Head Space Brain Work
Printed From: The Carbine Collector's Club
Category: The Club
Forum Name: Safety/ Accuracy/Shooting/Ammunition/Care and Maintenance
Forum Description: What to Check, Reloading, Tips for Accuracy, Competitive/Recreational Shooting
URL: http://www.uscarbinecal30.com/forum/forum_posts.asp?TID=4743
Printed Date: Apr 23 2024 at 10:42am
Software Version: Web Wiz Forums 12.01 - http://www.webwizforums.com
Topic: Head Space Brain Work
Posted By: floydthecat
Subject: Head Space Brain Work
Date Posted: Jun 03 2020 at 6:44pm
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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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Replies:
Posted By: W5USMC
Date Posted: Jun 03 2020 at 7:49pm
Nope! But maybe I'm just slow. ------------- Wayne USMC Retired NRA Life Member |
floydthecat wrote:Posted By: floydthecat
Date Posted: Jun 03 2020 at 8:06pm
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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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Posted By: W5USMC
Date Posted: Jun 03 2020 at 8:13pm
Now that I understand and also agree with. ------------- Wayne USMC Retired NRA Life Member |
Posted By: 03manV
Date Posted: Jun 03 2020 at 8:59pm
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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! JMHO  ------------- Don |
Posted By: jackp1028
Date Posted: Jun 03 2020 at 10:16pm
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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. ------------- JackP |
Posted By: 03manV
Date Posted: Jun 03 2020 at 11:33pm
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" 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 |
Posted By: jackp1028
Date Posted: Jun 04 2020 at 12:11am
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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. ------------- JackP |
Posted By: floydthecat
Date Posted: Jun 04 2020 at 4:22am
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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.
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Posted By: 1st M1 88
Date Posted: Jun 04 2020 at 7:54am
I found this for carbine chamber specs from sami.
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Posted By: 03manV
Date Posted: Jun 04 2020 at 8:07am
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 |
Posted By: 03manV
Date Posted: Jun 04 2020 at 8:16am
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.  ------------- Don |
Posted By: floydthecat
Date Posted: Jun 04 2020 at 9:15am
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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Posted By: jackp1028
Date Posted: Jun 04 2020 at 10:30am
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Don, just curious, how did your excessive head space end up being .080"? Sounds like a lot for even a worn out barrel. ------------- JackP |
Posted By: floydthecat
Date Posted: Jun 04 2020 at 10:43am
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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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Posted By: 03manV
Date Posted: Jun 04 2020 at 11:16am
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. 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!  ------------- Don |
Posted By: 03manV
Date Posted: Jun 04 2020 at 11:20am
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"!  ------------- Don |
Posted By: floydthecat
Date 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. |
Posted By: 03manV
Date Posted: Jun 04 2020 at 4:02pm
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. ------------- Don |
Posted By: floydthecat
Date Posted: Jun 04 2020 at 4:35pm
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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Posted By: 03manV
Date Posted: Jun 04 2020 at 10:36pm
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Floyd, we are in total agreement.  Military and commercial entities and their gauges are frequently different. I'm sure the "specified" numbers and limits are conservative- the statement that you made when you started this thread! ------------- Don |
Posted By: sling00
Date Posted: Jun 05 2020 at 6:39am
Prime example is .303 British Enfield. A SAAMI specs field gauge is .070 whereas the British mil spec field gauge is .074.
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Posted By: floydthecat
Date Posted: Jun 05 2020 at 8:02am
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I think it's a safe bet to assume there are many carbines out there shooting past 1.302, well past that. All the field gauge indicates is that head-space is longer than the gauge, it doesn't indicate how long it is and how long it's been shooting that way....unless it is accurately measured. I've looked at guns for owners in which a field gauge would practically rattle in the chamber and I am pretty sure they just keep shooting them. Like an ME gauge...it's a good indication of use, but doesn't necessarily condemn the carbine. These carbines were made to shoot rather lose with torso-shot accuracy to overcome action-pollution of mud, sand, salt-water, blood and guts. Ruptured cases due to sloppy head-space in a carbine have to be rare, if at all existent. Of course, one can blow-up any gun if he tries hard enough I suppose. These things were proofed at 140%. A tight muzzle and a 1.295 head-space might indicate a little fired gun, but I would gladly discount the right used carbine with an ME of 3 and head-space of 1.304 by $500-$600 and take it off your hands any day. Not for a range gun that is often fired, but for one to occasionally shoot, or that "closet gun".
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Posted By: Sawbones
Date Posted: Apr 12 2022 at 5:53pm
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Very interesting and informative thread. After careful scrutiny of all the opinions and facts presented I agree 100% w/ everything FloydtheCat has stated, particularly his last reply. Toss any cases fired 3 times otherwise risk splitting/disintegrating brass Ejector not locking? First, check the springs in the bolt. (see CMP Forum: M1 Carbine section, "Fail to chamber question", page 3, last reply 3-5-22
------------- "Always do sober what you said you'd do drunk. That will teach you to keep your mouth shut." Ernest Hemingway |
Posted By: JRBrid
Date Posted: Apr 24 2022 at 5:11pm
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Personal Experience with a Carbine that Was Out of Headspace I’ve been hesitant to jump into this discussion,
as I don’t feel it is my place to tell others what they can or should do with
their guns, but I have decided to go ahead and share this personal experience
with a carbine that was out of headspace.
Hope this helps … here is the story: A few years ago, my best friend and I were enjoying a day at the range shooting carbines. We had several with us, including one that he had recently purchased at a local gun show. We were having problems with the new gun, several failures to fire. Examination of the rounds showed light firing pin strikes, but the problem was not consistent and we were thinking that maybe the bolt needed a good cleaning. We continued shooting, when suddenly my buddy, yelled, dropped the new carbine, and backed away from the shooting bench. I’ll let him tell you what happened in his own words: “There was a flash in front of
my face and something hit me. I dropped the gun and stepped back from the
bench. The case had separated. The stock was split. The
magazine case was bulged and the base, along with the follower, spring and
remaining rounds were blown out. The extractor, firing pin, ejector and
spring were missing from the bolt. We found most of the pieces of
the magazine, bolt and brass. There was something embedded in my forehead about 1”
above my glasses. There was no sign of infection so I waited until the
swelling went down and finally pulled a BB sized piece of casing brass out of
my forehead.” Obviously, this was a traumatic
end to a day at the range – we were just thankful that it wasn’t much, much
worse. Using some thin washers and head
space gauges, we estimated the gun in question was as much as 0.030” out of
headspace. After this experience, we very
carefully checked the headspace on all of our carbines, and corrected any of
them that showed excessive headspace with a field gage. Fortunately, we were able to correct most of
the issues we found by swapping bolts, but the one did require a new
barrel. Bottom
line, for us at least, is that life is too short and precious to take the risk
of shooting a carbine that has excessive head space. We still have the pieces of brass
and the stock. If anyone would like pictures,
just PM me. Again, hope this helps. Respectively, Jim ------------- Jim B. |
Posted By: floydthecat
Date Posted: Apr 24 2022 at 5:43pm
| Spec’s give you 22 (1.302-1.280). My experience has been they start becoming problematic much past 1.310, which would be 8 out of spec. I would think the gun would stop firing at all. Other things could be at work. The only rupture I have ever read about was eventually blamed on faulty ammo (thin brass). |
Posted By: Smokpole
Date Posted: Apr 24 2022 at 8:05pm
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JRBrid, that sounds more like an out of battery explosion than a head space issue. Was that carbine by any chance a commercial carbine? Or more specifically a Universal? Universal took some shortcuts in their manufacture that allowed their carbines to fire out of battery and what you describe is classic out of battery firing. ------------- OGCA Life member NRA Life member Ashtabula Rod and Gun Life member |
Posted By: 03manV
Date Posted: Apr 24 2022 at 10:35pm
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Please tell us about the ammunition. I'm with Smokepole about an OOB firing, but a case head separation would do it too. ------------- Don |
Posted By: David Milisock
Date Posted: Apr 25 2022 at 9:32am
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I tend to lean toward an out of battery firing also. ------------- David Milisock |
Posted By: JRBrid
Date Posted: Apr 25 2022 at 3:41pm
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It was not a commercial carbine - actually a Winchester with mostly Winchester parts, including a nice Winchester stock that was split. We considered OOB, but found couldn't find any evidence on the bolt lugs or receiver to support that. This is the gun we replaced the barrel on, and have had no trouble with it since doing that - same bolt, same springs, etc. The one thing we could prove was that the gun was badly out of headspace, on the order of .030 inches beyond a Forster field gage. I will leave you to draw your own conclusions .... Jim ------------- Jim B. |
Posted By: floydthecat
Date Posted: Apr 25 2022 at 4:01pm
| Wow…30 past a 1.30 field gauge! I can’t see how that could not be noticed before it was fired. Not bashing anybody here….we just like to try and figure out what happened in cases like this. |
Posted By: Sawbones
Date Posted: Apr 25 2022 at 5:50pm
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Faulty ammo must be considered. Was the ammo used reloaded stuff? Must consider case separation due to metal fatigue. Case reloaded too many times renders case thin and brittle. That in conjunction with a way too long headspace and you've got the perfect ingredients for disaster. I refer all to Lyman Reloading Handbook, 49th edition , page 48. A new un-fired case will not disintegrate when fired unless other factors are present such as using too much powder, wrong powder, excessive crimping.
------------- "Always do sober what you said you'd do drunk. That will teach you to keep your mouth shut." Ernest Hemingway |
Posted By: Rebel92
Date Posted: Apr 25 2022 at 10:13pm
| So was the headspace on this carbine 1.36? That would swallow a field gauge right? |
Posted By: JRBrid
Date Posted: Apr 26 2022 at 12:33am
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Sorry, this happened some time ago, and we no longer remember the details on the ammo being used -- not sure if it was new commercial, surplus GI or reloads. Jim ------------- Jim B. |
Posted By: 03manV
Date Posted: Apr 26 2022 at 12:57am
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I'm sorry but without knowing the ammo involved and not measuring the headspace before the accident, one just cannot draw a firm conclusion as to the cause. The "cause" of the accident as you state was a case separation; what caused the case separation cannot be discerned from the information given. It is good your buddy wasn't more seriously injured. The real lesson here is to check any firearm before firing and when firing a "new, used" firearm for the first time use known "good" ammo. When encountering an unexpected series of malfunctions it is best to stop and determine the cause. ------------- Don |
Posted By: floydthecat
Date Posted: Apr 26 2022 at 5:02am
I don’t see how it would have fired at all under normal circumstances with the bolt closed and rotated on a chambered round. My experience is they stop shooting when it gets much past 10-12 thousandths longer than a field gauge. 30 thousandths longer than a 1.300 gauge would be 1.330.
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Posted By: JRBrid
Date Posted: Apr 28 2022 at 1:17am
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We have used ammo from that same box/lot in other carbines both the day of the incident and since, and have had no other problems. The only difference we have been able to identify is that the one carbine that we had the incident with was badly out of headspace, and all of the others gaged as good, again with all determinations being made with a Forster field headspace gage. Jim ------------- Jim B. |
Posted By: Smokpole
Date Posted: Apr 28 2022 at 11:38am
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Was part of the case stuck in the chamber? Or was the case simply split? ------------- OGCA Life member NRA Life member Ashtabula Rod and Gun Life member |
Posted By: JRBrid
Date Posted: Apr 28 2022 at 2:04pm
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Yes, part of the case was stuck in the chamber. The case completely separated just ahead of the base of the round. Jim ------------- Jim B. |
Posted By: floydthecat
Date Posted: Apr 28 2022 at 2:26pm
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Can we call this a case head separation as opposed to a rupture? The only time I have been able to find anything about a carbine case rupture was an individual who took his carbine to a gunsmith after he had one blow-up. Gunsmith determined it was a case head separation, because the owners ammo blew-up in his test gun as well…..bad ammo. I have been present or experienced two ruptures and one case head separation in my life. All were from semi-auto 9mm pistols with unsupported chambers. The separation left most of the brass in the chamber.
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Posted By: Smokpole
Date Posted: Apr 28 2022 at 5:22pm
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I had one case head separation in a carbine. I did not have any catastrophic blast. The carbine simply ejected the base but left the rest of the case in the chamber. Except for an unusual puff around the bolt nothing odd seemed to happen, except the next round wouldn't chamber. ------------- OGCA Life member NRA Life member Ashtabula Rod and Gun Life member |