He's using calipers to measure the throats. I suspect that .361" is not a true measurement.

[nhithaca]

Really need to use pin gauges to accurately measure the throats. If they truly are 0.361" then send it back to Ruger. A new gun should be right.

[35remington]

Yes...measuring cylinder throats with calipers guarantees an incorrect measurement.

[dogdoc]

You get a pin gage set .251 to .500 for around 100 bucks on amazon. I use mine all the time for measuring throats and other things. Well worth it if you are a serious hobbyist.



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[Kenstone]

My midwayusa calipers.

I have read all the recommendations to buy pin gages $$$
A $20 set of ball gages and a micrometer is really all you need.
Half ball gages can measure a diameter close to a shoulder/step and are what I prefer/use.
Ball gages can also check for taper, something a pin gage cannot do.
Here's some:
https://www.amazon.com/s?k=ball+gages&ref=nb_sb_noss_2
And there's telescoping hole gages for bigger holes.

[Forrest r]

Let your targets do the talking/thinking/measuring for you. Load some of your .358"/12bhn bullets up with old standard loads like 3.5gr of bullseye in some 38spl cases & hit the loud button a couple hundred times.

If you get patterns instead of groups you'll have your answer.