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Head Space Brain Work

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floydthecat View Drop Down
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    Posted: Jun 03 2020 at 6:44pm
Lets have a math lesson before you spend your hard earned $ for maybe no reason. Please feel free to comment.

Field gauge is...lets use 1.300. Minimum case length is 1.280. Do the math and arrive at .020. That is the maximum clearance allowed before your carbine is deemed out of spec. and dangerous.

Most off-the-shelf ammo I measure is from 1.284 to 1.286. Lets use 1.285 as an average. The difference between 1.300 and 1.285 is .015. That is .005 BELOW the allowable .020 when using a field gauge 1.300 and the SAAMI minimum case length of 1.280.

See where I am headed with this?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote W5USMC Quote  Post ReplyReply Direct Link To This Post Posted: Jun 03 2020 at 7:49pm
Originally posted by floydthecat floydthecat wrote:

See where I am headed with this?

Nope! But maybe I'm just slow.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote floydthecat Quote  Post ReplyReply Direct Link To This Post Posted: Jun 03 2020 at 8:06pm
Give an old man sitting at home with COVID in the family, watching all the crap going on across the country and being constantly told it’s my fault....a little pause to wander.😁

Point is, I don’t think a carbine is going to explode just because a field gauge says it’s bad. Pretty sure a lot of barrels and bolts have been replaced, when there was no need. I think if one watches his brass, it will keep shooting as long as the firing pin can punch the primer.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote W5USMC Quote  Post ReplyReply Direct Link To This Post Posted: Jun 03 2020 at 8:13pm
Originally posted by floydthecat floydthecat wrote:

I don’t think a carbine is going to explode just because a field gauge says it’s bad. Pretty sure a lot of barrels and bolts have been replaced, when there was no need. I think if one watches his brass, it will keep shooting as long as the firing pin can punch the primer. 

Now that I understand and also agree with.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 03manV Quote  Post ReplyReply Direct Link To This Post Posted: Jun 03 2020 at 8:59pm
It is not going to "explode"; one will probably first notice light primer strikes; and when headspace reaches really long- the extractor will not slip over the rim- then the cartridge won't fire at all.

Headspace is kind of "self limiting" when in excess! JMHOWink
Don
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jackp1028 Quote  Post ReplyReply Direct Link To This Post Posted: Jun 03 2020 at 10:16pm
Excessive head space will not necessarily lead to light primer strikes. The firing pin travels much farther than any potential excessive head space and can definitely reach the primer and discharge the round. What happens with excessive head space is the case will stretch to fit the chamber and possibly fail near the unsupported area near the rim. This will allow hot gasses at high pressure to blow back into the receiver possibly injuring the shooter. I guess someone has determined that .020" is the maximum amount of stretch that is safe before the case is likely to fail on a new case, thus the "field" gauge specification. This is particularly important for reloads as they become inherently weaker each time they're trimmed and reloaded.   
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 03manV Quote  Post ReplyReply Direct Link To This Post Posted: Jun 03 2020 at 11:33pm
" The firing pin travels much farther than any potential excessive head space and can definitely reach the primer and discharge the round." This statement is not applicable in the case of really long headspace that will not allow the extractor to snap over the rim of a cartridge. If the Cartridge is "ahead" of the extractor the firing pin cannot reach the primer and it will not fire. I had this unfortunate situation in just the last week or so. Required a barrel change.

As long as the extractor snaps over the rim, headspace is determined by the rim/bolt face/extractor 
space relationship, even IF the headspace is out of spec "long".

As said in the post above, if a case fails- whether it splits, the head separates, the primer is punctured, or the web fails due to stretching- hot gas is not a good thing to have escape.

Reloaders must for sure be vigilant regarding the condition of their weapon's chamber and examine fired brass for abnormal signs before reloading.
Don
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jackp1028 Quote  Post ReplyReply Direct Link To This Post Posted: Jun 04 2020 at 12:11am
If the bolt is in battery, the firing pin can travel up to .060" beyond the bolt face regardless of whether or not the extractor snaps over the rim. The cartridge can only be "ahead" of the bolt face by the amount of the actual head space which, if .020", leaves up to .040" of firing pin travel past the cartridge base. This is plenty to discharge the primer. If your extractor has not snapped over the rim, I suspect your bolt was out-of-battery resulting in the safety feature of the receiver firing pin cam preventing sufficient movement of the firing pin to reach the primer thus failing to discharge the cartridge.       
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Post Options Post Options   Thanks (0) Thanks(0)   Quote floydthecat Quote  Post ReplyReply Direct Link To This Post Posted: Jun 04 2020 at 4:22am
Good stuff guys. The conversation is developing just as I hoped.

My understanding is that head space gauges are engineered on the short side, conservative that is. SAAMI specs. usually include a chamber drawing and knowing what the max chamber dimensions are would be useful. The current SAAMI specs. don’t list the chamber dimensions for 7.62 x 33, at least I do not see any. Maybe they are in a publication that I have not seen?

I have seen the aftermath of a ruptured case and it wasn’t pretty. Luckily, just blew out a magazine and burned the shooters hand. I was able to repair the gun and upon interrogation, the shooter admitted to firing some gun show reloads. Only the good lord and the reloader knows how they were loaded.

I certainly do not want to be guilty of encouraging anyone to shoot a dangerous weapon. I just think in reality it’s not completely necessary to junk a carbine action based on head space gauges alone, especially for the casual shooter. I think it’s reasonable to expect you can take one a couple thousandths past 1.300 and remain completely safe. There is only .005 between a 1.295 no-go and a 1.300 field reject gauge and most carbines measure closer to 1.297 than a no-go of 1.295. Taking the max out to even 1.302 or 1.303 is another half-life left in the gun. Just watch your fired brass, use good quality ammo and avoid short-cased ammo.

I am not a fan of having to shop ammo to find something that works, but I have never measured a commercial 30 carbine round off the shelf of less than 1.284 in length, but I do check it all to make sure I get no long or short cases. I think if the action locks-up properly and one maintains the .020-or-less space allowance between the cartridge base and the bolt-face and uses good quality ammo, he can safely shoot-on a bit longer. Defer that $400 (or whatever) barrel change and purchase more ammo.


Edited by floydthecat - Jun 04 2020 at 5:16am
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Post Options Post Options   Thanks (1) Thanks(1)   Quote 1st M1 88 Quote  Post ReplyReply Direct Link To This Post Posted: Jun 04 2020 at 7:54am
I found this for carbine chamber specs from sami.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 03manV Quote  Post ReplyReply Direct Link To This Post Posted: Jun 04 2020 at 8:07am
Originally posted by jackp1028 jackp1028 wrote:

If the bolt is in battery, the firing pin can travel up to .060" beyond the bolt face regardless of whether or not the extractor snaps over the rim. The cartridge can only be "ahead" of the bolt face by the amount of the actual head space which, if .020", leaves up to .040" of firing pin travel past the cartridge base. This is plenty to discharge the primer. If your extractor has not snapped over the rim, I suspect your bolt was out-of-battery resulting in the safety feature of the receiver firing pin cam preventing sufficient movement of the firing pin to reach the primer thus failing to discharge the cartridge.       

No Jack, the head space was something like 0.080 ", way in excess; allowing the cartridge to move so far forward that the bolt would close behind it and never snap over the rim.


Don
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 03manV Quote  Post ReplyReply Direct Link To This Post Posted: Jun 04 2020 at 8:16am
Originally posted by floydthecat floydthecat wrote:

Good stuff guys. The conversation is developing just as I hoped.

My understanding is that head space gauges are engineered on the short side, conservative that is. SAAMI specs. usually include a chamber drawing and knowing what the max chamber dimensions are would be useful. The current SAAMI specs. don’t list the chamber dimensions for 7.62 x 33, at least I do not see any. Maybe they are in a publication that I have not seen?

I have seen the aftermath of a ruptured case and it wasn’t pretty. Luckily, just blew out a magazine and burned the shooters hand. I was able to repair the gun and upon interrogation, the shooter admitted to firing some gun show reloads. Only the good lord and the reloader knows how they were loaded.

I certainly do not want to be guilty of encouraging anyone to shoot a dangerous weapon. I just think in reality it’s not completely necessary to junk a carbine action based on head space gauges alone, especially for the casual shooter. I think it’s reasonable to expect you can take one a couple thousandths past 1.300 and remain completely safe. There is only .005 between a 1.295 no-go and a 1.300 field reject gauge and most carbines measure closer to 1.297 than a no-go of 1.295. Taking the max out to even 1.302 or 1.303 is another half-life left in the gun. Just watch your fired brass, use good quality ammo and avoid short-cased ammo.

I am not a fan of having to shop ammo to find something that works, but I have never measured a commercial 30 carbine round off the shelf of less than 1.284 in length, but I do check it all to make sure I get no long or short cases. I think if the action locks-up properly and one maintains the .020-or-less space allowance between the cartridge base and the bolt-face and uses good quality ammo, he can safely shoot-on a bit longer. Defer that $400 (or whatever) barrel change and purchase more ammo.

The various gage lengths vary depending on the source, commercial vs. military.
Military reject or barrel checking specs have varied over time.
The "Go" gage has remained constant at 1.290" as best I can tell.
The Field gage I have is marked 1.302" , which is the spec I see most often when reading.
The intermediate "No Go" gage has varied with time and circumstances; from 1.294 to 1.297, again
according to what I've read.

The Field gage is also used with a "test bolt" if the bolt/chamber in a particular carbine is found to be over/out of spec on the Field (reject) gage. Sometimes a bolt is minimum (or maximum) dimension; the field test bolt removes this variable. If a barrel passes when testing with a field test bolt and field reject gage, but not with the "normal" bolt and field gage- switching the bolt(maybe minimum-or worn) to a new or longer bolt will "Save" the barrel for more use.

It is tough to "generalize" about headspace and its manifestations, as each combination of receiver/bolt/barrel has it's own set of conditions- or problems. JMHO. Smile
Don
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Post Options Post Options   Thanks (0) Thanks(0)   Quote floydthecat Quote  Post ReplyReply Direct Link To This Post Posted: Jun 04 2020 at 9:15am
Originally posted by 1st M1 88 1st M1 88 wrote:

I found this for carbine chamber specs from sami.


Good job finding the chamber spec. M1 88. Looks like the max is 1.300. That sort of dispels what I read concerning field gauges. I have read that field gauges tend to be just short of chamber specs. In this case, doesn't look like it.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jackp1028 Quote  Post ReplyReply Direct Link To This Post Posted: Jun 04 2020 at 10:30am
Don, just curious, how did your excessive head space end up being .080"? Sounds like a lot for even a worn out barrel.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote floydthecat Quote  Post ReplyReply Direct Link To This Post Posted: Jun 04 2020 at 10:43am
80 thousandths is a crack big enough to throw a cat thru, as we say down here.

I think it would be obvious that if the extractor won’t slip over the rim, the action is a goner. Like somebody forgot to screw the barrel all way in. I am not suggesting anything close to that as being safe. I’m talking just a few thousands outside a field gauge.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 03manV Quote  Post ReplyReply Direct Link To This Post Posted: Jun 04 2020 at 11:16am
Originally posted by jackp1028 jackp1028 wrote:

Don, just curious, how did your excessive head space end up being .080"? Sounds like a lot for even a worn out barrel.

Someone got "over aggressive" when reaming the chamber; pure and simple "SNAFU", which ruined the barrel. 
Upon inspection of the "borrowed" reamer, it seems someone "sharpened" it the wrong way, and actually made a combination "thoat/chamber" reamer, when checking headspace with a flat face steel gage, the gage stopped on the wider first step, but a cartridge would move forward into the narrower part and would not head space; we're talking really small changes in diameter.
I never actually measured the "bad" headspace, but it must have been close to 0.080" , maybe more; as I said before the bolt would close behind the cartridge and never snap over the rim.

I might add that this was a commercial barrel, and required reducing the flange to index in the first place; so maybe not a good "example". I would not expect to find such a condition on a military barrel.


When single loaded with a cartridge placed under the extractor(not easy to do), the carbine would fire 
without any signs of a problem; in this condition, headspace was controlled by the extractor holding the rim of the cartridge. No case bulging, no primer piercing or set back.

My point is that one can have really long headspace that causes "unexpected" , unusual problems.Wink

One would "normally" be able to set back a barrel one thread and "save" most barrels.
But on a carbine that moves the gas piston back (.050") at 1 turn in 20 and thus reduces both forward and rearward space available for the slide to move.

I did not try setting the barrel back , as it was too much work just to see if it would still function, and perhaps one turn would not be enough to correct the head space anyway. I don't know how much leeway there is for slide motion to lock and unlock a bolt. Never needed to know before!Shocked
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 03manV Quote  Post ReplyReply Direct Link To This Post Posted: Jun 04 2020 at 11:20am
Originally posted by floydthecat floydthecat wrote:

80 thousandths is a crack big enough to throw a cat thru, as we say down here.

I think it would be obvious that if the extractor won’t slip over the rim, the action is a goner. Like somebody forgot to screw the barrel all way in. I am not suggesting anything close to that as being safe. I’m talking just a few thousands outside a field gauge.

Floyd,
I agree with your few thousandths not making any difference.

If 1.295 is "optimum", giving 0.005" headspace on the go gage;
then a minimum cartridge at 1.284" is then 0.011" in headspace, already 0.001" over
the SAAMI target of 0.010"!Confused
Don
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Post Options Post Options   Thanks (0) Thanks(0)   Quote floydthecat Quote  Post ReplyReply Direct Link To This Post Posted: Jun 04 2020 at 3:04pm
Some semi-auto pistols head space of the extractor. Early Colt 38-Super enthusiast get real puffed and refuse to use that term, but Colt did it up into the middle 80’s I think, finally manufacturing barrels to head space on the case mouth.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 03manV Quote  Post ReplyReply Direct Link To This Post Posted: Jun 04 2020 at 4:02pm
Originally posted by floydthecat floydthecat wrote:

Some semi-auto pistols head space of the extractor. Early Colt 38-Super enthusiast get real puffed and refuse to use that term, but Colt did it up into the middle 80’s I think, finally manufacturing barrels to head space on the case mouth.

Actually the .38 super is a semi-rimmed round, and head spaced on the rim(so is the .32 acp); but as you say, sometime later Colt "gave up" and started using a chamber that headspaced on the mouth of the cartridge.

I guess that reguired a drawing/specification change by the "keepers" of the cartridge spec data, either SAAMI or their predecessor(?), CIP in Europe, or other entities.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote floydthecat Quote  Post ReplyReply Direct Link To This Post Posted: Jun 04 2020 at 4:35pm
Originally posted by 03manV 03manV wrote:

[QUOTE Floyd,
I agree with your few thousandths not making any difference.
If 1.295 is "optimum", giving 0.005" headspace on the go gage;
then a minimum cartridge at 1.284" is then 0.011" in headspace, already 0.001" over
the SAAMI target of 0.010"!Confused

The SAMMI spec. does target .010, but in reality, a case length of 1.280 is allowed. This allows a clearance of .021 using a 1.301 reject gauge and more depending on who’s gauge you use. Just brings me back to my opinion that head spacing a bit longer than that likely poses no real danger....within reason of course. Nobody can hang their hat on a standard field gauge, apparently.
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