View Full Version : 338 WIn Mag brass from 300 Win Mag brass????

08-27-2014, 09:06 PM
If I want to try to make some 338WM brass out of 300WM brass, whats the best method?

Thanks for Any/All input...


08-27-2014, 09:48 PM
Short of the swapping section I would trim them down some and run them through your sizing die. I am not sure how much you will need to trim them so you might need to do some guess and check. I have done this with 300 RUM to 338 RUM. Works fine.

Jerry Jr.

08-27-2014, 10:14 PM
It can certainly be done..... with a substantial amount of effort.

I'd be inclined to see if a trade was available, your .300 Winchesters for someone else's 7mm Remington Magnums. That would be a far easier conversion, needing only moderate necking-up and then full-length sizing. I've actually been thinking about digging up some 7mm Mag brass and doing this for my own .338, since I'm a bit low on that brass right now.

Years ago, I "made" 7mm Remingtons from .264 Winchesters simply by putting 20 grains of Bullseye in the .264 cases... NO WADS, NO FILLER, NOTHING BUT LOOSE BULLSEYE!

Pointing the rifle straight up and touching it off made a HORRIFIC blast..... and neatly-expanded case necks. Instant 7mm Magnum cases! These days, Bullseye is too precious for such methods. I'd use the 'mechanical' method to expand case necks now.

08-27-2014, 11:05 PM
Before I`d try necking brass up, I`d put out a shout to see if anyone has any .338 mag. brass for sale or trade.Robert

08-28-2014, 05:07 AM
The best method is to trade for 338 or 7mm mag.

300 win mag is longer, and requires considerable trimming, as in an 1/8".

This quickly gets to be a pain in a manual trimmer.

08-28-2014, 06:16 AM
.338 brass is not easy to find, and those who have it tend to keep it.

The numbers of 7mm Magnum cases, compared to those of the .338, are probably at least ten times greater. The .338 is a wonderful cartridge, but its following among riflemen pales beside that of the 7mm Remington Magnum.

Necking up is a very simple operation and should not give anyone qualms. The results are fine, and it's a relatively easy way to get cases for a rifle which may otherwise be on short rations.

ANY usable brass is valuable, when the supply line can't furnish what we need. This is a time when we may have to fall back to using the skills we have available to keep our rifles shooting. To just state, "Find some .338 brass!" is very easy to say, but not so easy to accomplish.

08-28-2014, 10:36 AM
Okay guys, I'm ready to be schooled!
If you have 7mm Rem mag brass with a hole in the neck of @ .284, and you want that hole to be a little less than .338 (need some neck tension), do you just grab your .338 die with the standard expander plug, slap the lubed case in the press, and shove 'er up into the die? Or do you have to size up in stages - say first to .308 (use the plug from any of the .30 cal dies in the .338 sizer die) and then to .338? And should the brass be annealed first?
I'm actually looking to make some .358 NM brass from .300 WM, so I'm truly interested in what those with experience have to say!


08-28-2014, 01:46 PM
I'm actually looking to make some .358 NM brass from .300 WM, so I'm truly interested in what those with experience have to say! (QUOTE)


With the understanding that I have NOT performed this exact conversion, here's how I would approach it.

First, trim the .300 brass to just OVER the final case length you expect to use (2.5"-plus-a-tad.) This reduces the length of neck you have to deal with.

Next, flare the mouths of the trimmed cases large enough to accept the .35-caliber expander ball.

Lube the inside of the case necks, as well as the outside of the brass, and size the cases in the .358 full-length resizing die.

Measure the case lengths of the sized brass and do a final trim-to-length as required. This is where the final length is set, and this is why I suggested the initial trimming be to a slightly OVER-length dimension.

De-burr the new mouths, and load up your new brass!

You may want to consider annealing the necks and shoulders, but I've never found it necessary when the length reduction of the parent brass is so small.

That really should be about all it takes. I doubt that the brass at case mouths will be thick enough to need reaming.

08-28-2014, 01:54 PM
Thanks for the quick reply, Bruce!
If I get home early enough tonight, I'll give it a try and report back!


09-01-2014, 02:54 AM
Okay, so it took me a couple of days, but I proceeded as Bruce recommends above. The result was a passable example of the .358 Norma Mag brass, with a .300 Win Mag headstamp. It did take quite a bit of force to get the .35 cal expander plug through the .30 cal neck, but once that was accomplished, every thing else went smoothly.
To be truthful, the next time, I will probably start out with .338 brass to alleviate the trimming and expanding issues.

Thanks Bruce!

Have you tried your conversion yet??

09-01-2014, 12:05 PM
Check Monmouthbrass.com for fired .338 Win. mag. brass.Robert

09-01-2014, 03:18 PM
Personally, I found making 338 Win Mag from 300 Win Mag to be pretty easy. I ran the brass into a .338 Win Mag FL die. Trimmed them. Annealed them. Tested them in my rifle...set the shoulders back a hair. Now they fit fine