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GP100 action binding


Fishbreath

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I thought I'd solved this after the last match, but it appears I was wrong.

 

For the last several outings now, I've been getting extreme variation in trigger pull and extremely heavy trigger pulls after a stage or two. I'm utterly confused as to why--it only started toward the end of last year, and seems to be largely independent of gun cleanliness or ammo.

 

One thought may be debris under the star--the tolerance on this gun is very tight, to the point that even a 0.002" cylinder shim prevents the cylinder from closing. It looks clean to be, but 0.002" of something might be able to hide pretty easily.

 

Another thought may be bent moons. These are all 0.020" and have been through a pretty busy season. Maybe they're bent enough so they they're springing up against the frame? They don't look that bad, although some of them don't sit flush against the back of the cylinder if I press on one prong. Doesn't seem to come up at all in dry fire, though.

 

I'm not sure what else it might be. Any thoughts?

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It's a Super GP100 isn't it, in 9mm or a 38? 

Either way if it is a 9mm have you tried some factory ammo without moon clips, after firing several moon clips and it starts dragging?

Not done it with a Ruger but you can get a reamer and cut a little bit more end shake on the crane.

When it does it are you able to spin the cylinder freely?

Can you identify whether it's the cylinder dragging and causing it or is it an issue within the action itself.

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1 hour ago, pskys2 said:

It's a Super GP100 isn't it, in 9mm or a 38? 

 

Yup, Super GP in .38.

 

1 hour ago, pskys2 said:

When it does it are you able to spin the cylinder freely?

 

Not strictly sure—it seems to happen mainly on the clock, frustratingly. I'm going to fit a practice in this week and see if I can make it happen a few times for testing.

 

There's no real resistance to opening the cylinder, and in many cases, thumbing the hammer back will turn the cylinder and free things up again. Going by the safe table after each stage, I found that I could feel some notchiness in the trigger at certain points in the cylinder's rotation (I'd feel it again after counting to 8). Wiping the face of the cylinder beneath the ejector and the top face of the ratchet with a dry cloth always made the notchiness go away.

 

1 hour ago, pskys2 said:

Either way if it is a 9mm have you tried some factory ammo without moon clips, after firing several moon clips and it starts dragging?

 

I shot factory .38 Special for most of last year, and didn't have the same problem, but I also didn't have the same problem for the first few months of using reloaded .38 Special, either.

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What bullets and powder are you using? 

 

The likely culprit (had the same thing happen to a S&W 610 of mine) is powder residue buildup on the cylinder between the cylinder and forcing cone. Take a soft brass bristle brush and, after every stage, go to the safe table and brush the front of the cylinder and lightly brush the forcing cone. See if the drag goes away. 

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1 hour ago, GrumpyOne said:

What bullets and powder are you using? 

 

The likely culprit (had the same thing happen to a S&W 610 of mine) is powder residue buildup on the cylinder between the cylinder and forcing cone. Take a soft brass bristle brush and, after every stage, go to the safe table and brush the front of the cylinder and lightly brush the forcing cone. See if the drag goes away. 

 

Ibejiheads 160gr and Bullseye. Which is a notably dirty powder, and I do see some soot rings on the barrel-side face of the cylinder, around the front ends of the chambers. I'll make sure to take a brass brush up to the range with me this week.

 

Any recommendations for something cleaner?

 

1 hour ago, pskys2 said:

As I said earlier see if you can add a couple .001's by reaming the barrel of the crane  .002" is too tight of end shake.

 

While the shim suggests it's .002", feeler gauges at the barrel/cylinder gap put it at more like 0.005".

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4 hours ago, Fishbreath said:

 

Ibejiheads 160gr and Bullseye. Which is a notably dirty powder, and I do see some soot rings on the barrel-side face of the cylinder, around the front ends of the chambers. I'll make sure to take a brass brush up to the range with me this week.

 

Any recommendations for something cleaner?

 

 

While the shim suggests it's .002", feeler gauges at the barrel/cylinder gap put it at more like 0.005".

I use Clays/ClayDot at between 2.8 to 2.9.  Very clean.

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I had the same problem with an S&W 586 I had cut for moons about 15 years ago.

We found that the forcing cone was cut a couple thousands out of square. I had (I think) about .002 cut to make it square and the problem went away.

It only happened in a long string of fire and it seemed to be the heat in the cylinder binding it up.

Turns out a guy can fire rounds in a moon clipped gun way faster than speed loaders!

Who saw that coming??

 

I have some revos that are very likely to bind with stuff under the star.

Funny how some are and some are not.

 

I also use some .020 moons in a couple guns.

You can check them for bends by either stacking them on a post or laying them on a mirror or something hard and flat.

I straighten them with a small needle nose pliers one petal at a time. You can get them pretty flat that way.

 

Good luck with your search.

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7 hours ago, Fishbreath said:

 

Cold, 0.007-0.008" (cylinder pushed to rear). I'll measure hot next time I'm at the range.

If you can push the cylinder to the rear and get much of a different measurement that may be part of the problem.

I don't know much about Ruger's but I just fixed a friends 627 that had what I thought was a ton of end shake, the

gap went from 0 to .006, add some heat expansion after a wheel or 2 and the cylinder face was rubbing against the

barrel face. Doesn't take much to make them go hinky.

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I've got about 0.005" of endshake, it seems—it'll pass a 0.002" feeler gauge, but not quite a 0.0025" gauge, pushed forward. I think residue buildup might have been part of it. I gave the forcing cone and cylinder face a good scrub, and can see a bit more light between them than I could before. I'll have to add 'hot after 100 or 150 rounds' to my list of B/C gap measurements to take when I'm at the range.

 

6 hours ago, Dr. Phil said:

We found that the forcing cone was cut a couple thousands out of square. I had (I think) about .002 cut to make it square and the problem went away.

 

I don't think I have this problem, since it seems like the feeler gauge slides around without binding more at the top or bottom, but good to keep in mind either way.

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Hmm, you know when I had an issue with my cylinder release on my 66 it caused a lock-up issue for me. But only during live fire.

 

I have no idea how supers work but I would remove your custom release and give it a go under match conditions. It would at least remove a variable that you have introduced.

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It's a possibility for sure, especially since nylon expands 2-4x as much as aluminum with heating, but I don't think it's a strong contender.

 

The GP100-style cylinder release is a lever arm that pivots around a pin in the left side of the frame behind the recoil shield. Push inward on the release, and an arm behind the recoil shield levers forward. A peg on the lever presses a button on the back of the cylinder, which unlocks the cylinder. A spring and plunger in the latch hold it in the cylinder-open position when the cylinder isn't pushing back on it, and the lever arm interferes with the hand to keep the trigger from being pulled when the cylinder isn't closed.

 

One of the prime reasons I don't think it's a strong contender is because this particular extended release barely even keeps the hand from moving when the cylinder is unlocked, and the arm is in the forward position. It's way out of the way when the cylinder is in place.

 

A secondary reason is that I had the start of this problem at the end of last year, a month or two after switching mostly to handloads with Bullseye but before I came up with the extended cylinder release.

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17 hours ago, Fishbreath said:

The FLGS happened to have some Clay Dot on the shelf, so I bought the per-customer maximum of two pounds. Looks like I'm going to be changing things up soon.

good powder and 2 lb is a lot of it at the charge weights we use 🙂

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Check your moons on a piece of glass and if you feel any movement replace or straighten the clips.  Powder under the star is normal with most powders but the new Unique  I've found to be very clean and would probably stick with it.   

 

Test one thing at a time or you will be chasing your problem forever.

 

 

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