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GP100 Headspace Adjustment...

This is a discussion on GP100 Headspace Adjustment... within the Gunsmithing forums, part of the Firearm Forum category; Hi All, I've just been measuring my new GP100. BC = 0.005' (Tight) Endshake = 0.002' (Tight) Headspace, I measure at 0.004" The area of ...


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Old February 23rd, 2017, 09:41 PM   #1
 
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GP100 Headspace Adjustment...

Hi All,

I've just been measuring my new GP100.
BC = 0.005' (Tight)
Endshake = 0.002' (Tight)

Headspace, I measure at 0.004"

The area of the frame around the firing pin is pretty poorly finished. Can I simply dress this area down a little?



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Old February 24th, 2017, 01:40 AM   #2
 
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I'm not sure how you're coming up with that "headspace" measurement.

And NO, I would not take a file to the recoil shield or firing pin bushing.
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Old February 24th, 2017, 06:46 AM   #3
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Petrol and Powder, Revolver headspace is measured by using the thickest blade of a gap gauge that will fit between a chambered unfired case head and the recoil shield. For centerfire cartridges, .008~.012" is normal with .010" as optimum. Anything greater than .012" will start causing misfires. Anything less than .008" can cause the cylinder to bind.

BigGee, Before you dress down the firing pin bushing and recoil shield, try a couple different brands of 357 Mag brass because rim thickness can vary. If you find headspace is still too tight, use a fine single cut file and dress the area as smooth as possible. DO NOT use power tools!!! When you start dressing the recoil shield, chances are you will find a high spot and that will be your culprit. Once the entire surface is flush, I would stop, even if headspace is still too tight. Test fire the GP100 and if it functions OK .... especially in the DA mode .... where the case heads are not dragging, you should be good to go. If the cylinder binds up, take a couple more thousandths off.
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Old February 24th, 2017, 09:13 PM   #4
 
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Iowegan - thanks for your response !
I'll look at this at the weekend - I'm sure a little smoothing will sort it out.
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Old February 25th, 2017, 05:41 AM   #5
 
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I guess I should have been a little more clear in my response.

Yes, I understand that headspace on a revolver is measured between a casehead and the recoil shield. Since the thickness of the cartridge rim can vary from casing to casing, the casing used while taking the measurement affects the result.

.004" is very tight for that measurement, so the first thing the OP should do is use a different set of casings and re-take that measurement. I wouldn't take a file to the recoil shield based on that one measurement. That was what I attempted to convey and I did a poor job of that.
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Old February 25th, 2017, 06:59 AM   #6
 
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Iowegan, that reminds me of a revolver I had a few years ago that had a serious issue. It was a High Standard .22 LR that I believe was made in the 60's. I would load and start shooting, and most of the time, the cylinder would bind part way through, and would not want to turn.

I brought it back in, gave it a thorough cleaning, including under the ejection star, took it back out af the same thing happened. This did not happen without rounds in the chambers. It also did not happen with dummy rounds in the chambers. So, it only happened when live ammo was being fired.

The brand of ammo made no difference, and it was not jamming onone particular chamber.

I took it back to the store where I bought it, and they kept it for a few days to have their gunsmith look at it. They told me he did not know what was wrong, and gave me my money back.

I'm still puzzled, and disappointed, as I would have liked to have hept that little gun. I wonder what was wrong with it?
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Old February 25th, 2017, 07:54 AM   #7
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ditto1958, It is not unusual for Ruger or other revolver manufacturers to forget a step and not finish the recoil shield. As such, headspace will be tighter than normal and can result in the cylinder binding up when case heads rub against the recoil shield. As I mentioned earlier, centerfire revolvers typically have a headspace of .008~.012 with .010" as optimum. 22 rimfire revolvers run tighter .... .004~.008" with .006" as optimum. I think the reason why rimfires run tighter is because they don't have primers that are often seated a tad high.

I highly suspect this was the reason why your HS revolver bound up with live ammo. BTW, High Standard made some mighty fine pistols but their revolvers were pretty grim.
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Old February 26th, 2017, 05:16 PM   #8
 
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That's too bad. It was a cool little revolver. Just not so good.
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