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FN action: BOLT THRUST REDUCTION

 
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Old 05-11-2011, 03:20 PM
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FN action: BOLT THRUST REDUCTION

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
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Old 05-11-2011, 06:19 PM
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Re: FN action: BOLT THRUST REDUCTION

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.
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Old 05-11-2011, 09:23 PM
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Re: FN action: BOLT THRUST REDUCTION

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.
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Old 05-11-2011, 10:58 PM
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Re: FN action: BOLT THRUST REDUCTION

Quote:
Originally Posted by shortgrass View Post
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.
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.
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Old 05-12-2011, 03:00 AM
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Re: FN action: BOLT THRUST REDUCTION

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.....
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Old 05-12-2011, 05:48 AM
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Re: FN action: BOLT THRUST REDUCTION

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Originally Posted by Bigstrap3 View Post
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.....
Welcome, us Mauser fans gotta stick together. 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.
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Old 05-12-2011, 07:46 AM
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Re: FN action: BOLT THRUST REDUCTION

Quote:
Originally Posted by fj40mojo View Post
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.
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.
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