rear lug action bolt flex

Discussion in 'Advanced Gunsmithing & Engineering' started by Shawnba67, Apr 14, 2017.

  1. Shawnba67

    Shawnba67 Silver $$ Contributor

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    so in my free time I have been trying to figure out why my 22-250 Rem 788 occasionally has a sticky/ hard bolt lift on extraction.
    THIS IS VERY UNLIKELY TO BE HOT LOADS (read on)
    had to add that to to help this along productively

    Rifle was completely factory and would occasionally stick a little with factory ammo slightly more with reloads with no real change between starting or near max loads
    I REBARRELED rifle to 6-250 using new hornady brass basically same thing
    REBARRELED again to 6 Norma dasher and very first load of 28gr varget did it then kind of randomly through the whole load work up although the dasher did stick more than the 250's had been Now the sticking never required me to stand up but you would have to hold the rifle with one hand to coax the bolt on up and out with the other
    So we have factory ammo and reloads in 3 different barrels with 5 brands of brass this leads me to believe I have a rifle problem.
    My hypothesis is this I believe I have a bolt flex issue letting the bolt "spring" a little and if so it would let the brass be slightly to long. Remember it's a rear lug setup I have never had means until now to measure my brass for headspace growth so dasher is the only one I could measure
    What do you guys think? Am I barking up the right tree? This is more a problem solving exercise as this rifle is just my test mule for learning on.
     
  2. gunsandgunsmithing

    gunsandgunsmithing The best tuners and wind flags on the market Gold $$ Contributor

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    Is the bolt hard to lift at the very start of the upward movement or does it get hard at some other point in the upward travel?
     
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  3. Ggmac

    Ggmac

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    I think I asked you how many lugs made contact ? If you were checking bolt lugs you should also check receiver for lug set back , though I've never heard of a 788 doing that . I've played a lot and still do with them . I would check your cocking piece cam and bolt camming surface . If all checks out make sure the forcing pin bore , spring and check for rub marks or burrs .
    Eliminating all possible mechanical issues , do you full length resize ? If not I would try that .
    Like Gunsandgunsmithing asked , when does it get sticky ?
     
  4. Ggmac

    Ggmac

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    I've shot a 788 in silhouette using hot loads , I read all there's to rear lug bolts and bolt compression , I never experienced it . I think it'll be a simple fix , hope so . Did you change the main spring ( firing pin )

    Sorry , I keep remembering stuff , does the bolt lift get easier IF you pull the trigger while opening the bolt ?
     
  5. fguffey

    fguffey

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    Every reloader has the means; problem, the case does not have head space. All a reloader/smith/forum member needs as a means? is a good understanding of the datum. I make datums, Until I became a member on reloading forums I thought all reloaders, machinist and smiths made datums; I was wrong.

    I was at a gun show in Dallas, Texas when I saw a box of what looked like junk to most; I said "DATUMS!", the dealer said he did not have any datums, the happy shopper in front of me asked, "WHERE?" and the man behind me said he did not see any datums and finally the last shopper ask; "WHAT DOES A DATUM LOOK LIKE?". He was the only one of the four I could help.

    F. Guffey
     
  6. Shawnba67

    Shawnba67 Silver $$ Contributor

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    Has this rant made you feel better?
     
  7. Shawnba67

    Shawnba67 Silver $$ Contributor

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    Yes bolt is hard to start upward sometimes requires a tug to start rearward
    I have not checked the lugs for contact pattern am not sure of best way to do so? Dychem on them and pull back on handle while working it few times? Lapping compound for a few of same cycles? I will look over the cocking piece and It had a timney trigger put on after the second rebarreling if that could eliminate that as a culprit of why pulling trigger would affect it. I never tried the pulling trigger I will just haven't yet
     
  8. fguffey

    fguffey

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    Shawnba67, I am the only one that will tell you a reloader has the means. I am the only one that will tell you the case does not have head space. Members have called SAAMI to inform them I said the case does not have head space, I am sure SAAMI was impressed with their pedigrees and impressed with their accomplishments, none of that changed the fact the case does not have head space. If you are not interested in going beyond talking about it that is OK with me.

    Datum: when I started reloaders thought the datum was a line because the drawing had an arrow pointing to a line that was identified as a 'datum line'. It seems some of them are still mad.

    F. Guffey
     
  9. gunsandgunsmithing

    gunsandgunsmithing The best tuners and wind flags on the market Gold $$ Contributor

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    Difficulty starting upward implies the lugs are under a static load after firing. That may mean that the brass is still tight and may mean that the bolt is in fact compressing to some degree. As for it being sticky at the very top, not sure. If it's happening beyond the point where the extraction cam has done it's job, I'd think it's unrelated and may just be a minor hanging point. We're not working with a full custom action here and there may be a slight snag there that won't hurt anything. But, my point is that after the tapered cartridge is pried loose by the primary extraction cam, there should not be any resistance from that point on out of the chamber. Maybe a 788 expert will chime in. I'm not that person.
     
  10. Alex Wheeler

    Alex Wheeler Gold $$ Contributor

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    Take a case length measurement with a shoulder comparator of a fired case, preferable one that gave heavy lift. Make sure to re-seat the primer first so it doesnt give a false reading. Size the case so it will chamber with the lightest touch of a stripped bolt. Measure that case and note the difference. This should tell you how much the bolt is flexing and allowing the case to grow, if it is happening at all.
     
  11. Alex Wheeler

    Alex Wheeler Gold $$ Contributor

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    Definition of arrogance
    1. : an attitude of superiority manifested in an overbearing manner or in presumptuous claims or assumptions
     
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  12. Ggmac

    Ggmac

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    Shawn , the 788 has a 90 deg sear , if the cam timing or timing i should say isn't correct the cocking piece hasn't completely cleared the sear , by pulling the trigger it allows the sear to drop , thereby bypassing the problem . I may not be explaining it correct , but try it . If it works I'll tell you how to fix it .
     
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  13. dmoran

    dmoran Gold $$ Contributor

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    stalker10k3.jpg
     
  14. Shawnba67

    Shawnba67 Silver $$ Contributor

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    So I'm going back to rifle range sometime next week I'll report back if and what I can discover
     
  15. carlsbad

    carlsbad Details matter. Silver $$ Contributor

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    There are people around who will say that a rear lugged bolt will compress. I decided to crunch some numbers. with a bulk modulus of 23 E6 for steel and assuming a .700 od bolt with a .500 id and 4" long compressible volume, I got a force of over 100,000 lbs to compress it .001". This calculation has not been peer checked and was done in bed at 1am but it should be right.

    More likely, what is measured and looks like compression would be lug flex if one or 2 of the 9 lugs is high then it might flex a few .001" s. 9 lugs is cool but it means each is small so if only one engages, you can have a problem.

    Rear lugs also create the possibility of the bolt trying to cock sideways if the front fit of the bolt isn't tight, especially if only one lug is engaged. This could lead to case growth.

    the conventional wisdom is that rear lug bolts are harder on brass which makes sense to me since there is 4" of movement available. This seems to be backed up with real world data so it is somewhat "normal" to have more movement in a rear lugged bolt.

    Alex gave you a good way to take data to measure flex above.

    I would put some dykem on all the lugs and cycle the bolt many times, pulling back on it. If the blue only rubs off of one lug, then you have a high lug and lapping might be the right answer but is difficult to do right.

    Is your stock a good fit on the action? Could the action be under stress from a poor fit?


    --Jerry
     
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  16. Patch700

    Patch700

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    Lol , Jerry I am stealing that line..
     
  17. Robert

    Robert Silver $$ Contributor

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    Questions:
    1°) Do you full resize or neck size only?
    2°) If you rechanber a fired case who have presented hard extraction , do you experiment the same problem with it?.
    3°) Are you sure the primary extraction works correctly? Have you measured the primary extraction travel and timing?

    I do not think bolt flex or action stretch can be in cause for the 788. Case stretching with rear locking action is a notion of the past, generated because the spongy SMLE acctions and long thin bolts of the time.
    R.G.C
     
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  18. alintx

    alintx

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    Was having the same problem on a 22 250. Finally set it up REALLY tight, almost a crush fit on factory shells, and the problem went away. Let me know what you figure out!
     
  19. AckleymanII

    AckleymanII Gold $$ Contributor

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    Many years ago, I had a 788 in 22/250 that had pressure issues with lighter loads than my other two 788's in 22/250. I put dykem on all the lugs and was surprised at how many of the rear lugs had no contact at all. WE had great discussions with some very sharp aerospace engineers that I hunted p. dogs and chucks with as to whether the front lugs were more important than the rear lugs when it came to contact.

    We got to checking all our 788's, seemed like helter skelter on how many of the lugs had contact and how much contact. To say it was a goat roping engineering affair was an understatement. We had Canjar 2 oz triggers on all our 788's, so that took care of the trigger issue.

    Lugs engaging on 788's are helter skelter, check yours, be prepared to take a Zanex when you are done. It is not uncommon to have 50% of the lugs with no contact.

    Bottom line for us, was if you had a "good one" it would shoot like a house a fire and not exhibit extraction issues on warmish loads. If you had a bad one, consider it not worth the fix.

    Having rear lugs contacting where it kicks the bolt off center, creating off center primer strikes makes my head hurt.

    Case stretching on 243 Win in 788's is very, very common...lugs..warmish loads were a no no.

    I have had a flock of 788's in 222,223, 22/250, 243, couple of 7/08's and 308's. They are usually very accurate with tuned loads.
     
  20. jfseaman

    jfseaman Silver $$ Contributor

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    Clean your chamber and especially the neck and neck chamfer/taper to the free bore.

    Check your case length at the shoulder using standard measurement techniques. IE, Hornady case length gauge. If you don't have one. Use a 9mmx19 case over the shoulder. Measure as many cases as you can. Note the longest, shortest. If the longest was a sticky one and is difficult to rechamber you found the problem.

    The other thing to check is neck length. If you necks are not "straight" after coming out of the chamber as in has a little curve in at the mouth, your necks are long or too much crude built up in the neck chamfer/taper to free bore. Ref: first line of this reply.
     

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