I have a lot of .375 H&H brass and need more .458 Lott. Lube, put it in the 458 Lott full length die and trim?
Any tips or tricks or is it a bad idea?

imperial sizing wax after annealing necks .. don't bother to "fine" trim, as they'll stretch a bit --

if grossly oversized, of course trim ..

don't use max loads on first firing!!!

I've necked up .300 Weatherby brass to .375 after annealing the necks and using Imperial sizing die wax inside the necks. I fireformed to .375 H&H Ackley Improved. Didn't need to trim as Wby brass is a little short. If you use H&H brass do your final trimming after fireforming.

You should have no problem !

Good Luck
Hip

Offer a swap in the classified section. I got 100 Lott for 60 460 bee. I think both of us were happy, I got the Weatherby brass dirt cheap.

OK. Thanks. Need some imperial wax first. I normally use Lee lube. Never thought about annealing. Thanks again.

Instead of resizng, you can simply fire 375 H&H ammo (quite safely) and get fully formed cases which you can trim to length.
The cases will shorten by around .040” from their length as 375H&H.
Hope this helps.

Cheers.

Why waste the ammo? If you are going to do that just use some quick Pistol powder, corn meal and a dollop of wax.

Hip

Trim and resize in that order and no need to anneal, fireform if you must..and of course at some point you may need to anneal or buy new brass...

Another suggestion is sell the .375 brass and buy new marked 458 Lott brass..Always the better option IMO..

If the 375 H&H brass has been fired, you should definitely anneal it before necking it up. Brass shortens when you neck it up, so trim after you neck it up.

The reason for asking is that Lott brass is on backorder here. I have H4895, woodleigh 500grs softs and solids, F215M and even resizing wax now. Just want to play around. I will try to anneal with a torch and resize the 375 this weekeend. just wanted some info before ruining my die. Never formed some brass to another calibre before. Thanks guys