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Thread: Shorten Barrel Chamber to Accommadate Shorter Bullet OAL?

  1. #1

    Default Shorten Barrel Chamber to Accommadate Shorter Bullet OAL?

    If I were to let's say, rebarrel a 300 WM or 338 WM action with a 300 RUM barrel, would it be feasable to have the chamber reamed to the 300/338 WM length or just slightly longer? And then just seat the 300 RUM bullets to an OAL that would fit both the magazine and chamber?

    According to Barnes, the 300/338 WM COAL is 3.34, and the 300 RUM is 3.60, a difference of about .26

    I would of course, call this cartridge the 300 RUM Montana

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    Quote Originally Posted by MontanaRifleman View Post
    If I were to let's say, rebarrel a 300 WM or 338 WM action with a 300 RUM barrel, would it be feasable to have the chamber reamed to the 300/338 WM length or just slightly longer? And then just seat the 300 RUM bullets to an OAL that would fit both the magazine and chamber?

    According to Barnes, the 300/338 WM COAL is 3.34, and the 300 RUM is 3.60, a difference of about .26

    I would of course, call this cartridge the 300 RUM Montana
    You're kidding.

    The cartridge overall length isn't the issue here it is the length to the shoulder and the 338 Wm and the 300 WM are different and of course the RUM is different too. Look at the whole cartridge drawing dimensions and you'll see.

    I'm not sure what you want to do here but I think it is to make a 338 length on the 338 RUM case or keeping with 30 caliber ream a 300 Win Mag to accept a shortened 300 RUM cartridge.

    Or to be specific. A 2.5" long Ultra mag cartridge. I suppose we could do this by taking a 300 RUM barrel and setting it back, cutting it off to where it will only accept a 2.5" long Ultra mag case. Yes you could do that. It would likely be screw up in some way unless you do it yourself. (home machine shop) These rarely work because the smith doesn't really know what you want. Best bet would be to buy a new barrel and go from scratch. Have you given any thought to how you would make that brass? Shoving a shoulder back that much would have its own set of problems and die to do that would cost more than a new barrel in 300 Dakota caliber. That's what you would have. The Dakota is a better designed case and the RUM is rebated because Remington was avoiding the obvious 404 Jeffery based label and be cheap with machining cost.

    It is a good idea to use a 2.5" 404 case but you are about number 644 to come up with that idea. Look at the dimensions of the 300 Dakota and compare with your dream caliber. and yes the bolt head can be opened to accept the .545" rim vs the .532" rim.
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    Default ? already been done?

    I suspect he is thinking about a 338 Canadian and before that the 338 Imperial Mag....essentially the same at the 338 Edge. If able to rechamber the barrel there will still be magazine (and prob follower) issues as well as possible bolt face modifications depending on flavor of choice.

  4. #4

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    When I open the thread and the first thing I read from Murphy is *You're Kidding*, well then I know I'm really in for it

    Actually, my wording was poor. My idea was to take a *blank* barrel and ream it a little short so the magazine wouldn't have to be modified and just seat the bullet deeper into the case. But for some reason I didn't even consider the different shoulders. So I guess the next question is can the throat area be shortened to get the lands closer to the bullet if you seeated it to the lenght of a 300 WM.

    Reason I'm asking is I know a guy that had a blank barrel reamed to a 300 RUM and he had it put on a 300 WM action with no mods done to the action and still uses the magazine, seating the bullets to the 300 WM OAL, (maybe just slightly longer) and his bullets have a somewhat extended jump to the lands. They seem to work well for him, but I'm just wondering if there is a way to get a better fitting chamber or throat?

    Also considering this same question for the 338 RUM, Still trying to decide if my dream rifle will be the 300 RUM or 338 RUM, and by the time I do finally decide it will probably be something else

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    Quote Originally Posted by MontanaRifleman View Post
    When I open the thread and the first thing I read from Murphy is *You're Kidding*, well then I know I'm really in for it

    Actually, my wording was poor. My idea was to take a *blank* barrel and ream it a little short so the magazine wouldn't have to be modified and just seat the bullet deeper into the case. But for some reason I didn't even consider the different shoulders. So I guess the next question is can the throat area be shortened to get the lands closer to the bullet if you seeated it to the lenght of a 300 WM.

    Reason I'm asking is I know a guy that had a blank barrel reamed to a 300 RUM and he had it put on a 300 WM action with no mods done to the action and still uses the magazine, seating the bullets to the 300 WM OAL, (maybe just slightly longer) and his bullets have a somewhat extended jump to the lands. They seem to work well for him, but I'm just wondering if there is a way to get a better fitting chamber or throat?

    Also considering this same question for the 338 RUM, Still trying to decide if my dream rifle will be the 300 RUM or 338 RUM, and by the time I do finally decide it will probably be something else
    Oh, I see, you want a short throated 300 RUM, and want to know if the throat can be short enough to allow the RUM to fit a Win mag magazine.

    First; The throat dimension of the 300 RUM is a part of the SAAMI Specs.
    Second; What will keep anyone from trying to fire a factory spec'd round in this chamber.
    Third; Gunsmiths must stamp the barrel according to the reamer and reamer makers will only make a 300 RUM reamer to SAAMI specs, plus or minus a few tousands of throat length.
    Fourth; To makes such an extensive change they merchants would want a name change for you one of a kind reamer/chamber/barrel.
    Fifth; You cannot uncut the depth of a chamber throat, just don't cut it so deep to begin with.

    You can have reamers made with out the throat cutter (?) part and then use just a throat reamer to cut the throat to the desired depth. (normally used to increase the length of the throat) Or you can have a reamer made with a different throat length than is spec'd for the cartridge but here again it must be called something other than the original name because it ain't anymore. (Such as the 300 Montana Rummy)

    This would require very deeply seated bullet in the Ultra case and that generally isn't a good thing especially with that powder charge. You could seat the bullets normally as intended for the 300 RUM in your new single shot Remington M700.
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  6. #6

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    Quote Originally Posted by Murphy View Post
    First; The throat dimension of the 300 RUM is a part of the SAAMI Specs.
    Yup, understood.

    Second; What will keep anyone from trying to fire a factory spec'd round in this chamber.
    If they are using my rifle without my knowledge..... Also, they should know something is wrong when factory loads don't fit in the magazine and the bolt doesn't close.

    Third; Gunsmiths must stamp the barrel according to the reamer and reamer makers will only make a 300 RUM reamer to SAAMI specs, plus or minus a few tousands of throat length.
    Stamp it as the the 300 Montana Ultra Mag or whatever (MUM)

    Fourth; To makes such an extensive change they merchants would want a name change for you one of a kind reamer/chamber/barrel.
    See above.

    Fifth; You cannot uncut the depth of a chamber throat, just don't cut it so deep to begin with.
    Cool.

    This would require very deeply seated bullet in the Ultra case and that generally isn't a good thing especially with that powder charge.
    Why is this? Compressed charge? I would probably use about 95 gr of Retumbo. Higher pressures from deeper seating?

    You could seat the bullets normally as intended for the 300 RUM in your new single shot Remington M700.
    If I use the M700 I'll just use their RUM action and keep everything standard, and the reason I'm asking this is because I don't like Remington safetys... just a personal pet peeve.

    Thanks for the input Murphy

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    Seating bullets in the short necked RUM so that the bullets hang into the case about a half inch or more change the dynamics of pressure pushing up on the bullet. If that is the terminolgy. We aren't pushing the bullet we are just building pressure for the first few microseconds. Accuracy is the first to go and it does go. It will also increase the erosion of the throat which is bad enough as it is with this burner. I don't follow the logic with such a conversion anyway. I know we wanted more powder/velocity but the trade offs were extreme. Also, with such a short throat (to allow 300 WM COAL to touch the lands of this short throated RUM chamber) any factory ammo chambered in it will likely not allow the bolt to close, and if it does it would jam the bullet into the lands and up pressure way too much. (Try it sometime) This is how folks destroy rifles, not how we improve ballistics.

    If we look at those two cases 300 WM vs 300 RUM, there is a lot of difference in length and girth. To think that we could rechamber or rebarrel a rifle from the 300 WM to the 300 RUM is not sound. Everytime I've tried to do something cheaply it has come back to smack me in the face in one way or another. Every cut rate gunsmith or every cheaper rifle barrel was more costly in the end. Do it right the first time and do it with the right equipment. Sorry, I don't mean to be so abrasive but sometimes the direct approach is better. Basically what I'm saying here is if you are contemplating a similar project, re-think it. Just about all of us know somebody who has such a project and "it works". Well just because the bullet leaves the barrel and the rifle is still inone piece, doesn't mean it works. I have been given (loaned) dozens of project rifles to make ammo for some odd ball chambering. I've done a lot of them my self, but they need to be made on the right platform and they need to be made with some practicality in mind and with safety at the fore front.

    These projects should be started with some goal in mind, Some ballistic level or some accuracy level in mind. They also must be practical. A 16" barreled 300 RUM is always a bad idea just as is a short action RUM caliber I once had dropped in my a lap a 40 XB with a magazine and a 338 Lapua barrel on it. The use or non use of currently available ammo must also be considered if not for your use, maybe for the guy who gets it down the road. I have a rifle project now that is the 300 WBY case necked to 6.5. There have been at least three versions of this caliber and it isn't like any of them. It is a huge pain to make brass for it and it will not meet my 6.5 AI ballistics because of the throat. This rifle was inherited from "dad". If we've already done something like this we will get all we can out of it but try not to make the mistake again or learn from others mistakes and some of these, like some of mine, are mistakes.
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  8. #8

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    Montana Rifleman, what you are proposing is not a good idea.

  9. #9

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    Murphy, be as direct as you like. I sure don't want to do a bad idea. I'm sure we've all been down roads or up creeks we have regreted.

    But here's the thing, I know of a guy, and have corresponded with him, who had barrel chambered to 300 RUM and put it on 300 WM action and it works great. He had a well known gunsmith here in Montana put it together for him and this guy has an outstanding rep. He seats his bullets deep so they fit in the magazine. He has shot just about everything from 200 gr to 290gr (wildcat) bullets throught it, with many sub .5 MOA groups.

    The only thing different I am considering is shortening the throat for a more snug fit. Your point about throat erosion is something to be considered,

    My logic is that I don't particularly like Rem actions. Some guys do, I don't. Maybe someday I'll get over that, but not likely. And Rem is the only factory action available in the RUM that I know of.

    My goal is to build a good long range cabable hunting rifle and a 300 or 338 RUM fit the bill.

    As for safety, I don't see it being an issue. It will be my rifle and I wont be shooting factory ammo, (wouldn't be able to even load it and close the bolt) and I won't call it a RUM. If anyone else shoots it, they will be shooting my handloaded ammo and if I sell it (not likley), the buyer will know exactley what it is and what it is not.

    Anyway, I appriciate the input and that's why I asked the question. When you speak about guns, I listen. You have a ton more experience than I do.

  10. #10

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    Quote Originally Posted by jwp500 View Post
    Montana Rifleman, what you are proposing is not a good idea.
    jwp500, thanks for your opinion, if you had any data to back it up, I would weigh it more.

  11. #11
    Member 8x57 Mauser's Avatar
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    Quote Originally Posted by MontanaRifleman View Post
    ... My logic is that I don't particularly like Rem actions. Some guys do, I don't. Maybe someday I'll get over that, but not likely. And Rem is the only factory action available in the RUM that I know of.
    Dumb question: The keys to whether you can use a particular action are the length of the bolt movement and the size of the bolt face, right? So couldn't you rebarrel a 375 H&H action from whatever manufacturer you like? I can't imagine opening the bolt face from .532" to .534" would be a huge job. Or am I missing an important element?

    Quote Originally Posted by MontanaRifleman View Post
    As for safety, I don't see it being an issue. It will be my rifle and I wont be shooting factory ammo, (wouldn't be able to even load it and close the bolt) and I won't call it a RUM. If anyone else shoots it, they will be shooting my handloaded ammo and if I sell it (not likley), the buyer will know exactley what it is and what it is not.
    I never discount terminal stupidity as a safety factor. It's amazing what you can find people forcing into their rifles at the range sometimes. While factory ammo won't fit in the magazine of what you're envisioning, I'd think a friend could chamber it with a good hard shove, jamming the bullet well into the lands and back into the case.

    If you want a nifty wildcat with your own cool name on it, I'm behind you all the way. That has an indisputable cool factor.

    But from reading Murphy's thoughts, and mulling them over, I think this particular idea - the short-throated 300 RUM - would end up with lower velocities than the parent cartridge, and at potentially higher pressures. You may or may not get your acquaintance's accuracy, and you're likely to wear out the barrel faster. Your kids/heirs would have a hard time selling it, too.

    For my nickel (and I'm not one of those really experienced people you mentioned), I'd rebarrel a longer action of the sort you want for the caliber you like.

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    Much, much much more important than throat length, is the throat dimensions. Factory chambers and saami spec chambers have slop in them to allow for wear on tooling and dirt/gunk in the action. While this is all good and fine for manufacturing, it makes for less than ideal accuracy.

    If the throat is cut 0.001" over bullet dia, you'll get steller accuracy, even if the bullet has a fair jump before hitting the lands. Also a neck that is 0.002" over the neck on a loaded round helps everything align, and reduces brass working when firing. Custom barreled rifles are a great investment, because a good smith cuts a chamber much better than the factories.

    I wouldn't have a gun built with a chamber that wouldn't fit factory ammo.

    Asside from all that, if you're starting with a 300 win mag action, just stick with the 300 win mag chambering but have it custom made. You ultra mag doesn't gain much over the win mag unless you are using the heavy vld bullets. You'll also need to have the magazine and feed rails modified as the ultra mag is enough different in dimensions that you will compromise the feeding w/o having that work done.

  13. #13

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    Quote Originally Posted by 8x57 Mauser View Post
    Dumb question: The keys to whether you can use a particular action are the length of the bolt movement and the size of the bolt face, right? So couldn't you rebarrel a 375 H&H action from whatever manufacturer you like? I can't imagine opening the bolt face from .532" to .534" would be a huge job. Or am I missing an important element?
    The RUM cartridges are the longest that I know of, but the 375 is longer than the 300 WM or 338 WM and as far as opening the bolt face, it's already been done on other conversions includu=ing the one I mentioned.

    I never discount terminal stupidity as a safety factor. It's amazing what you can find people forcing into their rifles at the range sometimes. While factory ammo won't fit in the magazine of what you're envisioning, I'd think a friend could chamber it with a good hard shove, jamming the bullet well into the lands and back into the case.
    That's always possible, but at the same time there are numerous wildcats that this could happen in also. Any one who used my rifle would get a thorough briefing.


    If you want a nifty wildcat with your own cool name on it, I'm behind you all the way. That has an indisputable cool factor.
    The name really doesn't matter to me, I was just being humorous. But I would have to come up with something and it would probably be something like 300 Montana Mag (I bet that name has never been used....right)

    But from reading Murphy's thoughts, and mulling them over, I think this particular idea - the short-throated 300 RUM - would end up with lower velocities than the parent cartridge, and at potentially higher pressures. You may or may not get your acquaintance's accuracy, and you're likely to wear out the barrel faster. Your kids/heirs would have a hard time selling it, too.
    Good points here. Not sure why I would get lower velociteis or higher pressures if I kept the bullets off the lands, but then I know very little about the subject. And the throat life is definitely a big question. I'll also be getting the opinions of some gunsmiths here in Montana as well as Dan Lilja's opinion. I would be using one of his barrels.

    As far as resale, I figure any custom rifle will be hard to resell. My dream rifle is not likely to be someone elses dream rifle. So I'm going into this for a keeper gun.

    For my nickel (and I'm not one of those really experienced people you mentioned), I'd rebarrel a longer action of the sort you want for the caliber you like.
    As mentioned, I believe the RUM's are the longest?

    Thanks for the input

  14. #14

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    Quote Originally Posted by Paul H View Post
    Much, much much more important than throat length, is the throat dimensions. Factory chambers and saami spec chambers have slop in them to allow for wear on tooling and dirt/gunk in the action. While this is all good and fine for manufacturing, it makes for less than ideal accuracy.

    If the throat is cut 0.001" over bullet dia, you'll get steller accuracy, even if the bullet has a fair jump before hitting the lands. Also a neck that is 0.002" over the neck on a loaded round helps everything align, and reduces brass working when firing. Custom barreled rifles are a great investment, because a good smith cuts a chamber much better than the factories.
    I agree and if I do this, I'll be getting a real good smith.

    I wouldn't have a gun built with a chamber that wouldn't fit factory ammo.
    Yeah, I hear ya, too bad Rem is the only production RUM makers. But there are a lot of wild cats that don't fit factory ammo. Oh well...

    Asside from all that, if you're starting with a 300 win mag action, just stick with the 300 win mag chambering but have it custom made. You ultra mag doesn't gain much over the win mag unless you are using the heavy vld bullets.
    I might build on a 338 action? The RUM has about a 200 fps advantage over the WM, which isn't a whole lot, but significant. Another reason to consider the 338, but Not really sure i want a 338, it's really more than I need. The 300 gets me where I want to be.

    You'll also need to have the magazine and feed rails modified as the ultra mag is enough different in dimensions that you will compromise the feeding w/o having that work done.
    Agree.

    Thanks

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