PDA

View Full Version : Slugged my GP-100!!!



snuffy
11-28-2010, 07:46 PM
:veryconfu
My new GP-100 has only been shot a little. I tried both J-words and some 358156 GC. Groups were only so-so. No leading with loads of Unique and ww-231. J-words were winchester 158 JHP that I bought years ago in bulk from midway.

Anyway, it wanted to know what the bore and cylinder throats measured. I tapped a .390 soft lead ball down the entire BBL. It measured .3525! Did the same thing chamber forward with the throats, they were .358,(well one was). Then after reading the thread about also measuring the throat/forcing cone of the barrel, I tapped another ball in from in back with a brass rod at an angle. The bbl. throat starts at .374, and tapers down quickly to .352! Oh, I also just barely started a ball from the front with a length of brass rod already in the barrel that was long enough to push that ball back out. I wanted to see if the barrel threads were constricted, even though I didn't feel it on the full-length ball. It was also .3525.

Can this be made to work? I intend to shoot mostly lead boolits through it, but may burn through the 500 JHP'S I already have.

I'm tempted to return it to Ruger along with a note asking how in the heck they could let something like that get past inspection.:mad:

x101airborne
11-28-2010, 08:11 PM
My opinion based on limited experience is to send the piece back. Or, you could use a 9mm boolit!!

theperfessor
11-28-2010, 08:13 PM
My GP100 has a five groove barrel. Unless you have a V-anvil mike or a special gauge or use one of several special techniques you're not going to get an accurate measurement. With a regular mike you're reading from the land on one side to the groove on the other.

lwknight
11-28-2010, 08:15 PM
Thats a bummer to the max.
My GP-100 slugs at .356 and .3565 on all cylinders.
It shoots like a laser with .357 boolits and so-so with .358 boolits.
Also its great with the standard .357 bullets that actually measure about .3565-.3568

Lucky me I guess.

snuffy
11-28-2010, 09:35 PM
Well I'll be dammed, perfessor! You're correct, I just didn't see it until you said it! That means it's probably somewhat bigger. I'll see if there's any mics at work that I can use or somebody can mic it for me. I'll have to get one for my own use.

bhn22
11-28-2010, 09:46 PM
Try .358 to .359 & see how it works. One thing you're looking for when slugging is bore consistency. Pay special attention when the ball reaches the area of the barrel that screws into the frame. If you detect extra resistance when pushing the slug through this area you likely have the dreaded "thread choke". This requires lapping or barrel replacement to correct. It's not just a S&W disease, Rugers have it too.

geargnasher
11-28-2010, 10:47 PM
Snuffy, you're way ahead of the game taking the essential measurements.

If you don't have any significant amount of "thread choke" (I like that term, by the way, think I'll add it to my repertoir!) and the cylinder throats are bigger than the groove diameter, who cares? What I would probably look at next with regard to accuracy is getting the cylinder throats identical, which means reaming the undersized ones up to the largest size. Also, measuring fired cases from each chamber or "impact slugging" the chambers themselves can be important too, if the throats of some are undersized the chambers may be too, and that can affect pressures/accuracy.

A friend of mine has a 6" stainless GP100 that had a slight thread choke and .001-2" undersized cylinder throats (none exactly the same). The gun came to him used/cheap because the previous owner thought it had a bulge in the barrel. A good cleaning revealed it was an accumulation of lead that had been shot over with jacketed bullets so much that it was indistinguishable from the bore metal with a light. The leading had been caused by the grossly undersized cylinder throats. I reamed them to a uniform .358" with a bushed chucking reamer, firelapped the barrel, then polished the cylinder throats and forcing cone. Gun now shoots 1.5" at 25 yards and doesn't lead at all. Uniformity counts.

Gear

snuffy
11-30-2010, 09:30 PM
Is there a rule that says you can't quote yourself?


Well I'll be dammed, perfessor! You're correct, I just didn't see it until you said it! That means it's probably somewhat bigger. I'll see if there's any mics at work that I can use or somebody can mic it for me. I'll have to get one for my own use

I checked at work, there's no "V" anvil mikes there. I also checked the internet machinist supply houses, the cheapest would be a starret with multiple anvils, would be pushing $200.00!

Bottom line is I still don't know what the bore is. I'm not 200 bucks curious, so I'll have to just figure it's .357 and load accordingly. I'll also check all 6 cyl. throats, might just get lucky there, not need to have them reamed.

theperfessor
11-30-2010, 10:46 PM
Well, you do know how big your cylinder throats are, so why not see if the ball you used to slug the bore will go through the smallest throat? It is generally accepted that the throats should be slightly larger than the bore and the bullet should should be sized according to the throat size, so if the bore slug falls through then you at least know that the throats are larger than bore size. If it won't go through then the throats may need to be enlarged.

I've got a GP100 and it shoots almost anything I run through it sized to .358 without leading.

People talk about using pin gauge sets for checking cylinder throats. I have a set myself. But what would be really handy is a "hole gauge" set with precision bored holes in .001" increments over a size range that would cover .17 to .50 calibers. Then the issue of slugging a bore with an odd number of grooves would be easily solved to a reasonable level of accuracy.

lwknight
11-30-2010, 10:48 PM
I have seen several posts by others saying to wrap some .001 shin stock around the boolit and measure that way. Naturally , you deduct the thickness of coth sides of stock combined. In case of .001 deduct .002 from the reading.

snuffy
11-30-2010, 11:30 PM
Another of those "why didn't I think of that"! Or of course, then a slap to my own forehead!!:idea:

The slug that I pushed through the barrel slips like it MAY have a half thou clearance in all the throats. What I'd call a slip fit, not size-on-size. I'd call that good to go with .358 sizing of an as-cast .3585 as a best case scenario.

Yeah, a set of hole gauges like that would be the cat's meow. Sounds like the start of another internet search!

Thanks again perfessor.

ironhead7544
12-01-2010, 02:57 AM
Ive been using the slugs from LBT. Well worth the money. My Ruger Flat Top measures .430 bore and .432 throats. Had a .001 restriction at the barrel junction. You could feel the restriction when slugging with the LBT slugs. Also can feel the restrictions from dovetails on rifle barrels. LBT also has a service that measures slugs from your bore that you send in. Check out the site for more info on slugging. I learned a lot.

NHlever
12-01-2010, 09:50 AM
The "thread choking" or restriction that is found in many guns is the result of the face of the frame not being ground perfectly square with the barrel threads, so it is actually a slight "kink" caused by a slight bend in the barrel when it is torqued on. Many times, it will also be found that the front sight is too low......... another indication of that slight bend.

geargnasher
12-01-2010, 03:06 PM
I don't follow that explanation too well, if the barrel was bent by an unsquare frame, the restiction would be elliptical, wouldn't it? I have seen plenty of choked revolver barrels, but the slugs were all round.

Gear

MT Gianni
12-02-2010, 11:23 AM
Snuffy, if you have a set of calipers and can get a reading on the 5 groove bbl measure the back side with a mike. It may not be exact but should put you very close.

NHlever
12-02-2010, 05:54 PM
I don't follow that explanation too well, if the barrel was bent by an unsquare frame, the restiction would be elliptical, wouldn't it? I have seen plenty of choked revolver barrels, but the slugs were all round.

Gear

You are correct of course. How much of an elipse would being out of square .002-003 over a distance of .687-.700 give you? I hadn't taken the time to figure that out, and had over looked that since I have measured how much out of sqare many frames are before the barrels were installed....... I just made the assumption that the out of square condition was causing a problem. That will teach me not to make assumptions so easily. I was involved with the tooling those guns were made on for 36 years, give, or take a month, or two.

dicko
12-07-2010, 05:34 PM
But what would be really handy is a "hole gauge" set with precision bored holes in .001" increments over a size range that would cover .17 to .50 calibers. Then the issue of slugging a bore with an odd number of grooves would be easily solved to a reasonable level of accuracy.

I don't have a set like that, would take too long to make. But I have something close. Each 5 groove slug I need to measure, I drill and bore a hole in the end of a small piece of bar stock in my lathe. I open it up with very fine cuts until the slug just enters, the measure the hole with a small hole gauge and mike. Being more than one stage in the process increases cumulative eror, but it has proved, by cross checking with other methods, to be nevr more than three tenths out and usually a good bit closer. I mark these improvised gauges and keep them.

MtGun44
12-07-2010, 09:07 PM
Rather than out of square, the problem is more likely just the factory tightening the barrel
a bit too much to clock the front sight and this constricts the barrel in the thread area.

If you have one of these barrels and know a GOOD gunsmith, he can pull off the bbl and
take a HAIR [that is a technical term ;-) ] off the barrel rear shoulder and then reset the
bbl with red locktight and little torque. The constriction will usually be gone.

Or you can fire lap it out. Either way, you have to get rid of it.

Bill

NHlever
12-10-2010, 08:52 AM
I won't argue since I respect the judgement of many who have responded to this thread. The sights on the round barreled guns (Blackhawks, etc.) are installed / milled after the barrel is torqued on the frames, but that doesn't mean that the barrels could not be overtorqued. The restriction is, or can be a problem however it gets there.