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    Member CHUCKW's Avatar
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    Headspace on 1873 Springfield Trapdoor

    Gents,

    I have a trapdoor that I would like to shoot (with appropriate ammo) but do I need to check headspace? If so, how is this done?

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    Advisory Panel Patrick Chadwick's Avatar
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    Measure head/rim clearance, not headspace

    In my honest opinion, based on not inconsiderable experience with BP cartridge guns**.... don't bother. An 1873 Springfield has no bolt lugs to wear - it has the trapdoor. And it was made about half a century before SAAMI was created. Just check that it is clean, in particular that the recess for the cartridge rim is not clogged with prehistoric crud. And that the trapdoor latches properly. But don't let anyone convince you that if it doesn't meet a modern specification then the world is going to collapse in a puff of BP smoke. And for heavens sakes, don't go out and buy fancy expensive headspace gauges that provide no guarantee that the gun is safe.

    OK, so you're a wee bit nervous about it, otherwise you wouldn't have posted the query. So measure it the cheap and effective way:

    I am describing it here in detail, but once you have done it a couple of times you will find that you can do it in less time than it takes to read these instructions.

    Get a yard length of 1/4" x 26 threaded rod and a couple of good nuts.
    Mount the rifle horizontally in a nice, padded vice or clamp. Otherwise you are going to need three steady hands.
    With the trapdoor open, check that the firing pin can move back and does not protrude from the block. A broken firing pin, for instance, may jam in a protruding position and cause a slamfire when the block (trapdoor) is closed on a primed case.

    Half-cock the rifle, so that the firing pin is free to move back.
    Place an EMPTY unprimed case in the chamber and close the trapdoor.
    (An unprimed case, so there can be absolutely no question of it fouling the firing pin if ancient crud or damage causes the latter to protrude even a gnat's whisker from the trapdoor block when it is closed.)

    Push the threaded rod down the barrel, so that it enters the case and pushes it up against the front face of the block (trapdoor).
    Run a nut up until it is just touching the muzzle.
    With a little practice, and using one hand to hold the rod hard up against the block, you can feel when the nut starts to lift the rod off the face of the block.
    Now comes the tricky bit: without disturbing the position of the nut relative to the rod, pull rod + nut back a little and make a witness mark with a felt-tip pen on the nut and the rod. Now run up the second nut to lock the first nut so that the witness marks are still aligned.

    Push the threaded rod back up against the case. Open the trapdoor and push the case in as far as it will go. That will push the rod back out a teensy weensy bit. Use one hand to hold the rod still, and with the other hand EITHER measure the gap with a feeler gauge OR (my preferred method) run up the first nut until it is touching the muzzle once more. From the amount by which the nut needs to be turned you can calculate the width of the gap.

    This gap is the head clearance - the space between the front of the cartridge rim and the face of the recess. It is the relevant and real-life measurement of the clearance between the rim of the real cartridge case you are going to use and the real chamber recess into which it must fit. Not the theoretical space for a theoretical headspace gauge. Please forgive my sarcasm, but most people fire cartridges, not headspace gauges.

    My method tells you the head (or rather rim) clearance, and is accurate (with practice) to about 1/8 turn. With a 26 tpi threaded rod that is about 0.005" - and quite accurate enough for any BPCR that I have come across up to now. In fact, as long as there is a positive head clearance, the rifle will function, unless the clearance is so large that the strike of the firing pin is too short to ignite the primer. Or, of course the firing pin is too short!

    Can there be a negative head clearance? Yes, if the aforementioned chamber recess is full of dirt or you try to use a converted cartridge case from a type that has a much thicker rim. In both of these situations it will be difficult to close the trapdoor, a warning that something is wrong.

    Summarizing: For BP guns, which use rimmed cartridge cases, headspace gauges are a waste of money and provide no guarantee that the gun is safe to fire. Checking the head/rim clearance is also no guarantee of safety, but it tells you whether the thing is actually going to work.

    ** Belgian S&W Russianicon, Egyptian RB, Gasser M1870, Grenzaufsehergewehr, Martini-Henry, Mauser 1871, M71 Jäger, Peabody.41 RF, Sharps 1874, Springfield 1878, Snider, Swedishicon RB, Wesson & Harrington, Whitney-Laidley
    Last edited by Patrick Chadwick; 09-10-2019 at 06:17 PM.

  4. Thank You to Patrick Chadwick For This Useful Post:


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    Thread Starter
    Wow! Thanks for the reply. I do not reload 45-70 so I will have to find a case. Do you know of a company that makes 45-70 that is safe to fire in trapdoors?

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    Advisory Panel Patrick Chadwick's Avatar
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    Quote Originally Posted by CHUCKW View Post
    Do you know of a company that makes 45-70 that is safe to fire in trapdoors?
    Sorry, can't help you with suppliers. I live in Mauserland.

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    Thread Starter
    Didn't notice that. Thanks.

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    Buffalo Arms has black powder loads for the 45-70.

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    Really Senior Member Daan Kemp's Avatar
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    A lot of enjoyment in reloading, especially black powder. Little fuss, not critical as to nano grams of powder, nice big bullets to handle, big cases, cases last forever, etc.

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    Advisory Panel Patrick Chadwick's Avatar
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    Quote Originally Posted by Daan Kemp View Post
    cases last forever,
    ... as long as you
    ... ONLY neck size, if at all. If you are lucky with the chamber neck and only use soft to moderately hard lead bullets, then you may be able to avoid even neck sizing.

    ... IF you have more than one rifle with the same caliber, always use the neck sized cases in the same rifle. Distinguish by marking the base or using only one manufacturer per rifle.

    ... Always use the cases with the same orientation. I make a file mark at 12 o'clock and highlight it with a felt pen.
    Bolt actions are highly symmetrical about the bore axis. But all types of block action (Trapdoor, Snider, Martini-Henry...) are asymmetrical and can have a slight skew on the block face which worsens with wear. Full-sizing, by trying to force the case body to be square to the base again, is surely a contributory cause of that fatal bright ring about 1/2" above the base that tells you your case is on the way to a base separation.
    Last edited by Patrick Chadwick; 09-12-2019 at 06:04 AM.

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    [QUOTE=Patrick Chadwick;458968]... as long as you
    ... ONLY neck size, if at all. If you are lucky with the chamber neck and only use soft to moderately hard lead bullets, then you may be able to avoid even neck sizing.


    And anneal the cases!

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    Really Senior Member Sunray's Avatar
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    "...company that makes 45-70 that is safe to fire in trapdoors?..." Unless it's otherwise noted, all American factory .45-70 ammo is loaded for TD's. It's been like that since forever. And for the specific reason that they have no idea what rifle it'll be used in and they don't want to get sued. So they load down.
    Loaded for a vintage TD Carbine years ago(The Great BP Cartridge Experiment. There was no internet to tell me the Carbines did not use 70 grains of BP. Nobody to tell me BP is loaded by volume either. Shooting a cast 405 with 70 grains of BP out of a 6 pound TD Carbine hurts.) using a Lee Loader.
    The assorted loading manuals will give you smokeless powder loads that are safe in a TD too. Traded the Carbine to a buddy who said it shot well with smokeless.

    Spelling and Grammar count!

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