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Thread: My 45-70 barrel arrived...

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    Happy Camper hcpookie's Avatar
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    Default My 45-70 barrel arrived...

    ... and I'm not sure how I can make it work in this Yugo trunion.

    Issue #1:

    Notice that the chamber is threaded, and that it is thin enough that a normal-sized AK barrel pin would cut into the chamber.

    I think the correct answer is to shim the pin hole in the trunion, and drill it for a thinner barrel pin like those found on a G3 or VZ-58. That would let me pin it, and keep the chamber intact.

    Thoughts on a "skinny" barrel pin?

    http://pookieweb.dyndns.org:61129/45...win_barrel.htm

    ETA - The barrel is cut so that the Winchester tube magazine lays against it... well when the barrel is rotated into the alignment where that is on the bottom, the threads reach up to about 1/3 of the barrel pin hole. It is too close to call - the chamber may not be cut into, but I can't be sure. If it is not cut into, I do know that the wall of the chamber will be *very* thin. Only the barrel pin will support the chamber in that case. Therefore, perhaps a not-so-skinny barrel pin would be better... ?


    Issue #2:

    Pressure will increase if the bullet is forced into the rifling - correct? Therefore, I wonder if the 500 grain loads, being longer, need to have the throat reamed?

    I have yet to make a cast of the chamber, so this is speculative at this time.

    Lyman measurements:

    500 gr jacketed RN COL: 2.93" --- !!!
    350 gr jacketed SP COL: 2.71"
    400 gr jacketed FN COL: 2.55"

    So that's .4" longer for the 500 grainers. These are intended for a Ruger Mk 1 or similar bolt-action style "heavy frame" rifle that is capable of the higher pressures the 500 grain loads produce - up to ~ 39,000 CUP according to the Lyman book. Even higher with the (hot) Hornady loads. Compare this to the average mid-30's CUP pressure of the lighter loads. (the Yugo / RPK / PSL trunion design is rated for up to 45-49,000 CUP found in the 7.62x54R).

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    No Hope For Me 1biggun's Avatar
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    Interesting. sort of takes the fun out of it.
    really all you can do is thin the pin in the center and hope for the best. maybe it would be better to make the pin larger than 7mm that way there would be more area/strength in the part of the pin that dont need to be thinned and also give more ingagment. I am wondering how that guy with that big .50 did his????
    This is a build were a threaded insted of pinned trunoin would be nice.
    I am relly suprised at how thin the barrel in the first place.

  3. #3
    No Hope For Me 1biggun's Avatar
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    Other than to say it will do it I would be shying away from the 500Gr rounds. I really see no enjoyment in shooting that heavy of a bullet, do you plan on a muzzel break for it?? I wonder how this thing would be surpressed?? I have been pondering that possability with the 444???

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    Gunslinger jdgabbard's Avatar
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    What kind of 50 was it though? Was it a 50 AE, a 50 Beowulf, or 50 BMG? Because trust me, if he used a 50 bmg then he used a different trunnion. The round is almost larger then the trunnion's hole! 45-70 would be cool. But that is still going to be some ample pressure. Sure the chamber is going to be thick enough?

    Post pics?
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    Gunco Maniac sjohnson's Avatar
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    Would a sleeve threaded or sliver-soldered onto the barrel give you the meat necessary to install the barrel into a standard trunnion/standard barrel pin?

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    Happy Camper hcpookie's Avatar
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    Biggun, I want to have the 500 grain rounds available to me. They are obviously overkill for anything that won't eat you, but I don't want to discount the use of this build for a home defense situation or in same applications that a .50 Beowulf would be considered... I fancy that the 500 grainers could be used in those "stop that vehicle" scenarios. Therefore, I would rather keep the 500 grain rounds on the table.


    After looking at it more closely, I really do think I am OK with a thinner barrel pin. I'll have to insert a sleeve for the pin hole then drill it out closer to the top to ensure the chamber isn't affected. To keep it strong, I'll start with a bolt turned down to a barrel pin size and press it in, then drill it out at the top (not directly in the center). Since this will block the barrel, I'll obviously have cut the radius for the barrel to be inserted. That will give me a standard barrel pin arrangement with a smaller pin, and will avoid having to weld on the trunion. Then its a matter of finding some drill rod or other good steel for the new barrel pin.



    Here's the way the threaded barrel is intended for the Winchester receiver:



    If I think this through, the sleeve required for the trunion journal is going to actually have more material than the combined barrel and frame.

    However it does not answer the question as to whether the barrel, by itself, can handle the additional loads. Typically, the 45-70 loads are limited due to the strength of the rifle.

    So the question I really have is, if I were to use this in the heavier AK action, will the barrel be OK for use with the heaviest loads. I would think the answer is "yes" because "heavy" loads in this case are still in the range of the rest of the parts. I just can't see a MODERN 45-70 barrel being built in such a way that would cause it to open up like a banana if some bubba loaded a hot round for a "hey guys watch this" moment. However sustained fire may be different.

    Honestly, the 350 grain rounds produce more chamber pressure than the 500 grainers. That is according to all sources, not just Lyman. Unfortunately the Hodgdon data doesn't list anything over 400 grains.

    Now here's a question - given the same bolt size, how much difference will there be in bolt thrust for the larger, slower moving rounds? I seem to remember from the Chamber Pressure discussion that it makes a difference.


    Numrich is out of Ruger Mk1 barrels I may phone them to see if and when they will get more in stock.

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    No Hope For Me 1biggun's Avatar
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    HC I think the formula for the bolt thrust for Bolt face ID id in the original 308 AK build thread It was the first or second thread in this calibur conversion section. If I recall there is a considerable increase in bolt thrust. remember You use the ID of the case for that calculation. (Ill see If I can find it maybe it can be a sticky or added to the specifications thread)

    I was thinking that you reduce the ID on only one side of the trunion pin hole that way a steped pin could be used and be of full DIA on at least one side of the barrel an trunion. plug the hole and redrill it off center in the top half on one side only. (I wish I could draw a picture on the computer Of what IM thinking for You.)

    The other thing that concerns me is how thin the breach is. I have concerns that accuracy may really suffer once the barrel quickly heats up and and any any warpage at the thin breach is going to cause signifcant barrel whip and warp at the muzzel. This build would be great on a threaded trunion and a bareel threaded to fit it. (lot of work just thing out lout here.

    I hear what your saying about the 500 grainers. What barrel twist do you have?? will it even stabelize that heavy of a bullet? A 500 grain suppressed AK would be really cool.

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    No Hope For Me 1biggun's Avatar
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    What kind of 50 was it though? Was it a 50 AE, a 50 Beowulf, or 50 BMG? Because trust me, if he used a 50 bmg then he used a different trunnion. The round is almost larger then the trunnion's hole! 45-70 would be cool. But that is still going to be some ample pressure. Sure the chamber is going to be thick enough
    It was a 50-110 shown here (link not working for some reason) Looks like a 45-70 but even bigger. If the barrel pin is going through a 45-70 chamber how did this guy do it? diffferant trunion or perhaps threaded. Dragonov?? I forget.

    http://www.gunco.net/forums/f244/50-...ian-fpk-36134/

    I wonder how difficult it would be to thread a standard trunion????

  9. #9
    No Hope For Me 1biggun's Avatar
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    here is the formula for bolt thrust from the 308AK thread

    --------------------------------------------------------------------------------

    possorry for the long t just trying to get it easy to read, got some of the info from The Gun Zone -- .30 Caliber Ammo trying to do everything on the safe side hence the max proof pressures for the 308 stuff and the bolt head diameter instead of the true case head diameter

    i got some good solid works 3D models of a romy trunnion today trying to run the force application on it as we speak (test how the lugs will hold up under different pressures applied from the bolt thrust)

    not having much luck the software im using doesnt allow for heat treated steel so even the opperating pressures of the 7.62 x 39 arent giving safe results in the solid model stuff

    ill keep trying and let you know what happens


    Bolt thrust= area x psi

    Area = 3.146 x ( (case head diameter / 2)^2)

    Above is from http://www.riflebarrels.com/articles...g_strength.htm


    7.62 x 51 NATO

    "Proof" pressure = 67,500 psi
    Bolt head diameter = .475

    3.146 x ((.475 / 2)^2) = .1772

    67,500 x .1772 = +/- 12,000

    7.62 x 51 NATO = +/- 12,000 lbs bolt thrust max

    308 Winchester

    "Proof" pressure = 83,000 psi
    Bolt head diameter = .475

    3.146 x ((.475 / 2)^2) = .1772

    83,000 x .1772 = +/- 14,800

    308 Winchester = +/- 14,800 lbs bolt thrust max

    7.62 x 39

    Max "reload" pressure = 45,000 psi
    bolt head diameter = .458

    3.146 x ((.458 / 2)^2) = .1647

    45,000 x .1647 = +/- 7,420

    7.62 x 39 = +/- 7,420 lbs bolt thrust

    6.5 Grendel

    Case head diameter = .445

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    Happy Camper hcpookie's Avatar
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    Yeah that .50-110 is really interesting. I forgot I didn't post the scan of the article I had and just put it up there... It does make you wonder if he just but a brand-new barrel from a blank.

    It also makes you wonder how Meeper's .50 Beowulf SAR conversion worked.

    If I had an un-cut trunion I could relocate the hole higher. That may be what they did - TIG the hole closed and re-drill it. TIG probably would not impart that much heat. I'm not going to do that because the notion of using a thinner barrel pin should be sufficient. Plugging the original hole should not be a major issue... I don't see a problem with my plan, other than making sure I get the hole drilled properly!


    I'm honestly not too worried about the barrel whip because of the taper the barrel has. The .45-70 is intrinsically a short-to mid-range round anyway, and past 250 yards you need good sights to keep it effective. That keeps it at most any effective range for whatever I can imagine I'd need to shoot at those ranges. It is pretty fat on the breech end, even though it is notched for the feed tube on the bottom.

    I'm going to use a "fish gill" style compensator - think Barrett .50 - that pulls the gun away from your shoulder to help minimize recoil.


    I may give this barrel to my dad and buy a thick barrel blank from Midway. I hate to spend almost 2x the cost vs. this barrel however. The twist is the "standard" twist for a 45-70 - I think 1:20 - and I think he has a Winchester rifle he wants to rebarrel. I'm going to make a chamber casting tonight.

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