FN action: BOLT THRUST REDUCTION

Discussion in 'Gunsmithing' started by Bigstrap3, May 11, 2011.

  1. Bigstrap3

    Bigstrap3 Active Member

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    I am currentelly building a 6.5x55 on a FN Mauser action. Is there any way to quantify how much I can reduce the pressure on the bolt bye changing from the above mentioned cartridge to a straighter case with a steeper shoulder? I understand that a straighter case with 'grab' the chamber walls better , not sure if this is a old wives tale or a significant thing. I am using a kreiger 28.25 inch varmit contour barrel in 1-8.8 twist. I was thinking of the A.I. as a way to get some more velocity and maybe reduce the bolt thrust.Any input would be appreciated thanks........... George
     
  2. fj40mojo

    fj40mojo Well-Known Member

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    Hopefully Bob the Nailer will chime in here as I know he has built a 6.5x55BJAI but he sent me this link

    Article posted on Precision Shooting Website about Ackley Improved cartridges - Shooters Forum

    Some good info there. I built a 6.5x55 a while back and had I known of the existence of the BJAI version I would have built that instead. My accuarcy node falls around 2850fps and while I still have case capacity and no pressure signs I don't have enough to get to the next accuracy node. Going to the BJAI will reduce bolt thrust and boost charge capacity enough to get you there.
     

  3. shortgrass

    shortgrass Well-Known Member

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    The 6.5mmx55 is a relatively low pressured round, and the FN '98s, in good condition of course, are well made. Don't know why you're worried about 'bolt thrust' with that round in that action.
     
  4. fj40mojo

    fj40mojo Well-Known Member

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    That's great if you follow the rules and don't exceed maximum published data-EVER. But if you want to ring all the performance out of your rifle you aren't going to follow all the rules all the time, the Swede has a lot more potential than the reloading manuals would lead you to believe. I shouldn't be pushing 140grn pills at 2850fps, but I am and with out pressure signs. I can go faster yet but this is where I hit the accuracy node and don't have enough case capacity to reach the next node. Ackleyizing the Swede will get you there by reducing bolt thrust and increasing case capacity, I would think that 3100fps would be doable with 140grn bullets.
     
  5. Bigstrap3

    Bigstrap3 Active Member

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    thanks very much for the information. I shoot quite a bit so I am just trying to get the most I can from the rifle.....:D
     
  6. fj40mojo

    fj40mojo Well-Known Member

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    Welcome, us Mauser fans gotta stick together.:rolleyes: I'm in midstream of turning a Belgian '98 into a .280AI. Got a 1:9 Krieger #3 and a B&C Medalist Varmint. Smith is booked until end of June so everything is on hold for the moment. Hope to have it done before my elk season rolls around.
     
  7. shortgrass

    shortgrass Well-Known Member

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    The 'rules', as you call them, are there so folks don't hurt themselves. I realize published data is what it is out of respect for the '94/'96 Swedish Mausers. If you read the intro in the loading manuels, it says in an action more modern then the Swedish Mausers, more velocity w/stouter loads is possible. Modern made actions and brass make it possible. By Ackley-izing the 6.5 x 55 you may gain 150fps or so. If you need more, you might look at a different cartridge case. Your desired 3100fps. is a max load for the .264 Win. Mag., which a much larger volume. There have been 100s of thousands of Mauser '98s converted to feed and fire the Winchester Magnums, I happen to have a few of my own. Some, like your FN, came from the factory for the Win. Mags. So,to me, there's no question as to their strength. I have seen more than one, over the years (the last 35 or so) that have set back the locking lug abutments, turning an otherwise fine action to junk. To reach that 3100fps velocity in a case smaller than that the size of a Win.Mag. will drastically increase pressures and I don't think the straight walls of an Ackley case will be enough help in the Mauser. But, before I did it to a 6.5x55, I'd step up to a case with more capacity to try to keep pressures under control. And, by the way, that 2800 to 2850fps. seems to be the velocity range the guys shooting .260's, 6.5x47's and 6.5-284's in competition are at for their best accuracy. You might send it like Captain Kirks' 'phaser' but does it go there accurately? All of this has been tried 20, 30, 40 years ago. If you look around some of the gunsmithing books and on the web I'm sure you can find some pics of catastrophic failures. Floor plates blown out/off & mag boxes swelled, stocks splintered, bolts unremovable from the action because of set back. A well made '98 is a fine action, but not the one I'd want to try to 'hot rod' beyond its capabilities or past the strength of modern made brass cartridge cases.
     
  8. shortgrass

    shortgrass Well-Known Member

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    Out of an Oberndorf 1935 Peruvian chambered in .30-06 A.I. Owner allowed "he can get .300 Win. Mag. velocities with his handloads". Cost him partial vision loss in his right eye and a nice rifle!
     

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  9. Bigstrap3

    Bigstrap3 Active Member

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    Hello Short Grass I appreciate your response. The reason I posted this thread is attempt to quantify how much If any I can gain by going to an A.I. chamber. I am in no way shape or form interested in damaging my rifle/me. I just thought I might be able to reduce or keep the B. T. the same and pic up a few FPS. I was planning on using the max published 96 swede loads in a couple of my loading books. I may be a bit premature with posting this here as I dont even have it stocked yet. Any thoughts or input are appreciated thanks again..... George
     
  10. fj40mojo

    fj40mojo Well-Known Member

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    FYI, nobody "desired" 3100fps, I just stated that it might be attainable or approachable with the Ackleyized Swede. BTW I'm getting 2850 with 45.5grns of H4831, Lapua brass, Fed primers, and Hornady 140g SST from a 26" barrel, that is 1.5 grains<the published max load for this powder in Nosler #6 Manual. Hardly what I would call "hot rodding". Their test barrel was 23" and gave 2663fps with 45.0g, oh and yes it goes there accurately-3/8"@100yards. I'm not advocating that we all "hot rod" our rifles, but this example illustrates how different two rifles can be. By all means, practice safe reloading techniques, always approach max charge weights with caution, know how to recognize pressure signs, but on the same token know your cartridge/action history too and realize that factory ammo and reload data may not reflect actual cartridge potential and just because it's the newest uber magnum doesn't mean it's better. Funny that you mentioned the host of new 6.5/.264 cartridges, I'd say this was all done well over 100 years ago and that Rem and Lapua have reinvented the wheel. If these velocities (2800-2850) are safe in .260 (53g H2O) and 6.5x47L (49g H2O), why would they not be safe in the same action chambered for the larger volume 6.5x55 Swede (58g H2O)? I don't know how many grains of H2O you gain by going to the BJAI but it stands to reason that any gain would increase volume and thereby cartridge potential and in concert with the reduced bolt thrust, well sounds like a win win situation to me. Those darn Swedes were on to something and so was Ackley. Don't know if you've noticed but the Swede has a lot of body taper a common trait in military cartridges, but by blowing most of the taper out of the body of the case more pressure is exerted against the walls of the chamber and less rearwards into the bolt face reducing bolt thrust. This is because as pressure is exerted outwards against the walls of the case the tapered walls of the chamber act like a wedge redirecting forces and squeezing the case rearward into the bolt face. Less taper equals less bolt thrust. This and moving the shoulder forward and out to 40 degrees increases case volume. How much, I don't know, can't find it published anywhere. Is it enough to get to the next accuracy node beyond 2850fps? I don't know, I don't own one, wish I did. Just sayin.:rolleyes: Sorry for hijacking your thread Bigstrap.
     
  11. specweldtom

    specweldtom Well-Known Member

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    Ted, thanks for the "dirty pictures". One of the many things I like about the Mauser large ring actions is the 3rd "safety" lug. I'm guessing the action survived, but it looks like the bolt set back far enough to engage the safety lug. Also, the case must have ruptured badly enough for all kinds of crap to get in his face. I'm a little surprised the gas vents and shroud didn't catch or divert it. Can you share any more details? Did the floorplate blow open (or off)? If he had rounds in the magazine, did any of them go off?

    I was also wondering if maybe the stock blew apart and caused the injuries.

    A while back a friend wanted me to barrel up a Mexican small ring Mauser for him in 6.5 x .284. I just couldn't make myself do it. It might have been alright, but I balked. I did it for him on a F N commercial large ring with no hesitation. A 6.5 x 55 or an improved version should be a cake walk for that action.

    Good thread.

    Tom
     
    Last edited: May 12, 2011