Forming .300 Win Mag brass from 7mm Mag

[John Wayne]

I have a .300 Win Mag rifle and a lot of 7mm brass that I've picked up from the range...some of it consists of complete boxes of empty cases people stuck back in the original box and threw in the trash! Don't have a lot of .300 Win Mag brass, and even though it is easy to find, it's not cheap.

Can I just run the 7mm Mag cases through the .300 WM full-length sizing die to neck them up, then trim & load as usual? I realize case life may be shortened, but I have a lot of 7mm brass sitting around and would like to put it to good use. I do not own a 7mm rifle, so there would be no danger of the ammo with the wrong headstamp being used in a different gun (not that it would chamber anyway).

It seems like it should work fine to me; IIRC the .375 H&H is the parent case for both cartridges. Just checking here first before I ruin a bunch of cases or stick one in the die.

 

[rcmodel]

I have never tried it, but it should work, maybe.

The thing is though.
300 Win-mag case:
Max length = 2.620".
Trim too: = 2.610".

7mm Rem-mag case:
Max length = 2.500".
Trim too: = 2.490".

Whether you can gain back the missing .120" length when the necks are expanded I don't know.
Only way to find out is try it.

rc

 

[Funshooter45]

Or you could sell that 7 mm mag brass to me and then use the cash to buy brand new 300 Win brass.

 

[NCsmitty]

It'll probably be ok, but if you shoot a lot of the short cases, you may get some hard deposits built up at the end of the chamber, and bother if you use full length cases.



NCsmitty

 

[rcmodel]

Hard deposits, or worse, chamber erosion, which would be the end for that barrel.

rc

 

[John Wayne]

Ran a case through, it came out 2.496''.

I wonder if running a couple loads through it would stretch it out enough to use as a .300 WM case, but due to the issues mentioned (deposits, erosion), I don't think I want to risk it.

 

[FROGO207]

It would stretch some but would it move enough before the neck split? I would not chance it myself. Might be able to swap it for what you want in the swap/sell forum. Or sell it to Funshooter45 and buy some other.

 

[NeuseRvrRat]

i thought the "sell the 7mm brass and use the money for 300 win mag brass" suggestion was a pretty good idea.

 

[918v]

You'd wear out a 7mm case trying to stretch it to .300 length.

 

[Huckelberry75]

better to just sell and not have to worry. then you know that you are GTG.

 

[Sunray]

"...sell the 7mm brass and use the money for..." Yep. You're not going to stretch 'em 120 thou.
Or see it as a reason to buy another rifle.

 

[Steve Marshall]

No you can't. Not for the OAL reasons, but because the shoulder is way too short on the 7MM.

 

[NCsmitty]

Steve Marshall, the deal here is that the cases actually will headspace on the belt, and necked up to 308 caliber and loaded with a low powered load, will fireform the shoulder to fit the 300 WinMag chamber. The difference will be approximately .120" in case neck length.
It can be done but I don't consider it a wise decision. 300 WinMag brass isn't that hard or expensive to come by.

It's just an exercise that shows it can be done if it was necessary.


NCsmitty

 

[Steve Marshall]

That is semantics and you know it. The shoulder has to be moved forward by 5/32nds. This is not a practical thing. The OAL will be considerably shorter than .120 also. This in a case where the neck length is adjudged to be to short as is. Yeah, that's a good idea. A neck length of about 1/8". Good for bullet support.
In addition, belted cases already have a bad rep for case separation.
To the OP- Don't try this at home. Can it be done , once or twice successfully? Perhaps. Do you have a tight action that will vent gas efficiently?
Do you KNOW how to resize belted cases properly?
How many cases do you need? This is a BAD idea.

 

[fguffey]

When a case is necked up---------! it shortens, when a case is fired to form and is pinned to the rear the case shortens, firing it does not make it longer and as has been implied the 300 Win Mag neck is too short when 300 Win Mag cases are used.

F. Guffey

 

[gamestalker]

I have a bunch of once fired, primed, resized, and trimmed 300 win. and would be happy to trade for your 7 mag. brass. It was given to me by a guy who didn't need it any more, and since I never say no to good brass, it's available for sale or trade.

 

[ojibweindian]

Wouldn't stretching a 7mm Rem Mag case that much increase the probability of case head separation?

 

[fguffey]

Ojibweindian,, when the neck is necked up it shortens, when the case is fired to form the shoulder, body and neck the case shortens, when measuring the length of the two cases before starting it should be obvious to everyone the neck on the 300 Win Mag is too short before starting, after all the effort the neck on the necked up 7mm Remington Mag will be less than .144, back to keeping up with two thoughts at one time, neck tension (bullet hold) depends on the necks ability to hold the bullet, bullet hold depends on the length of the neck and neck tension, if the neck is shortened, bullet hold is reduced.

Back to stretch, the case shortened, the case is/was tied to the rear by the belt as is the case when held to the rear when the case has a rim, after forming it is important to apply the 'leaver policy', once the case is formed 'leaver formed', avoid full length sizing, get a companion tool to the press, a feeler gage. Back in the 'good ol' days'? what happened to the case body ahead of the rim/belt was of little consequence, think of 30-30, 30/40 Krag, 303 British and 300 H&H.

F. Guffey

 

[John Wayne]

To those who asked about trading brass--I appreciate the offers but I think I have found someone locally who wants to swap with me.