Dry Firing Jump

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by scsims, Apr 25, 2013.

  1. scsims

    scsims Well-Known Member

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    I've noticed that one of my rifles the crosshairs jump a little when dry firing. Also I did the coin on the end of the barrel test and this gun is the only one that the coin falls off.

    I don't think this can be good for consistent precision and accuracy. The rifle is a Rem 700 .308 stress free bedded into a Choate stock.

    I have other Rem 700s that don't jump near as much as this one.

    Any ideas on how to reduce it?
     
  2. Mikecr

    Mikecr Well-Known Member

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    You can reduce it and improve accuracy doing so.
    Contrary to popular belief, a firing pin should not be set to excess energy. Excess being more than needed for consistant ignition.
    If you load develop with firing pin adjustment you will find a sweet spot where performance is best, and it won't be at any extreme setting.

    I do this, after also backing off sear/cocking piece so that it does not bottom in the shroud on release.
    Also, if you whitness the bolt turning on dry firing(usually lifting) -that's evil energy right there.
    So I also set a bolt turn stop so that the cocking piece will fall freely without resistance, and the bolt ceases to so much as wiggle.
     

  3. scsims

    scsims Well-Known Member

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    Thanks for reply Mike, this sounds like it might need a trip to a gunsmith.
     
  4. Bart B

    Bart B Well-Known Member

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    Every fixed power scope I've ever used does that on all my rifles; rimfire and centerfire. Their windage and elevation adjustments are spring loaded pressing the erector tube in the scope hard against the flat of the adjustment screw. It's totally normal that that tube will vibrate a tiny bit and you see that when dry firing.

    In most variable power scopes, the two lens groups that slide back and forth when changing power will shift position a tiny bit when dry firing the rifle and that also makes the reticule jump a bit when the rifle's fired; dry or with live ammo. Their fit is not zero tolerance in the tube they slide back and forth in as the power is changed. They also can change position as the power's changed; the reticule moves in an irregular pattern about the collimator. Sometimes as much as 1 MOA. Which is why many folks wanting best accuracy with variable scopes set the power to one limit and keep it there; the scope's more repeatable that way.

    As long as the reticule goes back to the same position after jumping from mechanical shock, there's no problem. Put an optical collimator in the muzzle, zero the scope on it then dry fire watching the scope reticule on the collimator's reticule. If it goes back perfectly each time, no worries.

    And no rifle action I've used has its bolt jump when firing live ammo. The firing pin tip does not dimple primers enough to let the cocking piece strike the bolt camming surface to push the firing pin back. But when dry firing without a round in the chamber, it's as common as sunrise every morning. If this happens with live ammo, then something needs fixed; probably the cocking piece on the firing pin needs its cam point ground back a few thousandths.
     
  5. ultraedge

    ultraedge Well-Known Member

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    To reduce the dry firing jump , swap the firing pin and spring assembly. I have 7 custom rifles built on LA Rem 700 actions and Sendero stocks . They all had good accuracy potential, but when shooting from the bench I had to hold them differently to obtain the best groups.The ones with no dry firing jump could be held normally, but the ones with dry firing jump had to be held tighter with downward or side pressure applied to the stock to obtain best results. I discovered that by swapping firing pin and spring assemblies that the problem was corrected. This does not last forever, because the spring pressure changes over time. All springs do not work the same in all similar rifles. A firing pin and spring assembly that does not work well in one rifle may be entirely satisfactory in another. The assembly can be changed in a few minutes with only a dime. Gary
     
  6. backwoods83

    backwoods83 Well-Known Member

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    If its the factory spring on the firing pin, which I'm guess it is, i'd bet 50 bucks its way to long, thus it is binded up inside the bolt body. Check it, if it is have a smith cut it, or put a ptg or wolff spring in it and check the firing pin protrusion and the cocking piece. An aftermarket trigger can also help with the timing of things with tighter tolerances, especially a Jewell. I have had over a dozen 700s and only 1 has had a straight spring in it, the rest were as crooked as a poltician.
     
  7. Bart B

    Bart B Well-Known Member

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    All my firing pin springs scrape the inside of the bolt body. Nary a problem with that.

    And no trigger will change how the firing pin operates. It's a different mechanism and does not touch that spring in any way.

    And if it's a factory spring, there's no way it would be too long. Anybody want to bet against this?
     
  8. backwoods83

    backwoods83 Well-Known Member

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    Obviously the trigger doesn't touch the spring, however on a Jewell or any three lever trigger it takes a lot less force to drop the cocking piece, just a little snappier and a faster lock time not to mention a trigger in a whole different class. But since you're the resident wise a$$, I mean guru by all means let us know why the springs look like a double and drag on the bolt body when they are cocked or released, also you cannot convince me that the spring dragging on the bolt body doesn't affect the striking force and lock time, resistance is resistance no matter how you look at it. Other brand factory actions and custom actions (especially the ones off a remington footprint) do not do this, so explain away professor.
     
  9. Bart B

    Bart B Well-Known Member

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    Well of course the spring sliding against the bolt body reduces lock time and softens impact. Probably 2 or 3 percent. Go measure yours then you'll know for sure. I don't care about mine. But I use heavier than factory spec springs anyway and they more than make up the difference as well as making primers perform more uniformly. And all springs on firing pins drag on something; no rifle action on this planet has "free floating" firing pin springs that do not touch either the pin or bolt body. To say nothing about the firing pin and/or its cocking piece dragging on its shroud or sear or even the bolt face around its hole; they ain't free floating either.

    Do you know how much your springs weaken every year?
     
  10. backwoods83

    backwoods83 Well-Known Member

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    The short answer, enough to justify buying a 10$ spring once a year, unless you have a Bat action and your a tight wad, of course 75$+ is a little pricey for a spring.
     
  11. Mikecr

    Mikecr Well-Known Member

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    I may be taking this out of context, but on it's own this is incorrect.
    Triggers change EVERYTHING about bolt timing, including much in OPs concern.
     
  12. Bart B

    Bart B Well-Known Member

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    While a trigger may effect lock time a small amount, it's the firing pin impact on something (the bolt or its sleeve?) that causes virtually all of the scope's reticule jump that's seen by humans. At that time, the trigger has no effect on it whatsoever.

    I don't think triggers have anything to do with the OP's concern.
     
  13. Mikecr

    Mikecr Well-Known Member

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    The firing pin is held back and released by the trigger.
    Therefore, the trigger directly affects firing pin impact. It affects the amount of pin fall on release, as well as friction in this fall(as is solved by the Kelby replacement for Jewells with certain actions), and yes lock time for sure. Bolt TURN/POSITION is also affected, and you should mind the consistent stopping of bolt turn. Don't assume it.

    If, with a given trigger(not model but actual), you have not set the pin strike for best grouping, you probably have not reached the best from your gun. I have whitnessed a perfectly firing mis-setting of a firing pin cause ugly performance. Flyers..

    This is an area rarely looked at or discussed. I believe there is a lot to learn here.
    Ever wonder why we're left to swap primers trial & error during load development? Why a primer that sucked for you works great for someone else in the same cartridge & load?
    The answer is primer striking. Well actually, the ABSTRACT is primer striking..
     
  14. backwoods83

    backwoods83 Well-Known Member

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    I had a very similar problem with my 6br, Mike. I had to shorten the firing pin by .020" and use a PTG reduced power spring in a trued sa 700 with a Jewell trigger. Before I done this it would pop every 3rd or 4th primer and send a flyer, now it shoots 1/3moa at 600yards.