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Old April 25, 2006, 08:15 PM   #1
Ammo Junky
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excessive head space?

Hears the deal, I have a rem .243 adl. The fired cases measure 1.625" with a stony point headspace guage. My go guage measures 1.617 and the no go guage measures 1.621. My fired brass is .004 over the no go guage and the chamber is probably a little more than that. I figure the gun is defective. Well I try the no go guage in

.243 win coyote less than 100rd
2 different ruger .243 both less than 100rd
rem .308 vs, new unfired

the bolt closed on the no go guage on all of them
Sooooo, whats the deal? It would have to be a coincidence of coincidences for all five guns from 3 manufactures to happen to be defective. An unfired rem brass measured 1.620 so I am thinking the guages must have been on the very min of tolorances. I am lookin to get a field guage, but mid way does not cary that one.
Ideas?
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Old April 30, 2006, 03:32 PM   #2
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AJ,

Go and no-go are something of a judgement call. If I look at the SAMMI drawing for the .308 chamber, it gives a minimum of 1.630" and a maximum of 1.640". But 1.640" is a worst case field length. A set of armorer's gauges I have for .308 say the go is 1.630" and the no-go is 1.634" and the field no-go is 1.637". A set of gauges I got with a reamer from Dave Manson for the 6.5-284 uses a no-go that is only 0.0025" above go (he is really trying to keep that match chamber within tight limits). Those gauge ranges of 0.0025"-.0040" chamber tolerance are reasonable to expect a gunsmith to meet, but factory work doesn't always stay within them.

Anyway, the first question is whether your Stoney Point gauge is meauring accurately or whether you have a bad set of headspace gauges? See if you can borrow an RCBS precision mic and check the gauges and case in that for verification? "0" on the precision mic is 1.630", though it is not a true precision instrument and mine is off about 0.001". Still, if it gives you numbers that are different by more than, say, 0.002", you definitely have a measuring tool issue. You could call Stoney Point about it.

I've had bad headspace gauges before from Forster that were out by up to 0.002". Buy gauges from Dave Manson or another reputable reamer maker. I also suspect your measuring gear because the gauges are showing short of SAMMI numbers and your case is 0.005" below the SAMMI minimum case dimension. I could see a little spring-back, but not that much.

Nick
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Old April 30, 2006, 04:02 PM   #3
Jim Watson
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Technique

Quote:
the bolt closed on the no go guage on all of them
Tell us how you did that.

Did you run the gauge into the chamber like a cartridge and see if you could close the bolt like loading the gun?

Or did you strip the bolt, slide the gauge into the chamber and see if the bolt handle would go down with fingertip pressure?
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Old April 30, 2006, 04:08 PM   #4
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Jim,

Are you thinking a cold forged chamber? That would be bad. Should show up in case marks.

AJ,

When you've stripped the bolt per Jim's point, and assuming the gauges still go in with no bolt stopage, can you hear them fall against the shoulder and bolt face when you tip the muzzle up and down? Any rattle?

Nick
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Old April 30, 2006, 04:39 PM   #5
Jim Watson
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Nick,

I don't know what you mean by cold forged chamber. Many barrels are made that way now, are they more subject to headspace variation than reamed chambers? Or do you mean that he might have bashed out the chamber with the gauge itself? I think it likelier that he might have bent the gauge, but that assumes the gauges are not as hard as the barrel and I don't know about that.

I found a reference where one guy said he could chamber any gauge in any barrel... with a heavy enough hand on the bolt.
Hatcher's notebook describes an alarum over headspace in 1917s because people thought they if could just ram in a no-go gauge like it was a cartridge that it was proof of excess chamber headspace.
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Old April 30, 2006, 06:07 PM   #6
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Jim,

I was thinking the second choice by "cold forging" - a dent in the chamber. That's why I thought it should show up on the case somewhere. Perhaps I should have said "cold-reforming". The headspace gauges I have are hardened and ground. About RC 50, like 1911 rail plates. Hard enough to keep their shape, so the gauge remains accurate, but not so hard they tend to chip when dropped or are otherwise accidentally abused. Rifle barrels that are heat treated are usually about RC 32 at the most. Any harder and they become too brittle and are in danger of shattering. Anyway, short of breakage, the gauge will win in a contest with the chamber.

Your point on the light touch is well-taken, though. There are normally several thousandths of draw when a barrel is tightened to the receiver, so there is some flexibility in the whole system. Forcing a gauge in mostly stretches everything, but I would be watching out for chamber marks on the case to indicate this happened. Worst case, AJ can have a gunsmith pull the barrel, set the shoulder back a few mils and clean the chamber to a desired shorter dimension. That will make brass last longer if he full-length resizes for a magazine.

Nick
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Old April 30, 2006, 06:16 PM   #7
Jim Watson
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You will note he got the no-go into FIVE different rifles. Nyekulturny.

Check Hatcher, chapter X, pg 247 (in my edition) for an account of the same phenomenon in a shop setting up to sporterize 1917 Enfields.
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Old April 30, 2006, 06:58 PM   #8
918v
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If virgin brass measures .005" under your fired brass, then your headspace is perfect, nevermind what your gauges say. They could be on the tight side to account for FL resized brass. Some people like to FL size every round, so the gunsmith will set the headspace on the tight side to maximize brass life. This way there is no adjustment of the FL die, just screw down to the shellholder and forget it.
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Old April 30, 2006, 07:27 PM   #9
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Jim,

Yep. Agree. They were camming the gages in on those Enfields and that fit is made possible by stretching steel. It was interesting to go back and re-read the parts of Hatcher leading up to the headspace gaging portion; that slamming a bolt will forshorten the case enough to change its headspace. This reminded me that when you look at the SAMMI specs for the .308, that the maximum headspace for the .308 case is 1.634", while the minimum headspace for the chamber is 1.630". The assumption that the action will have no problem shortening the case that extra 0.004" by forcing it to fill out into the chamber is inherent in that spec, and of course there is about 0.011" of extra diameter everywhere in the chamber except the neck. That's what gets filled.

If someone wants to download the relevent pages from Hatcher's Notebook on how to gauge headspace, I have a temporary download link to a .PDF file here.

Nick
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Old April 30, 2006, 08:27 PM   #10
Ammo Junky
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I droped the guage in like a rd and closed the bolt. No bolt stripping, no unusual resistance to closeing the bolt. Unfired rem brass measures in the .007" range shorter than fired brass. That is a little more head space than I care for, but not what I would expect to be dangerous. I have a growing suspision that my guages are on the short side. I bought them on midway and one of the reviewers commented that they were on the short side. Head space quages in genral measure .010" shorter on the stony point than what is printed on the guage. That being the case my fired brass is 1.635", which is right in the middle of the range UN gave for sammi chambers. I know it is off topic, but my bbl picks up copper fouling like a kid picking up candy at a parade. UN, thanks for the tip on Dave Manson guages. I had no idea what brand was good or bad.
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Old April 30, 2006, 10:01 PM   #11
918v
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.007" is not bad. More than .011" is excessive.
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