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Mosin headspacing

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  • Brains

    One of the idiots
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    Anyone in the Houston area have a field gauge I could borrow for a few seconds?
     

    Brains

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    Yeah it closes nice and easy, so I'm now concerned about how much space is between the bolt and the back of that chambered round. I guess if it fires, it's not an awful amount, right? What's the worst that could happen?
    :boom:
    :)
     

    Mikewood

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    Big boom.
    It should not close on a field gauge. Sorry I can't help you. Did you buy it new? If it's new surplus it should have been checked. Pasadena gun center has a smith and I bet they have a field gauge and will check it for a nominal fee. Be sure to call ahead. The smith is not there full time.
     

    benenglish

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    What's the worst that could happen?
    I own a gun with excess headspace that I've never fixed.

    Accuracy is degraded.

    I get some misfires when the firing pin energy is wasted pushing the cartridge further into the chamber instead of setting off the primer.

    Sometimes cases stick in the front of the chamber and the primers back out. (Obviously, this isn't a high-pressure round.) With factory ammo when the cases don't stick towards the front, they display flattened primers that look like the ragged edge of high pressure, almost more than I will tolerate when working up loads. It's weird to fire two factory rounds in succession where one has a flat primer, looking like a high pressure load, and the other has a primer backed 1/3 of the way out and still possessing a rounded profile at the rear. It's very weird.

    Overall, I can never escape the feeling that I'm on the verge of something really bad. I don't shoot that gun any more. Maybe someday I'll get around to getting it fixed.

    My conclusion is that firearms with excess headspace should not be fired even though I've done it with no particularly bad results. Besides, if you think there's a problem, then there's a problem. Find the gauges and put your mind at ease.
     

    itchin

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    I doubt you'll have any problems. Maybe a split case. If that bolt head came with the mosin I'm sure its fine. The mosin fires a rimmed cartridge (54R). Its pretty forgiving on head spacing. I'd fire it and check the case to see if it splits.
     

    mikeofcontex

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    There are many ways to lengthen a cartridge... duct tape... clay... use your imagination. I'd think you'll know if it's close without any real trouble. The 303 British generally is long in the chamber and the round simply grows to fill the chamber without issue since both the 7.62x54R and the 303 British headspace on the rim.

    I've actually seen a 300 Win Mag after it was shot in a 300 Weatherby Mag chamber. No damage or signs of any trouble, but it sure is "funny" looking.
     

    Brains

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    I bought this one from a forum member, he stated he's fired it a few times. The numbers do all match.

    I want to know it's at least in the ballpark before firing it, maybe I'll stick some of my daughters play-doh to the back of a cartridge and close the bolt on it :) Ideally I want it dead on, but I'd live with it failing only a field gauge. Since it's "only" a Mosin, I may cobble together a field gauge. I have some acrylic sheet I could get within a half thousandth by sanding and mic'ing, or there's this thread I stumbled across:

    Surplus Rifle Forum
     

    mikeofcontex

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    The play-doh needs to be a ring in front of the shoulder. Unless something is drastically wrong, the distance between the front of the rim and the face of the bolt should be very close to the correct measurement.
     

    Mikeinhistory

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    There is really very little possibility that the headspace is off. Especially if it is a refurbished example. Most likely it went through an arsenal and sat in a crate for 30-40 years before coming to the states. There is almost no chance it has had enough owners to have been shot enough to change any of that. The only thing I'd be worried about would be if the bolt didn't match/ had a lot of issues when working the action/ or visually looked like something was wrong. I have personally owned well over 20 Mosins and fired way more than that and I've never had a problem.
     

    Brains

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    There is really very little possibility that the headspace is off. Especially if it is a refurbished example. Most likely it went through an arsenal and sat in a crate for 30-40 years before coming to the states. There is almost no chance it has had enough owners to have been shot enough to change any of that. The only thing I'd be worried about would be if the bolt didn't match/ had a lot of issues when working the action/ or visually looked like something was wrong. I have personally owned well over 20 Mosins and fired way more than that and I've never had a problem.
    ... and you'd be correct on this one as well :)

    I ended up, uh, "machining" myself a field gauge tonight. I'm well within spec, time to go see how she shoots. Thanks to everyone for your tips.


    Mosin Field Gauge 2.jpg Mosin Field Gauge 3.jpg Mosin Field Gauge 1.jpg
     

    Josh Smith

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    Hello,

    The only way you're going to have excessive headspace in a Mosin-Nagant is with lug setback.

    The 7.62x54r is rimmed, and headspacing occurs on the rim.

    If I find my set I'll send you the field gauge from it, but if it's been fired without any pressure signs (incipient case-head separation being one) I'd say you're good to go.

    Gas handling in a Mosin is not the best, but it will vent out the bolt. You'll get a puff in your face, but likely no debris. That said, don't take chances and always wear shooting glasses.

    The action is one of the strongest, if not the strongest, out there. It locks on the bolt head, but that massive thing the bolt handle attaches to is the backup lug.

    The result of overcharging? Well, a gunsmithing school found that the action will not let go under a compressed load of Bulleye! It stretched the action too far to make firing pin contact, true, but the shooter would have been safe.

    One of the YouTube Yoohoos did the compressed pistol powder test, then managed to open the action. He then swept up mixed powder from the firing range floor and compressed that into a load. He lit that off and the action still held together -- in fact, if I recall correctly, it cold welded the bolt lugs to the action. I may be wrong on that last one, but such a charge would be strong enough to do it.

    Of course, if you have doubts, test the headspace. Lord knows there are a bunch of Lee-Enfields out there that close on field gauges.

    I have not ever, once, heard of a Mosin that did, though.

    Regards,

    Josh
     

    Brains

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    Hello,
    If I find my set I'll send you the field gauge from it, but if it's been fired without any pressure signs (incipient case-head separation being one) I'd say you're good to go.
    Wow Josh, that's quite generous. I really appreciate the offer, but based on the field gauge I cobbled up last night, I'm very confident on the measurements.

    Gas handling in a Mosin is not the best, but it will vent out the bolt. You'll get a puff in your face, but likely no debris. That said, don't take chances and always wear shooting glasses.
    Always wear them anyway, that was pounded into my head at an early age. :)

    The action is one of the strongest, if not the strongest, out there. It locks on the bolt head, but that massive thing the bolt handle attaches to is the backup lug.
    You're absolutely right, there's a lot of metal holding that sucker closed. I wasn't so much worried about the receiver blowing apart or the bolt impaling my shoulder as I was just wanting to know it was 'right.'

    The result of overcharging? Well, a gunsmithing school found that the action will not let go under a compressed load of Bulleye! It stretched the action too far to make firing pin contact, true, but the shooter would have been safe.

    One of the YouTube Yoohoos did the compressed pistol powder test, then managed to open the action. He then swept up mixed powder from the firing range floor and compressed that into a load. He lit that off and the action still held together -- in fact, if I recall correctly, it cold welded the bolt lugs to the action. I may be wrong on that last one, but such a charge would be strong enough to do it.
    I need to find that yahoo's video, sounds entertaining :)

    Of course, if you have doubts, test the headspace. Lord knows there are a bunch of Lee-Enfields out there that close on field gauges.

    I have not ever, once, heard of a Mosin that did, though.
    Tested, and it's nowhere near closing on a field gauge. I didn't make one, but based on how far I have left to go to close the bolt, I guarantee this one won't close on a no-go gauge either. On the other end, it chambers surplus ammo smooth as silk. I'm happy to know it's in spec.
     

    ROGER4314

    Been Called "Flash" Since I Was A Kid!
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    The poor boy method of checking head space uses masking tape on the rear of the cartridge. Closing the bolt with zero or one piece of tape may be OK, two is excessive and three....well, forget it.

    Flash
     

    Brains

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    Guess y'all aren't reading the previous posts :)

    Headspace was dead on. Checked the rest of everything, and took it to the range on Sunday. Functioned flawlessly, didn't bother trying for accuracy but it puts the holes where it should. Got it home, and found the bore really dirty. Maybe previous owner(s) cleaned it, maybe not, who knows. After soaking, brushing, jagging about 12 times it's reasonably clean now though, and has a good coat of oil. Maybe not squeaky, but the bore is finally looking bright.

    I still need to check the barrel contacts in the stock, and the crown needs a little polish to be perfect, but all in all this one was easily worth the sub-100 bucks I paid.
     
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