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Thread: 7mm rem to 338 win mag resize

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    Default 7mm rem to 338 win mag resize

    have a load of 7mm rem mag brass that i wont be using soon. would it be possible to resize the neck up to .338 for my 338 win mag? i am new to reloading an not to worried about doing this any time soon but was just wondering about the possibility.

    thanks in advance

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    Member Diesel Nut's Avatar
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    They'll work, but it's a lot of effort to go from .284 up to .338. On the other hand, I know a friend (AKRoadkill) who resized .264 Mag brass to .338 with the simple pull of a trigger so it can be done.

    If you're serious about it, it would also be beneficial for you to learn about annealing. The necks may become a bit brittle and work-hardened.
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    Sorry to revive an old thread....but I ran across a dozen or so pieces of once fired 7mm rem mag brass and since I don't have a 7mm, I figured I'd turn them into 338WM. I ran the well lubed necks into my 308 expander button first, then full length size them in my 338 WM die. It didn't seem to take too much force on the ram to expand the necks and dimensionally they are pretty darn close to 338 specs. The distance to the top of the shoulder is maybe 0.020" short, but the dist to the base of the shoulder is right on.

    The overall length to the case mouth is about 0.010" short of the trim to length dim of 2.49". I can't imagine that would be a problem...

    Anyone see any reason why I would have any issues using this brass for my 338? It chambers in my m77 fine, just starts to tighten up as I cam over the bolt.

    thanks

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    Both cartridges headspace on the belt, so I don't think it would be a problem if you fireformed them, and THEN checked the case length, and trim if necessary.

    If the necks end up short that COULD be an issue as the 338 doesn't have much neck either.

    Interesting project. Did the necks get too thin?

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    Member dkwarthog's Avatar
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    I dont think the necks are short, but I will check. Good call on checking the neck thickness, I'll do that tonight just to make sure they are ok. Otherwise, it was a piece of cake to resize them. I dont expect to make a habit out of it, and I doubt that I would form them the other direction (from 338 down to 7MM). I wouldnt want to take a chance that someone might look at the headstamp and inadvertantly put a 7mm case into a 338 chamber and pull the trigger...

    Thanks SOTN.

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    Quote Originally Posted by dkwarthog View Post
    I dont think the necks are short, but I will check. Good call on checking the neck thickness, I'll do that tonight just to make sure they are ok. Otherwise, it was a piece of cake to resize them. I dont expect to make a habit out of it, and I doubt that I would form them the other direction (from 338 down to 7MM). I wouldnt want to take a chance that someone might look at the headstamp and inadvertantly put a 7mm case into a 338 chamber and pull the trigger...

    Thanks SOTN.
    You could seat a bullet and measure the neck diameter, then subtract the bullet diameter.

    I don't know how thin is too thin. If it's thick enough for good neck tension, I'd call it good, I think.

    Belted cases have fallen out of favor with some folks, of late, I think, because of a lot of negative nonsense written about them.

    One thing I like about them is that if you DID chamber and fire a 7mm Rem Mag (or other smaller than 338 cal. belted cartridge) rnd. in a 338 , it would be relatively safe, BECAUSE of the headspacing on the belt. (Not 100% safe, because of other possibilities.)

    SOTN
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    Thanks for the information. I'll skip the fire forming, but it is nice to know about using the midstep .308 IF I needed to use 7RM brass for .338 loads. In these (and future?) times of few components such information might be needed.

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    Necks are about 14 thousandths and vary by about .001" I apparently didn't have the lube even on the necks or something because a couple of them pulled unevenly so that the case mouths will need to be trimmed square and the neck thickness varied more on those two. I did ruin the first one when I ran it through the 308 expander. Didn't have the button extended far enough out of the die and gave it a double shoulder in 308...

    yea I guess firing a 7mm in a 338 may not be a disaster, but definantly not a good plan

    I'll load them up with a moderate load and report if anything interesting happens...

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    Quote Originally Posted by LeonardC View Post
    Thanks for the information. I'll skip the fire forming, but it is nice to know about using the midstep .308 IF I needed to use 7RM brass for .338 loads. In these (and future?) times of few components such information might be needed.
    No expert here, but I would definitely fire form. (Before you use them) They're gonna form anyway the first time you fire them.

    I fireform when I make 7mm Wbys outa 7mm Rem.

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    Just a follow up, I fired this brass with a middle of the road load with 200 gr Speer hot cores and A4350. 2750 fps, and a group around 1.5". No signs of trouble with the brass, but I'll likely just use them for starting loads at the range just the same...

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    Actually fire forming them is a very good way to turn 7mm Mag cases into 338Win Mag cases. Its a little more expensive than die forming considering the cost of bullets and powder now days but it works very well. If you could load the 7mm cases with about 15grs of red dot and a cast bullet it would fire form very well and not waste such expensive components. The necks will be a little thin but shouldn't be any problem.

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    Quote Originally Posted by rbuck351 View Post
    Actually fire forming them is a very good way to turn 7mm Mag cases into 338Win Mag cases. Its a little more expensive than die forming considering the cost of bullets and powder now days but it works very well. If you could load the 7mm cases with about 15grs of red dot and a cast bullet it would fire form very well and not waste such expensive components. The necks will be a little thin but shouldn't be any problem.
    Do you mean, expand the necks to 338, and load a 338 cast bullet?

    Couldn't you FF without expanding the neck, and without a bullet? It seems I've heard of that.

    Thanks,
    SOTN
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    There is a couple of ways of fire forming. Some would use a 7mm cast bullet and about 15grs of red dot or some similar fast powder. Others will use about the same powder charge and just fill the case with cream of wheat, uncooked, point it up and pull the trigger. It doesn't take a lot of pressure to form cases and you just need the pressure to get high enough to blow the case out to fill the chamber which happens before the bullet even moves. Same with the cream of wheat. You can go to the trouble to expand the neck out and use a 338 bullet, but fire forming before expanding with a die will usually be more even.

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