What if Bolt Locking lugs not bearing Fully/Symmetrically ?

Discussion in 'Gun Project Questions & Gunsmithing' started by pemo, Jun 24, 2018.

  1. pemo

    pemo

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    Which problem(s) would you generally see with a rifle where one, both or more of the locking lugs were not bearing fully or symmetrically on matching action surfaces ?

    Changing POI, vertical/horizontal stringing, larger difference when changing projectile/load ?

    Some people suggest using polishing/grinding paste to get to better bearing surface, could that work ?

    Specifically I have a few "hysterical ones" in the cabinet and I'm wandering if the observed lack of bearing surfaces and symmetricality could explain some of the issues (wandering POI (2-3cm), occasional flier, 4-6cm POI difference when changing ammo). Bedding, bases, rings, scopes and load components have been checked/changed.
    The other good shooting ones show >75% and symmetrical bearing surfaces.

    Many thanks in advance :)
     
  2. Ggmac

    Ggmac

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    Before you do anything I would ask a smith to evaluate. Once you remove metal it's not easily or cheaply fixed .
    Many more questions need to be answered . Bench , target , competition or hunting ? What cartridge. Barrels, are they factory or aftermarket? Stocks are they free floated , bedded . Any trigger , bolt , screws hitting the stock ? Factory loads or other ? Crowns good ? What triggers ? Factory ? What's the pull weight ?
    Now for your shooting . Have you let others try ? Bipod , front rest rear bag or other ?
    Scope , power ? Rings lapped and tight . Base tight ? No screws hitting the barrel ?
    More after thought
    What's the shape of the groups , one out 4 in a cluster , 2 distinct groups , horizontal, diagonal, vertical ?
     
    Last edited: Jun 24, 2018
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  3. butchlambert

    butchlambert Site $$ Sponsor

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    This has been discussed many times. Unless your bolt is fitted to a reamed bolt bore, you can't achieve full bearing. As mentioned a few times, when you cock your bolt the rear of the bolt rises and unloads the upper lug. How much does it hurt? I don't know.
     
  4. Ggmac

    Ggmac

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    Only if angled sear . Remington 722-700 , not the 783 ,Winchesters , not Mausers , springfields, others ?
    Not Marlin Xl/S-7 , Mossberg ATR,
    Agree with how much does it hurt ?
     
    Last edited: Jun 25, 2018
  5. pemo

    pemo

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    Many thanks for constructive comments.

    The 2 rifles with <50% bearing surface on right/lower lug are a brand new Tikka T3x CTR and a Husqvarna 1900 clone (Viking).
     
  6. Alex Wheeler

    Alex Wheeler Gold $$ Contributor

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    Lug contact must be checked with a stripped bolt. The trigger does un seat the top lug but by the time the bullet is entering the throat both lugs are fully seated. So in my book lug contact is CRITICAL. It is one of the things I am very picky about.
     
  7. DaveTooley

    DaveTooley Silver $$ Contributor

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    Butch

    Just a thought. If the rifle is a repeater with rounds in the magazine wouldn't that push the midpoint of the body to the top of the action? If so then the debate is over. At least for repeaters.
     
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  8. STOMP442

    STOMP442 Silver $$ Contributor

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    You say you are seeing POI changes when changing ammo. This could easily be barrel harmonics. Different weight bullets, burning rates of powder and velocity all play big parts on how the barrel vibrates and at which point of the vibration the bullet actually exits the muzzle. Making sure you have good lug contact is important but I think that is relatively hard to see on paper as compared to proper loading and shooting techniques.
     
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  9. Ggmac

    Ggmac

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    If that were true each consecutive shot would reduce the pressure and give even more erratic shots
     
  10. butchlambert

    butchlambert Site $$ Sponsor

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    I believe he is looking for the wrong gremlin.
     
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  11. DaveTooley

    DaveTooley Silver $$ Contributor

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    Reducing pressure on the bolt wouldn't change the location until the weight of the bolt exceeded the spring pressure. Never looked to see if or when that happens on my own rifles. Not a priority for me.
     
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  12. Ggmac

    Ggmac

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    It happens on the FAL , that's the only reason I brought it up.
    Somewhere in my library an article was written about the tilting bolt accuracy problems . I did limited testing with a few FALs . Each one showed a better group loading one at a time in a sled. Barrels cooled , cleaned and as many variables eliminated as possible .
     
  13. DaveTooley

    DaveTooley Silver $$ Contributor

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    About a hundred more things going on with FAL than a bolt gun.
     
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  14. Ggmac

    Ggmac

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    Yup ! But I only eliminated one factor !
     
  15. SamLS

    SamLS

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    The location of a spring loaded ejector can cause variability. Some rifles use a plunger ejector which is spring loaded. It can cause the front of the bolt to cam down upon closing. Make sure the corners are round and smooth. Basically it pushes the bolt back against the lugs. Can also take the plunger ejector out and shoot to see if the problem goes away.
     
  16. butchlambert

    butchlambert Site $$ Sponsor

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    Sam,
    Where is the plunger when the rifle is cocked?
     
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  17. Alex Wheeler

    Alex Wheeler Gold $$ Contributor

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    Nothing is overcoming the 60k psi in the chamber that is forcing the case against the bolt face and seating the lugs in that first .5 ms.
     
  18. SamLS

    SamLS

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    Actually the brass grips the chamber wall pretty effectively. The bolt doesn't see 60,000 psi unless extenuating circumstances (ruptured case head or similar event). Ackley did some tests confirming this way back when. Have you have ever seen a primer back out.

    The point your missing is what happens when the firing pin is released and the case pushes back against the bolt face from pressure (if it does). Do you want those parts moving as the firing pin drops or do you want them to be in the same place when its cocked and when the firing pin strikes the primer.

    Sam,
    Where is the plunger when the rifle is cocked? Depends on the rifle
     
  19. Dusty Stevens

    Dusty Stevens COVFEFE Gold $$ Contributor

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    So if it doesnt go against the boltface with full pressure why can you see ejector marks on some loads? And it goes back so hard on some loads when you lift the bolt that hole shaves the brass off that flowed into that hole
     
  20. Alex Wheeler

    Alex Wheeler Gold $$ Contributor

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    Exactly, by the time the bullet has entered the throat the lugs are seated. The ejector spring is so weak it has no effect on the lugs, and could never overcome the mechanical advantage the sear/cocking piece relationship has. Every Benchrest action used today has rear lift of the bolt. Just some more than others. Does it matter to accuracy? Not that I have seen. And I have been watching.
     

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