Leading in my GP100

[robertbank]

I purchased my GP-100 .357 several years ago and have used it for IDPA Revolver division using my cast bullets in the main. Mu cylinders were so tight 3 would not let a .356 bullet pass through them and three would not let a .357 bullet through. I just had all six reamed out to .358. I had my forcing cone cut to 11 degrees. Since doing this I have yet to have any leading shooting my RCBS 158 gr RN bullets sized .358 or any other lead bullet iteration sized .358.

In the process of dealing with my new GP-100 10MM. It has leading just past the forcing cone. I intend to shoot lead through it so it will spend time with my gunsmith to have the cylinders checked out and teh forcing cone cut to 11 degrees.

Take Care

I know all guns can be different but if you are looking for best accuracy and no leading I think the quick fix is what I described above. I doubt your cylinders are greater than .358 at the throat.

[Bazoo]

I have a new report. I fire lapped probably 40 rounds (can’t recall without my notes). Upon doing this I could still barely feel a resistance when slugging. Leading was not improved with specials but was somewhat improved with magnums.

I recut the forcing cone to 11 degrees. I fired 6 of the previous 38 specials and leading was not improved. Without cleaning I fired 6 357 magnums and the previous leading was gone. It shoots without leading with full power 357 magnums.

The load was
38-150-SWC WW+Sn alloy sized .357
50/50 lube
W296 16.8 grains
CCI 550

I’m still thinking it through and experimenting slowly. I am going to slug again as it’s been so long I can’t remember. I may firelap more. I think my issue is cured by obturation, so I’m going to try some softer bullets for 38 loads. I probably will try some special level loads in 357 brass just to rule that out.

I also tried bullseye for 38 special loads with the same results as unique; severe leading in the forcing cone and rifling beginning.

Another question that hasn’t been answered, will bullets larger than throat diameter cause leading in of itself.

Thanks for everyone’s suggestions and ideas.

[robertbank]

How about we tale a deep breath. First stop lapping your barrel not sure why you are doing it but stop.

To answer your question about cylinder throats. It has been my experience Ruger cylinder throats are for the GP-100 are spot on at .358 OR more likely less than .358. Measure each one. If they are smaller than .358 have a gunsmith ream them out to .358. If any or all of them are smaller than .358 the bullets will be sized down by the cylinder throats to what ever diameter the individual throat is. You are not going to size your bullets to individual cylinders hence my suggestion to have them all cut to .358.

Size all your bullets.358. I am assuming your barrel is .357 in diameter. You can slug it and measure the slug. Use a sift lead slug. Check to see if your sizing die is sizing your bullets /358/

If the diameter of the barrel measures .357 you then should size your bullets .358. I have had dies that were not what they said they were ir at least not sizing my bullets as indicated on the die/

Your forcing cone you say is now cut to 11 degrees.

Observation: IF you had leading in the grooves of your gun you will not shoot it out by shooting lead .357 bullets. I have never been able to do is with plated or FMJ bullets either.

Backk to your GP-100. I don't water quench my 38 spl lead bullets cast from WW alloy. They shoot well in my gun. I do get occasionally some leading in the first 1/8th inch of the rifling. I credit that to slightly undersized bullets. Ignoring those few occasions I do not incur leading with my GP-100 using ww alloy in 38spl. For .357 I do try to water quench them. Again I do not get any leading in the rifling. If I shoot a lot IU do get lead build up outside the forcing cone. That lead build up can be blown away with some stiff plated/FMJ .357 mag rounds.

Assuming you do not have a barrel that looks like a railway track row of sleepers if your cylinders are .358m your barrel .357 and the forcing cone is cut to 11 degrees you should not and will not get leading in your gun using WW alloy air dried bullets in 38spl or water quenched bullets in your .357Mag.

Personally I think you are over thinking this issue. TRake a deep breath. Start with your cylinders. You have already cut your forcing cone. Just for the hell of it measure your bullets you are using out of the mold. I have a 44mag Lyman mold that casts ww alloy bullets .428 when they should be.431. I could not figure out why my Rossi 44 mag lever gun was shooting all over the park. Undersized bullets, leading and one ticked off caster. ie me.

Different powders will not have any effect on leading, assuming your loads are within reason/

Tale Care

Bob

[RedHawk357Mag]

Just spitballing... maybe change up the dies your using. 357 doesn't lead, 38 Special does. If you don't have a buddies dies to borrow, maybe load up a couple cylinders worth of 38s with just the belling of cartridge removed. A Redding Profile crimp die and I went round and round with a similar issue a few years ago. It somehow stacked the tolerance against me to cause leading no matter how loose a crimp I attempted to utilize. Bought enough second hand dies cheap enough to determine they aren't all the same; minor difference can stack tolerance against you and befuddle you. Good luck finding a solution. Maybe segregate headstamps of 38 brass to reduce variance while pinpointing the cause. Case length of your test brass within .003" till you pinpoint what is causing the issue. Check to see if crude rings forward of the throat are maybe reducing bullet diameter. Maybe use a nonsense alloy certified by Rotometal to see if alloy is gotten out of whack. Have purchased RAW bullets from Missouri Bullets to chase down alloy shenanigans and sized lubed or coated them. Wish primers were more plentiful so you didn't have that aspect to consider while figuring the problem out. Best of luck in chasing it down.

Sent from my SM-G986U using Tapatalk

[Bazoo]

Robert, thanks for the suggestions.

I fire lapped it because I felt a constriction when slugging. I don’t have a way to accurately measure the bullet as it’s 5 grooves. However, a slugged bullet goes through my throats easily.

My throats measured .3575 or thereabouts but measured with my calipers. I’m sizing to .357.

My bullets are .3585/.359 out of the mould and my sizing die is right on .357. My expander plug is long enough and .356 diameter and a pulled bullet isn’t swaged down.

I don’t see how opening up the throats to .358 will do anything given these parameters.

I don’t water drop.

I crimp both 38 special and 357 magnum. RCBS dies. I don’t see how a roll crimp could cause an issue but I’ll think on it.

I use same headstamped brass trimmed for consistency. Winchester brass in this case.

I don’t have commercial bullets, either cast or jacketed, nor desire to use them for 38special/357 magnum.

I will reslug both bore and cylinder. And I’ll try the shim measuring thing on my slug again since I have a mic now. Which btw confirms my calipers and my feel for them are spot on.

[robertbank]

1. With your revolver you can be certain your barrel is .357 in diameter. The proper size for your cast bullets sized is [B].358[B] not .357. .358 sized bullets will ensure a tight fit in the barrel and prevent gas cutting, the primary cause for leading. lead bullets sized .357 are to small in diameter and will lead your barrel as they will allow gas to blow buy your barrel.

2. Size your bullets .358 with a .358 sizing die. Take those sized bullets and see if they pass through your cylinders. If they do your leading problems should end. At the very least you will eliminate one of the major cause of leading and poor accuracy ie under sized bullets.

3. If your bullets fail to drop/push through your cylinders when sized .358 then have them reamed out to .358. If you don't ream them out you will be shooting bullets sized down to what ever diameter each cylinder throat is. I don't think crimping your bullets is going to have the effect of reducing bullet size as they leave the case. Use your bullets to determine throat size. Use bullets sized .358.

4. I'll assume you have had your cylinder throats all reamed to .358. The reaming will not affect plated or FMJ commercial bullets. Accuracy using those type of bullets will not be affected.

5. I generally do not roll crimp my 38spl cartridges. I say generally. When I do, the crimp is not excessive. I am not one to hotrod .38spl. If I want to build what you would describe as a hunting/slef defense round I water quench my bullets as they fall from the mold and use them in .357 Mag cases. I then use a a more aggressive roll crimp.

6. Lastly, load a cartridge with a measured bullet sized .358 as you normally would then pull the bullet and measure it. The bullet should not have swaged down .001". If they do harden the bullets up a bit. This exercise is just a way of eliminating any question that your bullets are smaller .358 as they enter your barrel. The barrel will size the bullets down to it's diameter but in so doing provide a tight seal to prevent gasses from getting by the bullet and causing gas cutting and the inevitable leading of your barrel. The OAL of your case is important as to wide a difference can affect the quality of your crimp. For what I use my revolvers for eg action shooting sports and bear/cougar protection while in the bush I don't spend a lot of attention in case OAL I just shoot to much to take the time to concern myself about OAL of my cases. They will all shorten over time if they have not cracked before get a chane to shorten where it would be a concern.

I shoot IDPA, steel and some IPSC. Off a bench and over my Chrony I have never noticed a significant difference in accuracy or bullet velocity using mixed brass. I am working with 76 year old eyes now. Fine accuracy beyond 20 yards with a handgun using iron sights is not now happening. I am not a candidate for bullseye shooting anymore. Not that I ever was. LOL The above two posts should cure any leading problems you have.

Take Care

Bob
ps If after all of the above steps have been taken and you still are getting small amounts of leading in your gun in the first 1/16th to 1/i18th of the barrel you might be experiencing barrel constriction. At that point you might choose to live with the slight annoyance or attempt to remove the possible constriction in the barrel. I'll pass you off to some else who has some experience with barrel constriction. I should ask is your GP-100 the regular model or the Match Champion. If the latter your barrel might be a sleeve barrel. If you have a sleeve barrel you won't have any barrel constriction. My 10MM MC has a sleeve barrel, I am not certain the 9MM MC does.