I bought a 6x45 (6mm on a .223 case) and needed dies for it. I placed a WTB ad here on AR's classified and a good guy from Vermont supplied me with a nice set of Bonanza 6x47 dies (will work just fine for the 6x45, too).

Bonanza was apparently absorbed by Forster Products, which still makes the same or similar dies. I was surprised to see a factory note, original to the dies, in the die case. The note said that with the Bonanza seater (which holds the case firmly in a floating chamber), that you could use a boattail bullet to neck up your .224" cases to .243 by simply seating it in a case, then pull the bullet. I reasoned if you could do that, why couldn't you simply prepare a load in the .223 case then seat the 6mm bullet on top of it and be ready to shoot?

I had a supply of commercial .223 brass already FL sized and primed, so I chamfered the inside mouth, dropped the appropriate amount of an appropriate powder, and proceeded to seat a 58 grain Hornady V-Max in the case. There was little resistance (not as much as full length sizing) and the resulting cartridges are as pretty as you please.

I won't get a chance to test these loads for a couple of weeks, but I see absolutely nothing wrong with them.

Sounds good to me. Might pay not to try for a crimp, though.

I negligently screwed the die rather than the bullet-seater down once. Did it just expand the shoulder? No, it rolled it in so the neck was inside and the case was about 3mm shorter than before

I agree, no crimp on these (although I crimp nothing that I load other than perhaps some .30-30's for a Model 94, and handgun rounds, of course). Going from .224 to .243 they should have plenty of neck tension, anyway.

The 8mm/06 used to be a popular round when there were lots of surplus military Mausers around. I wonder if going from .308 to .323 would be too much of a stretch (using the proper type seating die, of course)? Or all the way to .338 for the somewhat popular .338/06?

That should work, too, Stonecreek if percentages have anything to do with it: .224" to .243" is a more than 9% diameter expansion and .308 to .338 is still under 10%.

Another way to approach this neck expansion might be to push a .338 bullet in backwards some distance and then pull it. I have done that to make dummy rounds for my son's 35 Whelen from .30-06 cases. Not as quick as putting a boat-tail on top of powder - but if you want to finish up using a plain-based bullet ...

Stonecreek you can go to 323 from .308 with a set of dies..Mine are 8x57 and 8x60 RCBS with a tapered expander balls...SAme with my 6x45 with a benchrest 0 tolerance neck that must be outside neck turned...