GP100 and cast

[double bogey]

I have looked at a few threads, regarding gp100's and cast boolits. My pistol has poor accuracy and heavy leading.

Boolits: lee round nose flat point 158gr. dropping at .358, 159gr ac clip on wheel weights. Xlox lube.

Cyl throats: Best I can measure is .3570 to .3575, measured with telescoping gauge and micrometer. (I need to get some pin gauges)

Barrel: Slugged with lead .44 round ball, wrapped with .0015 feeler gauge, miked .353, subtracted .003, came up with .350.
this seems very small. Is this within spec's, or is my measuring technique off?

In any case, what is my next step? Thought about sizing boolits to .357 and trying again. Help, I really like this pistol, but if it won't shoot cast, I can't use it.

[GP100man]

info on the bhn & load will help answer a part of the puzzle.

First , is fit , do they fall/push thru the throats ??

trying to reach mag velocitys with "fast" powders is a sure way to lead a barrel also!

GP

[double bogey]

The boolits will not go thru the throats. Obviously larger. The loads have been full range of H110, full range of 2400, full range of a couple of others. Not really trying to push velocity levels, powder availability is somewhat of an issue. Same boolit, around 11.5 - 12 bhn, with staedtler pencil test. Also a couple of loads in .38 special, same results.
Am thinking of ordering some pin gauges to confirm cyl throats.
Is that barrel dia. within spec, or is it more likely my slug is wrong ? The barrel didn't seem to have any tight spots.

[Whit Spurzon]

I just picked up a GP100 and the cylinder throats measure much the same as yours. I haven't slugged the barrel. Yesterday I shot almost 100 Lee 358-158 RF and 358-125 RF sized .358" in both 38 Special and 357 Mag (700-1200 fps) that I had previously worked up for my SP101. I was very impressed by how easy the GP is to shoot. I detected no leading. I pan lubed with Beeswax/Criso/Vaseline mix.

http://i1115.photobucket.com/albums/k542/WhitSpurzon/Ruger/1-007_zps3c2cc6fc.jpg

[double bogey]

Think I will size to .357 and try again. I wish mine would shoot like that. Mine looks like buckshot at 25 yds. While my Blackhawk shoots 2 - 3 inches at 25.

[GaryN]

I would size to .358 if you have the right sizer. I suspect you need to redo your slug. Use pure lead and lots of lubricant. Only use a micrometer to measure. My GP100 never leads and is very accurate. I size to .358 with an RCBS LAM. I use 50/50 for pistols. I shoot light target loads all the way up to full bore magnums. It shoots great. I don't think it would be possible to wrap a feeler gauge around a lead ball and measure your bore and come out with anything useful.

[slughammer]

Am thinking of ordering some pin gauges to confirm cyl throats.
Is that barrel dia. within spec, or is it more likely my slug is wrong ? The barrel didn't seem to have any tight spots.

Groove dia seems way too low for a Ruger. Did you try pushing the barrel slug through the throats with a pencil?

Instead of pin gauges or telescoping gauges, I use a set of small hole gauges. Much less technique involved compared to telescoping gauges.

Why are you using the feeler stock? I've measured slugs from 5 groove barrels with a micrometer before. There should be a sweet spot you can get a measurement from. (Also, just try your barrel slug in your throats).

[catboat]

Boolits: lee round nose flat point 158gr. dropping at .358, 159gr ac clip on wheel weights. Xlox lube.

Cyl throats: Best I can measure is .3570 to .3575, measured with telescoping gauge and micrometer. (I need to get some pin gauges)

Barrel: Slugged with lead .44 round ball, wrapped with .0015 feeler gauge, miked .353, subtracted .003, came up with .350.
this seems very small. Is this within spec's, or is my measuring technique off? .

I must be missing something-and it wouldn't be the first time. I don't understand why you are slugging your bore with a .0015" feeler gauge wrapped aroung a .44 ball.

Why don't you simply use a lead slug (your cast slug would be fine, unsized). Lube your bore with something like Marvel Mystery Oil, or Imperial sizing die wax/lube (and your slug). start the slug through the muzzle (tap it in). Use a non-marring rod (brass/wood) and push the slug through the barrel. Once the slug is inside the bore, give it a few quick raps (on the push rod) to expand the slug into the bore. Continue adding controlled force (tapping or pushing the rod) to push the slug through the bore, into the barrel/frame/cylinder area (with the cylinder out). Capture the slug and measure the diameter with a calibrated caliper or suitable measuring device. You are looking for the GROOVE diameter (the widest part of the bullet, in the bore), not the bore diameter. You would like the cylinder throats (and the lubed bullet) to be .001" -.002" larger than the GROOVE diameter. This will minimize gas blow by and reduce/eliminate barrel leading.

Pay particular attention as you pass the barrel slug through the part of the barrel where it goes through/threaded onto the frame. It is possible the frame is tight, resulting in the barrel's bore/groove diameter to be smaller here, as compared to the rest of the barrel. This isn't good. This constriction, early in the barrel (if present) will reduced the bullet fired from the cylinder as it passes through the frame area. As the fired bullet "clears" the frame area of the barrel, into the rest of the "larger diameter barrel", you now have a bad situation: an bullet that is smaller diameter than the barrel-permitting hot gases to blow by, causing leading. Even if you size your bullets to a "larger size" (ie .360", if the barrel constriction in the frame area is tight, ie .355", then your "large" bullet (.360"), will be sized down to .355" as it passes through this area. If the rest of the barrel has a .357" GROOVE diameter, that means your "large" (originally .360" diamter) bullet, is now .355", and in a .357bore. That means the bullet is .002" undersized.

Not saying this is the case on your gp100, but it's possible. It's something you need to be aware of when you do your bore slugging check. If this is the case (barrel constriction present), you may need to firelap the constriction open, to eliminate the excessive sizing down of any diameter bullet in the loaded cartridge. If the barrel is pinched at the frame area, it doesn't matter what you size the original loaded bullet in the cartridge. If there is a barrel constriction, THAT CONSTRICTION diameter, will be the bullet diameter. Too small a bullet in a too big bore is a recipe for leading.

Where did the technique of "wrap a .430 round ball with a .015" feeler gauge" come from, to get a groove diameter on a ~ .358" barrel?

[double bogey]

Sorry I wasn't clear. The 44 round ball is the only pure lead I have. After pushing thu the bore I measured. Seemed small, so I assumed I was in a groove. So I pinched a .0015 feeler gauge around the slug and measured, then subtracted the feeler gauge. I have seen this recommended for 5 groove barrel. Tried it with the slugs from my .44 SBH, and it measures the same as it mikes after subtracting the feeler gauge. This is the third slug I have done in this pistol, all the same. Doesn't seem to have a restriction at the frame. I ordered a .357 sizer already, so I will try that. Tried the slug in the throats, it falls through.

[silverado]

My GP slugged at 357. I have shot 357 and 358 commercial cast with w231 and never leaded. Recently with cast, some pc boolits all sized 358 on top of titegroup, cfe pistol, and h110 with no leading. I'm sure smarter people will speak up but I highly suspect your slugging should be suspect.

[Sgtonory]

I also have had 0 issue with my GP100 size to .358 and push up to 1150 FPS with Alliant 300MP. Love that gun.

[double bogey]

Maybe my slugging size issue comes from starting with too large of a slug. I'll look for a smaller pure lead slug to start with, and try again. Thanks for all the input.

[silverado]

Are you rotating the slug and using the largest reading or measuring the point between lands? You should probably take the largest reading from your slug.

[GaryN]

Another thing. If the .357 die isn't working you can use the bullets as dropped from the mold with 45/45/10. It works in my 357 and 44 magnum.

[freebullet]

Pull one of your loaded boolits from the case. Measure it, is your crimping sizing the boolit down? I did that even though I knew better and it leaded up quick. Like 6-12 shots turned it into a smooth bore quick. It might do better with .359, judging by the throat size you have. I would not size down from .358 you might be doing it already and that could be your problem.

[double bogey]

Ok, I oiled up my barrel and one of my boolits that measured .3585. Pushed it thru and came up with the widest point of.3552. Closer, but still seems small. The whole barrel was tight, not noticeably tighter at the frame. The slug will not go into 2 of the cyl throats.
I didn't pull a loaded boolit, but I am roll crimping in the crimp groove. Pretty sure I'm not sizing the boolit down any. I can do this after I get some work done today.

[freebullet]

I was roll crimping too, swaging down a cast boolit during loading happens easily.

Your bore slug seems very small. What are you measuring it with?

[double bogey]

Pulled a few boolits and compared with new. All measure the same. I have used 2 micrometers, a Chinese and a starrett. They both measure the same .358. Tried 2 different types, some bought, and some I cast. I am ordering pin gauges tonight, want to check cyl throats. Seems like a couple are tighter using the slug.

And my slug measurement is at the widest. Also used feeler gauge and subtracted, got the same measurement. Using the lighted magnifying glass and reading glasses to insure not reading between lands.

[freebullet]

At this point I would think there may be a constriction somewhere down your barrel. That just seems really small to me.

[double bogey]

I think so too. I am going to find a smaller round ball of pure, and try again. I just ordered some pin gauges to check my throats, could be the 2 tight ones are causing my problems. The slug won't enter 2 of them.

[legi0n]

I think so too. I am going to find a smaller round ball of pure, and try again. I just ordered some pin gauges to check my throats, could be the 2 tight ones are causing my problems. The slug won't enter 2 of them.

How are you measuring the bore slug?
Do you use a v-shaped anvil?
Being a 5 groove barrel, it's not possible to measure the slug with a regular mic.

[silverado]

How are you measuring the bore slug?
Do you use a v-shaped anvil?
Being a 5 groove barrel, it's not possible to measure the slug with a regular mic.

Whoa.... my GP is a 6 groove.... are some 5 grooves? If it is 5 groove I have been trying to figure how to measure without a special v clamp for my smith.

[legi0n]

Whoa.... my GP is a 6 groove.... are some 5 grooves? If it is 5 groove I have been trying to figure how to measure without a special v clamp for my smith.

try this (read thread to the end)

http://castboolits.gunloads.com/showthread.php?119952-how-I-measured-a-5-groove-bore

[legi0n]

Whoa.... my GP is a 6 groove.... are some 5 grooves? If it is 5 groove I have been trying to figure how to measure without a special v clamp for my smith.

I just checked with Ruger: all current production GP100 are 5 groove.

[silverado]

I just checked with Ruger: all current production GP100 are 5 groove.

I bought mine maybe 18 months ago and it is definitely a 6 groove. I did measure trailing/leading edges of lands and got .358 on my 6904 (woohoo same as my beretta 92) but didn't feel comfortable with the method since my 358 sized boolets don't chamber too well, even when loaded short. Sorry op... back to original programming....

[double bogey]

I have one of those articulating lighted magnifying glasses on my bench. carefully working with the mike I got .355, using the .0015 feeler gauge around it, and subtracting .003 I got the same thing. I remember seeing an anvil mike somewhere, with mine or my dads tools, never occurred to try that. if I can't locate I think I can borrow one.

I looked at the post with the formula, I might be able to work the last one, but I think if I tried the first one I would probably come up with a 8.3718 or so :-?

[captaint]

I recently bought a new GP100. First thing I did was clean it good and go to work with the pin gauges. With patience I can just get the .358 minus pin through all 6 holes. Very happy. With that, I haven't worried about barrel dimensions. The gun shoots very well with the 3 different boolits I have put through it. Likes the Lyman 358156 especially. Also, the barrel definately has 5 grooves. One of the first things I checked. Thought sure it would have 6. Nope.
OP - I would try sizing to .358 and let them fly. Let us know. Mike

[GP100man]

Use 36 cal. round balls for C&P revolvers.

Slug the cone end `bout 1" then the muzzle the same.

Use short peices to push the slug in the cone end.

ya got a constriction or a very ruff barrel, does it snag on patches when ya push 1 thru ??
1 of mine was snaggy when I pushed a patch thru on a jag & many strokes with Flitz done the deed.

& just where is the barrel leading after 1 2 3 4 5 6 shots???

[Maximumbob54]

Is Xlox up to full power loads? I would think you would need at least 50/50 for that use. I've shot a lot of my own cast through my GP100 and SP101 and when I used WLL's BAC I never had any leading in either gun. But that is a stiffer lube than just straight alox.

[Sig]

I pour soft lead into a piece of spent brass from said gun. Let cool & use a kinetic bullet puller to remove the slug. I'll give the slug a couple of taps on the end with a hammer to enlarge the diameter if necessary. Now I have a tapered slug that starts easily in the barrel.

[GP100man]

^^^^^^ Brilliant !!!!

[double bogey]

Been busy all day today, but got pin gauges ordered. Also got a .357 sizer. Have not opened it yet. Boolits come out of the mold at .358 to .359. It didn't only not shoot higher powered loads, it wouldn't shoot all the way down to mild .38sp. I do hvac, so I may be tied up a bit from here on in (imagine that). I may not have a lot of time to pursue this, at least not as much as I would like. But I will keep this updated. And thanks to all who have contributed, I'll get it fixed yet.

[double bogey]

Got pin gauges. Throats will take a .357 (under) pin easily, a .358 (under) will not go. I am sizing to .357, will try a few and see how it works. Can't locate an anvil mic. yet.