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Sep 21, 2014
United Kingdom
Okay guys, I know what headspace is, anyone chambering a barrel should know what headspace is and God knows, I have done dozens. My question is quite serious and borne of a real curiosity: What is going to happen if your headspace is +.100" so over by a hundred thou? I do realise that some actions, Remington for instance, there is a real likelihood that the cartridge will go too far into the chamber and the firing pin will not ignite the primer but if it was controlled feed like a Mauser that could ignite the charge, right?
My thoughts are that the case will become unuseable as it will elongate too much out of spec but as far as "blowing the gun up" is concerned, well that is just not going to happen. I welcome all of your comments.
0.100" is a LOT.

most weapons simply would not function. Most firing pins only extend about 0.070" or so so they would never hit the primer hard enough to ignite it. The extractor is just not going to hold the round tight enough.

Now the occasional round that does ignite would deform so badly that it would not eject. It might be possible on some weapons for the bolt locking device to be severely damaged. Perhaps repeated firing would break this and you would have the dreaded explosion - very rare event but possible.
The case would probably rupture and it would vent the expanding gas somewhere, depending on the design of the action.
Once long ago I managed to accidentally chamber a .308 in a 30-06 US Remington 1903-A3. That's probably like 0.600 headspace. Nothing bad happened, the bullet actually hit the target, and the case extracted, but it looked like a 45-70 when it came out. I wish I'd taken a picture, but camera-phones didn't exist back then.
but if it was controlled feed like a Mauser
If you search the internet you will find a video by Dunlap where they progressively increase the headspace and continue firing without blowing anything up. If I remember correctly he does make a statement about thicker military brass. However too much headspace can be caused by a multitude of things some of which can cause catastrophic failure.

When I was an armourer we checked weapons on the range for matching part numbers, no match no fire. I had a sergeant major tell me it was safe I said cool we will retire from the range and you can proceed but it will be recorded, he decided not to argue the point. This check was to ensure correct headspace.

We had a group of officers firing AK's and R4's which are a South African version of the Galil which is a AK copy in 5.56. They brought in an AK that was jammed, I stripped it and there was something in the chamber, ran a rod down to check length and ensure it wasn't a live round then tapped it out. 5.56 fire formed to 7.62x39 with rebated head. Pretty cool looking new round. I was really surprised nothing gave way.
Very interesting comments guys, I really can't see a .223 round in a 7.62 x 39 chamber blowing the gun up, but I agree with the comment, a bloody interesting case shape. How about a more modern design though, like the AR15 platform? Or the AR-10? Surely by their very design the cartridge cannot destroy the gun, even with an outrageous headspace error?
There's a whole chapter in Hatcher's Notebook about headspace. That was back when there was a bunch of "I wonder what happens if we do this.." being done by the Army Ordnance department. Looks like under $20 can get you a copy from eBay. Very interesting reading.
That's not really accurate. The case protrudes from the barrel at least enough for the extractor to engage it with zero headspace.
There's a whole chapter in Hatcher's Notebook about headspace. That was back when there was a bunch of "I wonder what happens if we do this.." being done by the Army Ordnance department. Looks like under $20 can get you a copy from eBay. Very interesting reading.
There are free downloads around. I know I've got one somewhere safe, so safe I'll struggle to find it myself.
Seen the 308 in a 3006 and I myself let a 222 get fired in a 223 ar, no safety issues, but noticed extra gas from the charging handle cutout escape, rounds look cylindrical with little to no throat left.

That said when I was a kid my uncle fired a 270 win out of a 7mm mag sako , massive gas escape, powder freckle burns on his face, had to flush out his eyes with water and he said they bothered him like welders flash for a few days but was fine.
Gunstock cracked in half at the mag box cuttout, but gun was otherwise fine. Examination of the case showed it split down the side.. This was all factory ammo. He had the gun checked out by a local smith who stated it was fine, and sako even sent him a new stock for free because it "should not have cracked" . he stated the gun never shot that great after that but was still huntable. Fast forward 25 years later I re barrel the gun for my cousin as it no longer shoots worth a crap. Pulling off the old barrel I was able to inspect the lug seats, they looked perfect, inspection of the bolt lugs and face, perfect, and bolt lug engagement was even. the replacement stock was not fit properly so that was corrected and borescoping the old barrel showed it to be a rusty tube about 1/2 down from the muzzle from rain , dew, and lack of maintenance New lija barrel got it shooting great again. These modern bolt guns can take a lot, and modern ammo is pretty strong too. Headspace is more of an issue to handloaders and if youre having misfires.
Back in the 1970s I was a civilian Armourer in an Australian army Base Workshop.
I used to do a lot of outside jobs for units requiring the services of an armourer.
The most interesting by far was when I was sent as the expert witness on a Naval Board of Enquiry into an accident with a.50 Browning MG on board a RAN patrol boat.
If it seems odd that a civilian from an Army unit was called on to sort things out for the Navy, the explanation is simple, I was the only one available with the necessary qualifications for the Browning.
To cut a long story, when the Board assembled on the boat, I had the gunners go through the drill of setting up the gun that they had fired successfully, no faults.
Then I requested that they do the same on the gun that had burst a case and wounded the gunner with bits of brass.
They were headspacing by screwing the barrel home and then backing off two turns, all seemed OK then I noticed a 3/8 in hole in the side of the gun and all became instantly apparent.
The hole was there to allow the block on the back end of the new barrel locking spring to move in and out as the barrel was screwed in and then when the barrel was in battery it locked the barrel and stopped it from unscrewing whilst being fired which was a minor embarrassment in armoured vehicles but a disaster on a ship when a.barrel went into the drink.
I had just modified the first ten guns converted in Australia, [vide the instructions from the States] and they were distinguished by a 1 in white square painted around the hole and 2 white bands painted around the body.
The gun that was the focus had neither and the barrel had stopped turning one inch out of battery plus two full turns. So the rear inch + of the case was unsupported, not the usual excessive headspace problem but headspace none the less.
The gun had fired 13 rounds and the 14th ruptured which says heaps for the quality of USA ammunition.
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Having just fixed a Model 70 (Montana Rifle model 1999) .300 Win Mag that had a 30-06 fired in it, I can tell you that it does do some damage. The extractor claw was blown off, the magazine floor plate bent and the case ruptured right at the pressure ring on the case head. The action did as designed and vented the gasses out the mag well, so there is that. I did replace the barrel just so I know its good, took every action measurement I could find a reference for, to see what else happened and found nothing else. Bolt body is true, bolt lugs are fine, bolt face is discolored but fine, action is straight and front ring did not expand. The original barrel headspaces fine, measured the chamber with a cast and its well within tolerance, but got swapped just to be sure.
I had a free 243 barrel from a Model 70 that blew up ...........barrel fell in the dirt just in front of the gun............it was undamaged ,in perfect condition .............the reciever ring had the top half missing...........no one was hurt ,surprisingly.