GP100 Forcing cone, can someone check this pic for me

[gray wolf]

Here is a Picture of my GP 100 forcing cone, Can someone comment on the shape of it please.
The reason I ask is because I have been getting lead build up at the beginning of the rifling.
at the 6 O-clock position, looking at it with the pistol orientated correctly.
It starts as a single streak and gets wider and spreads along the barrel for about 2.5 inches,
the remaining 4 inches of the barrel stays lead free to the muzzle.

Chore boy brings out lead dust and tiny particles, no strips of lead. 6 strokes with the chore boy and the bulk is removed, a little JB cleans up the rest. The reason I ask about the Forcing cone is because there does not seem to be one, Just a long taper that goes right into the start of the rifling, The rifling is very soft at the beginning of it. As in a very long lead
before it actually grabs the lands and grooves.

My thinking is I am getting gas blow by at the start of the rifling because it is larger in that area do to being reamed ( lack of a better term )

My metal is ACWW ACWW+2%Tin 50/50 ACWW and Pure with 2% tin
My lube is home made and has always worked fine, Accuracy is very acceptable.

Cylinder throats are .3575/6 Bullets are sized.358 RCBS 158 grain SWC mold.
Barrel slugs at .3563

.359 bullets did not make a change,

Loads are 4.2 tight group
6.9 Unique
12 grains of 2400
7.5 Longshot
9.8 Blue Dot
I hope the pic is OK, we only have a 1.2 Pixel camera

110612

[Messy bear]

do all the loads cause leading?
can you measure the major diameter of the cone?

[gray wolf]

can you measure the major diameter of the cone?
At the very beginning I can measure it with a caliper,
inside edge to inside edge at the start of the barrel seems to be .380
If I put a .358 sized bullet into the cone backwards ( base first ) it enters for .390 before It meets any resistance.

All the loads develop the same kind of leading, with the mild 900 FPS tight group load taking the longest time to happen, but it starts to show after 6 or 8 rounds and gets worse as I shoot. The unique load is about 1000 FPS and I takes a little longer for the leading to happen.

[Harry O]

Looks to me like you have a 11 degree forcing cone at the start of the barrel. Most of my guns have that, and I have had it put on some of them that don't. I would strongly doubt that it is the cause of the leading.

I don't have that particular mould, but from memory, it is plain base. And, the bullet hardness sounds fairly soft. If so, I would look there. I do know that I had leading problems with 12.0gr of 2400 with a plain base bullet. When I went to a 358156-GC bullet, I was able to go considerably higher without problems. I eventually settled on 13.5gr as accurate and reasonable recoil. Absolutely no problems with leading.

[randyrat]

I'm watching this thread..My son dropped off a GP100 off and asked if I could develop loads for it and his did the same thing..All the loads leaded except some real HOT 180 grain GC boolits. I think they were so hot they blew the lead out from each shot.
He sold the gun before I could figure it out.
I often wondered what caused the leading..I still think there was some sort of barrel obstruction/tight spot where the barrel was attached.
Maybe you have his old GP100..God bless ya:-o

[Outpost75]

Check the muzzle and and the breech end of the barrel with pin gages.

I expect that you may have a slight constriction at the root of the thread where the barrel was torqued in.

Best way to remove this is to cast a lead lap just ahead of the forcing cone and work it out by hand using AA Clover.

[runfiverun]

look from the side of the frame where the leading starts, it's quite possible even with the forcing cone being cut you still have a small tight spot there under the frame.

the hole being bigger right before the rifling is also a great way to get gas blow bye, and if the cylinders don't line up quite right you have a slightly tipped boolit to help it a little more, the cylinder gap being where it is doesn't help either.

maybe a longer boolit would help, or as was mentioned a gas blocker for the base.

[gray wolf]

*****the cylinder gap being where it is doesn't help either. ****** I don't recall mentioning the cylinder gap, but for the record it's .003 with the cylinder held back, .0025 with it pushed forward. As for a forcing cone that I would think should look something like this \ / well mine Is just a hole, with a very shallow taper, and the rifling lead is long, the beginning of the rifling is very shallow and then gradually gets deeper, like it was scooped out, (looks like a tapered dowel with sand paper on it was spun inside the first 3/4" of the barrel ) The pistol was bought new. The forcing cone does not look like a forcing cone. It reminds me more of a Taylor throat. ( long gradual taper to the rifling ) I don't feel any tight spots when I slug it. Tight spot ?? I was thinking more along the lines of to big and letting hot gas blow by the base. Am I wrong in my thinking that a thread choke would reduce the bullet in the area of the choke, and then cause the barrel to lead up after the choke from the smaller bullet going down the barrel. Also like I said the leading starts with a Small streak in a groove at the 5:30 position. As I shoot it gets wider and starts getting longer and maxes out to about 2 1/2 inches with the rest of the barrel being clean, I get a little lead wash in the cylinder throats also.

[runfiverun]

you didn't.
my line of thinking is you are still shoving gas around the base of the boolit, and the cylinder gap being where it is [right in front of the open funnel of the throat there] is helping any gas cutting get started from the cylinder gap all the way into the rifling.
I think your getting some leading on the front of the cylinder too...

[h8dirt]

I'd look for thread crush (choke) just forward of the forcing cone. To check: Oil your bore. Then use a small brass hammer to gently drive a soft lead ball (or you can use a 38 Special casing to cast a slug) into the bore at the muzzle end and push it through the barrel using a cleaning rod with a brass jag. If you have thread choke you will feel it stop just before it exits the bore and you will have to tap it out. It's worth checking. Just IMHO.

[rockshooter]

I just check the same area on 2 GP100s- they look the same as yours. I don't have any leading issues to speak of using a 158gr cast boolet (BHN 9.4) pushed by 12.5gr of 2400.
Loren

[zomby woof]

My GP100 leaded up bad. I had thread choke. I think I had a .348" pin gauge would stop at the threads. I shot a few lapping compound boolits down the tube and opened up the choke. No more leading. I also opened up the cylinder throat.

[gray wolf]

I'd look for thread crush (choke) just forward of the forcing cone. To check: Oil your bore. Then use a small brass hammer to gently drive a soft lead ball (or you can use a 38 Special casing to cast a slug) into the bore at the muzzle end and push it through the barrel using a cleaning rod with a brass jag. If you have thread choke you will feel it stop just before it exits the bore and you will have to tap it out. It's worth checking. Just IMHO.

Yes I have done that and I can't feel any tightness at or near the barrel frame interface. Not to say it can't be,
just saying if it's a tight spot I can't feel it.

I do not get any lead splash on the front of the cylinder or anyplace inside the frame window.
The portion of the barrel you see in the picture ( forcing cone ) does not show any leading, just burnt powder.
The leading starts just after the rifling begins, like I said in one groove at the 5:30 / 6 O-clock position

I do not have pin gauges so I can't use that method.

[L Ross]

To the OP, you say the accuracy is very acceptable. Does the accuracy deteriorate as the leading progresses?

[gray wolf]

To the OP, you say the accuracy is very acceptable. Does the accuracy deteriorate as the leading progresses?

I think accuracy goes south with most barrels that lead up, To what degree ? I guess each barrel has it's own limit.



To make life easy what I have been doing is to run a brush with Chore Boy down the barrel every 25 rounds. 3 or 4 strokes and it's good for another 25 rounds.

When I get home it only takes 10 Min. to clean up the barrel.



But to answer your question, if I don't do the above the accuracy will fall off.

[HeavyMetal]

I think your boolits are to soft, if your WW metal is clip on stuff run straight cows add 2% tin add drop from the mold into cold water.

Shoot 100 and see if that changes your leading.

[runfiverun]

Sam:
the water dropping of your alloy is about the cheapest thing to try right off.

[gray wolf]

OK men, I will water drop the next time I cast, It may not be in the next few days but I will do it and report back.

Thank you all for the help.

[44man]

I think your boolits are to soft, if your WW metal is clip on stuff run straight cows add 2% tin add drop from the mold into cold water.

Shoot 100 and see if that changes your leading.
My take on it too, boolits are skidding. let the boolits age at least a week before loading.

[HATCH]

Hitek coat the boolits and see if that clears up your leading issue.

I would go out on a limb and say your issue has to do with the alloy of you boolits.
Coating them would basically cover up this issue and most likely solve your problem.

[tomme boy]

Didn't you already have to send this pistol back to Ruger for issues?

[gray wolf]

Didn't you already have to send this pistol back to Ruger for issues?
Yes , for a totally unrelated problem, cylinder needed a part replaced.
Back to me in 4 days.
No Money for powder coating.

[gray wolf]

It's good to know I have a well cut cone. I don't size .359, it was a one time try to see if it would help. I size .358, I don't think .0005 over throat diameter is an issue.
I also don't think I should need an alloy of 15-18 hardness to shoot at 900 FPS
But I will try it.

Thank you

Sam

[GP100man]

Sam , everyone of my GPs have a longer/deeper throat than the 1 in the pic.

My 6" SS GP does have a very slight thread choke & with mid loadings with isotope alloy 11-12 bhn I get an even lead wash , I imagine if ya shot 100 rounds it would build up more , but for 50 rounds it does`nt affect accuracy that I can tell.

Harder does`nt stop the leading ,but it takes more rounds to reach the same foulin as the 50 rounds.
GC boolits will clean the bore up & stop the wash build up .

Also a swc with a wide front band or a fnrp design lessens it to the point ya have to look close.

What I think is happening is a small bit of skidding & gas blowby (not neccarily gas cutting) in the transition from throats to bore.

GP

[gray wolf]

Sam , everyone of my GPs have a longer/deeper throat than the 1 in the pic.

My 6" SS GP does have a very slight thread choke & with mid loadings with isotope alloy 11-12 bhn I get an even lead wash , I imagine if ya shot 100 rounds it would build up more , but for 50 rounds it does`nt affect accuracy that I can tell.

Harder does`nt stop the leading ,but it takes more rounds to reach the same foulin as the 50 rounds.
GC boolits will clean the bore up & stop the wash build up .

Also a swc with a wide front band or a fnrp design lessens it to the point ya have to look close.

What I think is happening is a small bit of skidding & gas blowby (not neccarily gas cutting) in the transition from throats to bore.

GP

Makes a lot of sense, We haven't spoke for a while, I will PM you with My # and maybe we can talk a bit. ( it it's OK with you )

[Petrol & Powder]

I've had several GP's and a few had noticeable thread choke and similar leading. I would push a tight fitting patch through and see if you can feel a restriction near the breach end of the barrel. I'll second Outpost on the fix, although some will suggest fire lapping. That Ruger steel is some very hard stuff, so act accordingly. If you have access to pin gages, I would check both the throats and the barrel. What ever you do, cutting metal should only be done after a lot of careful measuring.

[Echo]

What I zero in on is the .380 throat origin. That is .022 larger than the cast boolit diameter, and I cannot see how this can be a good thing. How it would cause leading only @ 0600 is beyond me, but that much clearance cannot be good - IMHO...

[gray wolf]

Yes if I measure across the barrel opening ( beginning of the cone ) it is .380 from one side to the other.

How it would cause leading only @ 0600 is beyond me,
I believe I said 900 FPS

[Wayne Smith]

Sam, do you have a GC mold for that gun? If not I can send you some to try. I size .359 so you can resize if you want. Mine are an old Lee HP design.

[gray wolf]

No Sir, I do not have a gas check mold, I have one for the 44 magnum that GP100 was so nice to let me use but nothing for the 357 magnum.

I think that is a nice gesture on your part to let me try some GC bullets. I will PM you my ADDY.

Thank you and thank you all for the help.

[Jim Flinchbaugh]

So, is your throat diameter of the cylinder bores smaller than the groove of the barrel?
If so, there is your problem. If your boolits are getting sized in the throats (as it should) but smaller than the groove diameter
you gonna get gas cutting/blowby=leading. My GP100 shoots everything well. My Redhawk 44 mag, has gone down the road as a Ruger
failure in manufacturing

[RobS]

Sam:
I've run this road before and if your cylinder throats are smaller than the throat (where the lands start) then it can be an issue in particular if you are shooting a shorter boolit. I remedied this issue in a particular revolver by simply running a longer bearing surfaced boolit. The other option is to use a GC mold however then you have another expense. Now a days I prefer boolits that will still be in the cylinder as the nose engraves the rifling as accuracy tends to be better. I also have spent a bit of time designing boolit molds that will have maximal bearing surface (why I don't shoot/design Keith style SWC's or SWC's anymore) while yet providing me the nose profile that is accurate to the ranges I prefer to shoot and at a wider velocity or better put, RPM range to stabilize.

[gray wolf]

OK, my cylinder throats are .3575 / .3576 my bullets are a perfect .358 ( with a Mic )
Best I can tell my barrel is .356 perhaps .3563 ( with a Mic. )

I tried .359 bullets one time with no help,
Cylinder throats decide what size they comes out.

I am shooting the RCBS 38-150 SWC that drops at 160 grains, it has a very wide rear band, .150 narrow front band .070 the bearing surface is .455
Second bullet is a RNFP also 160 grains .120 rear band and the front band is .110
Bearing surface of the this bullet is .440

Cylinder gap is .003 with the cylinder pushed back.

I really don't feel a thread choke. But what I do see is when they made such a long lead at the beginning of the barrel they carried the long taper of the cone into the rifling.
and in doing so they reduced the size of the rifling at the point that it begins.

Let me say it this way, if my barrel is .356 then the beginning of the barrel must be
bigger at the start of the rifling. It has to be bigger it's been polished out when they made the forcing cone. If in fact the barrel is bigger at the start of the rifling then couldn't gas cutting be starting at that point.

Fact is the leading starts with a small thin streak at 6 O-clock in one groove and as you shoot it grows to about 2" in length and gets wider. The rest of the barrel does not lead.

If I push a bullet nose first into the barrel from the back ( into the forcing cone )
about .285 of the bullet is sticking out the back of the barrel.

So it's definitely still in the cylinder throat as it transitions from the cylinder throat into the barrel.

[RobS]

If I push a bullet nose first into the barrel from the back ( into the forcing cone )
about .285 of the bullet is sticking out the back of the barrel.

So it's definitely still in the cylinder throat as it transitions from the cylinder throat into the barrel.

Yes but how long does it take for the boolit to actually stop skidding or gas cutting (I don't know which is possibly happening) before the base of the boolit leaves the cylinder. When you press the bullet into the forcing cone/barrel do you see any rifle engraving? Just because the boolit has made contact on the nose and is sticking out the barrel doesn't mean it has engraved the rifling as the base of the boolit clears the cylinder. I like to take a freshly cast bullet that has just cooled and work it down into the forcing cone/barrel and then with a good object to push with such as a 1X1/2" by 6" long piece of oak board I will set the muzzle on folded rag on a solid table and push the boolit as deep as I can so I can see pronounced rifle engraving. Now how much of the boolit is sticking out of the barrel is the next question. With your situation having the lands somewhat polished down less than the rest of the barrel it may take even more entrance of the boolit to stop either the bullet skid or the gas cutting. Oh, yeah of course you will need to take a cleaning rod and push the boolit back out as it won't come out with your hand.

[gray wolf]

I pushed a bullet into the forcing cone and fully engraved the first drive band. The drive band is about .070 wide. I had .085 sticking out the back of the barrel.

[RobS]

I am shooting the RCBS 38-150 SWC that drops at 160 grains, it has a very wide rear band, .150 narrow front band .070 the bearing surface is .455
Second bullet is a RNFP also 160 grains .120 rear band and the front band is .110
Bearing surface of the this bullet is .440



The bearing surface is not just the base of the boolit to the top of the front drive band on the RNFP (assuming it is not a front drive band that is offset from the nose ogive like a SWC). The bearing surface is easily seen from a recovered boolit or one that is pushed through the bore. A SWC is the obvious in seeing bearing surface as the front drive band is it's stopping point. The ogive on most RNFP, LFN and WFN designs will provide some additional bearing surface.

[RobS]

As a comparison although not a 357. My Ruger 45 Colt Bisley has the following measurements of boolits protruding from the start of the 11 degree forcing cone cut barrel.

260 grain LFN style: .1895"
310 grain LFN style: .3490"
345 grain LFN style: .4546"

Now I know that the start of my lands the groove diameter is .451 like the rest of my barrel. I have a new Ruger Flat Top in 45 Colt that has the original 5% forcing cone and all of these measurement are even longer (more boolit sticking out) with the above boolits. I also have a Ruger SRH 454 Casull that I cut the forcing cone on and had to go deeper than I wanted to take out the compound angle from the factory cone. This didn't bother me as I knew this gun was going to be shooting primarily 310-345 grain bullets with longer bearing surface. This firearm of course has less length sticking out of the above bullets. To no surprise the 260 grain LFN shoot accurately however will lead the bore just a touch similar to yours but this doesn't with the 310/345 grainers. Both my Ruger Bisley and Ruger Flat Top 45 Colts shoot the 260's without a hitch. I do have a 235 grain LFN style design that is flawless in the BH Flat Top with the 5 degree forcing cone but will give a bit of leading in the Bisley 11 degree cone and is not a good one at all for the SRH 454 Casull regards to the leading similarity you describe with yours. I still can use all of these boolits though because I have a PB gas check maker that I can install for the situations that need a GC.

I don't know how far in your barrel the lands and rifling are opened up from the cone cutting/polishing so it's hard to say how long a boolit may need to be to aid in stopping the leading from either the skidding of the lands or the gas cutting. I really suspect that you are skidding the lands/rifling but that's my gut feeling.

[gray wolf]

My RNFP has an obviouse front band, very, very small step, but still a step.

[RobS]

I pushed a bullet into the forcing cone and fully engraved the first drive band. The drive band is about .070 wide. I had .085 sticking out the back of the barrel.

Whats the groove diameter........did the front drive band shine all the way around it i.e. did the front drive band fill the groove diameter of the bore?

[RobS]

My RNFP has an obviouse front band, very, very small step, but still a step.

Ahhh.....I see. Not better for bearing surface but the design is good for helping with cycling in lever actions. This is also considered good by some when sizing smaller as the bearing surface won't "grow" on you vs a similar design that doesn't have the shouldered front drive band or smooth transition to the ogive. I myself am not in favor of the shouldered RNFP or any design for that matter other than a SWC or Keith.