Fast pistol powder and cream of wheat or cornmeal.....I think you need 300 win mag length brass not 7mm or 338 wm

 

If you are mechanically expanding you'll need to anneal as part of the expansion for best results less case loss, depending with the case you start with at least 2 times maybe 3 times between sizing up, Mandrels straight and or tapering work well and are good to have on hand, If you use the COW method annealing is still the first step and COW is incredibly messy, At the end of this procedure wall thickness will determine how many times you can reuse the case, If the neck fails splits your gun wont blow up but you may have some extra cleaning to do, All up to you to get your measurements needed before hand, I would start with the closest diameter neck I could

 

If I could get some 375 H&H.or 416 Rem mag at a good price I would use it. They are the closest diameter. SEM you mentioned mandrels are they tools for a press? I have been using dies with the expander extended . First I used a 300 mag die, then a 35 Whelen. My problem is I have nothing to expand from 35 to 375 to 40 to 416 etc.

 

Lyman makes M dies they are expander mandrels I use the long and short bodies 22 thru 45 cal they are really for truing and slightly flaring for easier seating but they work great for doing what you are in need of

 

I built an expander to go from 30-06 to straight wall .430 with non annealed used brass I loose a few.....but not bad, after fire forming the shoulder wrinkles are mostly gone.....you just can't buy(reasonably) any of the good stuff you used to be able to get like -06 basic brass or magnum basic brass.....so, ya gotta get crafty......I could build you an expander.....I need to build one for myself as I too need some 458 win mag brass......I have a couple new bags of winchester 300 win mag to use...the method I use takes about 5 to 6 handle cycles to get fully expanded but I do it all at once, if that makes sense .... shipping probably cost you more than I would charge to build it. Pm me if your interested, I can get you some pictures.

 

Thanks for info and offer on making an expander Crashdoyle. I just found multiple universal die expanders from 22-45 cal for around 20.00 on ebay . Cheap solution. I know exactly what you are taking about. I used to be able to buy the best powder for each caliber pre covid. Now it's 2nd, 3rd,4th,5th,or last choice on the shelf for powder and other components. Another reason for casting your own bullets.

 

If you find it isn't quite what you need, holler, shout, scream, curse a little, and when you calm down, pm me.

 

Peterson Brass makes a "belted mag" brass that is 2.89" long so can be easily transformed. Grafs has them in 250 lot but at $502 which Holy Cow! But you may find them in smaller 50 piece lot somewhere. Much less effort to get to final form. Peterson also has another option that might work for you. They have a "shooter evaluation" process that might work if they have this brass available.

Grafs:

The Reloading Authority - Graf & Sons

Graf & Sons, the reloading authority, has a wide selection of reloading supplies, shooting supplies, ammo and more.

www.grafs.com

Grafs 50 count out of stock can be notified when back in stock.

The Reloading Authority - Graf & Sons

Graf & Sons, the reloading authority, has a wide selection of reloading supplies, shooting supplies, ammo and more.

www.grafs.com

Peterson Brass shooter evaluation page:

Evaluation Shooters | Peterson Cartridge

Help us help you by providing direct feedback to our team about brass rifle casings from Peterson Cartridge.

www.petersoncartridge.com

Peterson Cartridge | Match-Grade Brass Rifle Casings

American-made brass rifle casings designed by, and for, long-distance shooters and reloaders.

www.petersoncartridge.com