Leading in my GP100

[Bazoo]

I am getting a bit of leading with the gun. It starts in the forcing cone and seems to build if I fire a couple boxes. But it also has a trace of lead in the rifling down the bore. It brushes out with a reasonable amount of effort.

Both of my loads I've tried are 5 grains unique and either the RCBS 38-150-SWC or the Lee358-140-SWC. Both in special brass and with BAC lube. I've tried both ww alloy and a softer alloy, 50/50 ww/Pb I'd guess. Sizing them .358

I slugged the bore which is 5 grooves. Using a soda can shim the best I can figure it's .354. I've re-measured it and really tried to get a good feel and a light measurement. I can feel some change in the resistance when going through the area of the barrel where it threads into the frame. It's not a huge change though.

So I run as tight a fitted patch on a brass jag(didn't have a plastic one) as I could get started down the bore and it didn't get tighter at the threads. Now if I'm not mistaken Doug was suggesting this method with a plastic jag to feel for tightness. Will it not work with a brass jag or is the danger just getting the jag stuck?

I slugged just the muzzle by inserting a brass rod in the bore to tap the slug back out and it was .354 too. Seems small for a 357 bore, don't ya think? Is this common for Rugers? Looking back, I'm going to remeasure this.

I slugged the cylinders they all measure .3575. Yes all this with calipers as I don't have a micrometer.

So my query, Will sizing the bullets over throat diameter cause leading in of itself? Where should I go from here? Straight to having the throats opened up?

Should I buy a selection of pin gages? And if so, which sizes. Minus Z gages right?

Megasupermagnum has suggested I pull a bullet make sure it isn't being swaged down by the case which I will do.

Thanks for your help.

Bazoo

[Neverhome]

I have nothing to offer except to commiserate with you about measuring a slugged 5 groove barrel. Absolute pain. I used the soda can shim too and never felt 100% positive about any measurements I got.
Good luck figuring out your leading!

[Bazoo]

Thanks. It's not horrible leading but it's there. I'm not confident with my shimmed measurements either.

[Cosmic_Charlie]

About the best you can do is size your boolits to fit through the cylinder throats with slight resistance. If the throats or forcing cone is tighter than the bore it will need work.

[BigAlofPa.]

I had that issue with my SW 25-5 5 45 colt. Lubed and coated cast. Jacketed and plated no issues. I was shooting BHN 18 in it. I ordered some BHN 12 coated cast then. They work fine. I have not tried 38 special in my GP 100 yet. That i can run the BHN 18 loaded to 357 mag. Leading is very agravating. I know how you feel.

[Petrol & Powder]

That groove diameter sounds small even with all of the efforts to measure it correctly. I wouldn't put much faith it that number.
I'm with Cosmic Charlie, size your bullets to fit the throats (because that's what size they will be anyway after passing through the throat) and go from there.

I would almost bet you have some thread choke based on your description. I fire lapped a GP-100 once and that steel is HARD. So if you go that route be forewarned that it takes more than a couple of rounds.

I will offer this - I was using BAC lube for most of my handgun bullets and getting some leading with the tame loads. I switched to NRA 50/50 and that made a huge improvement. So, before you start removing metal, I would try a softer lube and bullets sized to the throats.

[Piedmont]

Bazoo, I have nothing to comment on your leading but wanted to suggest something on measuring that barrel. Several years back I was really into milsurp rifles and slugged a pile of them. Some of them were 5 groove and I am confident I was accurate to .001 whereas with even numbers I'm confident to half a thousandth. Take the soft slug that you pounded down the barrel and roll it in your micrometer. Let the jaws kind of float open when the slug catches and doesn't want to go around as you roll it. It seemed pretty simple when I got the hang of it and lacking a "v" block to compare my results to I'm still confident I was getting very close to reality. This only gives you a groove measurement.

[Bazoo]

I appreciate the replies.

Well BAC lube for whatever reason hasn't impressed me. I can't quite put a finger on it. The lee bullet leads more. Maybe it has something to do with the amount of lube that cavernous RCBS lube groove carries.

I do have some 50/50 that someone made which I got out of the swap box. I will see about switching lubes and see what improvement I get.

[C.F.Plinker]

Try taking one of the boolits you slugged the barrel with and pass it through each of the cylinder throats. Since it is now groove diameter it should pass through the throats easily. If it doesn't then that throat could be sizing it down to less than the groove diameter.

Sometimes you can measure the diameter of a 5 groove boolit by catching the edge of one land with one of the anvils of the micrometer and the edge of a different land with the other anvil. It helps if you pad the frame of the micrometer with a piece of leather and use a vise to hold the micrometer so you can use both hands to manipulate the boolit and adjust the micrometer thimble.

[Petrol & Powder]

I appreciate the replies.

Well BAC lube for whatever reason hasn't impressed me. I can't quite put a finger on it. The lee bullet leads more. Maybe it has something to do with the amount of lube that cavernous RCBS lube groove carries.

I do have some 50/50 that someone made which I got out of the swap box. I will see about switching lubes and see what improvement I get.

As much as I like BAC for some magnum loads (and it makes the bullets easier to handle before seating) it is just a little too hard for standard velocity. I tried to have "one lube for all handgun cartridges" and BAC was almost it. I really resisted going back to NRA 50/50 for standard loads but when I did, it was the right call.

BAC has its place and I still keep a little on hand for the few occasions I need it. For the vast majority of my loading, 50/50 has proven to be better.

[dannyd]

I use Carnauba Red,Blue and Commercial 160 in five GP100's with no problems. Mid range target loads. 125 and 158 grain bullets.

[nannyhammer]

I use Carnauba Red and size to .359 in my GP100, no issues to date.

[Petrol & Powder]

.359" seems a bit larger than Ruger GP throats tend to run. Can you even push a .359" bullet through the throats?

[megasupermagnum]

.359" seems a bit larger than Ruger GP throats tend to run. Can you even push a .359" bullet through the throats?

It appears he is sizing to .358", which is normal to me. I would actually be sizing to .357" myself, but being slightly oversize should not cause any problems except possibly not chambering fully on a bullet with more front nose.

[dannyd]

It appears he is sizing to .358", which is normal to me. I would actually be sizing to .357" myself, but being slightly oversize should not cause any problems except possibly not chambering fully on a bullet with more front nose.

I was using.358 for about 25 years and then switched to .359 groups got better.

[ABJ]

My Ruger double actions have been problems as well. sp-101 and gp-100 both stainless. Gas check bullets helped but I finally found a gunsmith that would clean up the rough forcing cones which was the major source of the leading in mine.
That steel is beyond hard, most pistol smiths charge extra if they will do it at all. Take a good look at the forcing cone first.
Tony

[Petrol & Powder]

I use Carnauba Red and size to .359 in my GP100, no issues to date.

This is the post I was referring to and he's using .359"

[megasupermagnum]

I was using.358 for about 25 years and then switched to .359 groups got better.

I've found that many, if not most 38 caliber revolvers will take a .359" bullet. My own GP100 takes a .358", although my other loads (some don't fit) with a .359" shoot just as well. Being as Bazoo says his throats are .3575", a bullet sized .357" would be ideal, although no problem with .358" if they chamber.

[Bazoo]

I appreciate the help folks. My .358 bullets do chamber okay in both 38 and 357 brass. The bullet will not pass through my throats however. The bullet I slug drops through the throats with room to spare.

I am going to first try changing lube. And I have a line on a .357 sizer.

[243winxb]

Add linotype for a better buller.