mosin head space

Discussion in 'Technical Questions & Information' started by sluggermn, Apr 9, 2005.

  1. sluggermn

    sluggermn New Member

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    I took my mosin-naget in to get the head space checked and he told me it is 12 thousndths out of spec :eek: :eek: .
    My "great deal" is turning out to be not so great of a deal!!
    the gunsmith told me he could fix it for about $250 to $300. its a $70 dollar rifle for crying out loud.
    I have decided I am going to have to fix it my self. he told me there are two ways to do it.
    1st is to take the barrel off and take one thread off then screw it back on and adjust for proper head space. he said this is so the sights dont get messed up

    2nd is to screw the barrel out and take 1 thousandths at a time untill the head space is in the proper spec. Im not sure what this will do for my sights.

    SOOO HERE IS THE QUESTION ....FINALLY.
    what do you recommend I do ?? do I try #1 above or do # 2 or just hang it over the mantel and chalk it up to experience or put it in the recycle bin???
    what say you???
  2. mtnboomer

    mtnboomer New Member

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    Buy a No-Go gauge before you buy your next Mosin.
  3. LDBennett

    LDBennett Active Member

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    sluggermn:

    Firstly most guns imported by responsible importers come head spaced correctly. Secondly, his price is a little high. Thirdly, sometimes you can find a takeoff barrel with a good bore that headspaces correctly by just installing it. It has happen to me on a Mauser. I think that interchangeability might be a higher priority on a military weapon than a commercial one so the chances might be more in your favor.

    I would not trust that the gunsmith measured the headspace correctly or at all. Get your own headspace gage and check for yourself.

    If you set back the barrel anything less than a full thread you will get the sights rotated around the barrel to a position that will make them impossible to use (like on the bottom of the barrel??). The correct way is to move the seating shoulder on the barrel back one thread width and face off the end of the barrel to get to the correct head space. Then you have to run a chamber reamer into the chamber to deepen the chamber since you moved it back too. I have fit a barrel to a receiver on my home lathe and it is not a task for amatuers like me. Mine came out OK but I sweated bullets the whole time and spent more time making special tooling than actually doing the job. If indeed the head space is too much find a different gunsmith as yours is too highly priced. Brownells price survey lists installing and chambering a barrel (what is necessary to set the barrel back) as $150 to $250. Your guy is a little out of bed, price wise! And 0.012 inches is a lot more than wear. It sounds to me like a measuring error!

    LDBennett
  4. sluggermn

    sluggermn New Member

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    the guy I took it to works for gander mountain. he did not have a go/nogo guage so he took masking tape and taped it to the primer end of a live round. he then added tape until the bolt would not close. that was 4 pieces of tape later...
    He said each piece of masking tape is approx. .003 to .004 thick so my head spacing was about (approxmently) 12 to 16 thousandths out of speck and he was guessing 12.
    I have yet to find a gun smith in this area that has a go/no go guage for the mosin rifle. though I do have two more to check with next week when they are open. I will also check with these two about price but I was hoping for a cheaper fix. the wife and I are BOTH going back to college and we have two kids which = no funds for the toys right now. :(
    my f.i.l. has a shopsmith and would be willing to help me with the gun but Ive never done this kind of thing so I'm a little hesitant and dont even know where to start. I guess its no big deal if I screw it up its only a $50.oo rifle I can always use it as a boyfirend scarer when my daughter gets older, you know answer the door with it in my hands and procede to clean it while she finishes getting ready. :D :D
  5. Ross95966

    Ross95966 New Member

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    Sluggermn,
    Instead of correcting the gun, why not simply correct the ammunition? That route would be free.
    The MN, as all rimmed cases, headspaces on the rim - usually. You can thicken the rim by denting the flange forward so that it stops the case from seating too far forward.
    And/or you can distort the body of the case to prevent it from being easily inserted, thus lodging tightly against the bolt face when it is closed.
    In either case, the first shot will expand the case to custom fit your chamber perfectly. Thereafter all you need to do is necksize or partially size the case for your gun.
    Probably you could just shoot factory loads safely enough as they are, then just size them enough to barely fit your chamber.
    I just saved you several hundred dollars, so if you do not now reload, you will have plenty of money to buy a Lee Classic Loader for $12.89 from Midway and still take your wife out for dinner. In addition you will save a bundle by continuing to reload at a saving over factory ammo. At least that is what I tell my wife.
    Cheers from Darkest California,
    Ross
  6. sluggermn

    sluggermn New Member

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    I am a total rookie in this area so PLEASE correct me if I am wrong. ok

    the way I understand it is my barrel is too far out ( not threaded in all the way??) and what this causes is a gap between the bolt face and the base of the bullet. ( right so far??) then,what this does is create a space for the base of the bullet to expand upon firing.( does it also force the bullet backwards at this point??) which in turn, due to the pressure of the expolsion (shot being fired) the base of the bullet casing swells to the point that it causes the case to stick in the chamber. am I still on track or not????

    now a question : how does lengthening the bullet help cure this problem?
    no I am not being a smart a$$ I am not sure I understand this problem.
    wouldnt this create more of a gap and make the casing expansion more of a problem and possibily a kabooom in the chamber??

    another question : would tightening the barrel one full turn help. ....no wait this "cure" would shorten the chamber right?
    so if Im going to do that I would have to get a chamber reamer and re-chamber it correct?? ( that does not sound correct to me the chamber should stay the same right)
    what is the thickness of one turn of threads?

    is it possable that the barrel was never tightened down correctly but the chamber is correct for the gun, needing just to have one more turn of the threads ( about 12 thousandths) to seat it correctly?
    and how would I know if this is a possibility?
  7. gamachinist

    gamachinist New Member

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    That is a half a'ed way to check headspace,especially for a shop.

    One thing to keep in mind is that almost all rims are not as thick as the minimum headspace dimension.
    Measure the rim thickness ( make sure to measure inside of the tapered rim area)and remember that all tape will compress several thousands of an inch.If you must use tape,use "Scotch" type tape,as it won't compress as much as masking tape will.
    My advise would be to buy a NoGo gauge and see it the bolt will close on it.If it won't,or will just barely close on it,it should be fine to shoot.
    (I made my own rather than buy a rimmed case headspace gauge,since I don't think I will ever wear one out as a hobbiest).
    For what it is worth:
    A Forster brand 30-40 Krag Field reject gauge (.070")is slightly smaller on the rim diam (.019" difference),and close to the NOGO spec for a 7.62x54R (.071").
    Generally,Clymer uses max dimension for NoGo btw,where Forster calls that a Field reject gauge.
    Hope this helps,
    Robert.
  8. LDBennett

    LDBennett Active Member

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    sluggermn:

    It works like this. The firing pin falls and pushes the case forward so that the shoulder of the case is pushed into the chamber's shoulder. If the head space is too great the firing pin doesn't impact the primer hard enough (some of energy expended moving case forward and a softening of the primer impact) and the cartridge does not fire. Assuming it fires reguardless of the excessive head space, the expanding gases pressurizes the case and forces it into the chamber wall (in the forward position). The pressure pushes internally until the chamber pushes back which happens when the case stretches to meet the face of the bolt. The area of the case that gets stretched and thinned is the area just above the the head where the case head tappers into the case thin walls. If the head space is too great the brass case will seperate at that juction. When you extract the case you get only the head and the rest of the case sticks inside the chamber. That is worse case. The case may just be damaged (bright ring around the case just above the head or internal thinning of the case wall) or open up a crack. But the gases can escape into the action and perhaps into your face (??).

    If the cases are fired with reduced loads, the cases may expand to fill the chamber without spliting. Then if you reload them yourself, you can neck size only (not pushing the shoulder back to the specification point) so that the formed case entirely fills the chamber. Now you have made a case perfect for your long head space chamber.

    When a barrel is "set back" it is done one thread (about 1/16 inch, depending on the threads-per-inch of the barrel threads). That moves the existing chamber to the rear as well as the end of the barrel. The barrel must be shortened on the bolt face area something a little less than one thread depth, and the chamber reamed to get to the correct head space. The point of setting back the barrel is to get the chamber to bolt specification correct by reaming a new chamber (not as deeply as when you started).

    The measurement technique you described for determining head space is bogus. It is highly unlikely that the chamber is 0.12 inches too deep.If you are out of money just take the gun to a range, tie it to a tire and set it off with a string from a safe distance away. Extract the case and look for signs of case head spliting or thinning of the case near the head (internally, using a bent wire). If you find that indeed there are signs of case splitting at the case head then your rifle needs work. You may get away with reduced load case forming as described above. If no sign of case stretching or head splitting, just shoot the thing and kiss off the "gunsmith", that used tape to determine headspace, for any future work.

    LDBennett
  9. Xracer

    Xracer *TFF Admin Staff Mediator*

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    Minn-eeee-sota, ya, sure, you bet!
    Sounds to me like you have a mismatched bolt to the rifle.....which doesn't surprise me. A lot of these inexpensive MilSurp rifles are made up on mix-'n-match parts.

    If you'll check out the schematic here:

    http://www.e-gunparts.com/productschem.asp?chrMasterModel=1920z91/30

    ....looks like you might be able to shim up the bolt head (part # 1A).
  10. clcreek

    clcreek New Member

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    Robert, where did you get the specs on the head space for the Mosin Nagant? I have been searching for a while.

    so 0.071 should be the fail thickness of the rim?

    cl creek
  11. TRAP55

    TRAP55 New Member

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    The bolt "head" is the part used to correct the head space on a Mosin. You can change every other bolt part, including the bolt body, and leave the same bolt head, and the head space doesn't change.
  12. clcreek

    clcreek New Member

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    I am trying to find the specs on headspace for a Mosin Nagaent 7.62x54r.

    I think I have found these, but I am not sure If I am correct:

    go 0.064"
    no-go 0.071"

    I cant find find the spec for the field gauge.

    If anyone has the information and can post it or email it that would be great. If you have a set of gauges and can measure them that would be great (please provide the name of that gauge manufacture as I understand they are all slightly differnt).

    email is clcreek@gmail.com

    Thank you in advance
  13. Willie

    Willie Active Member

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    I tried that. Never had any success. Teenage boys are not afraid when chasing teenage girls. :cool:
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