.280 GNR

up. I have gone straight down to 366 from the 405. But if you already have the dies it is better to go down in the order Sean posted. And remember you only have to do this one time. After the first firing you can go right down to the 280.

What if ... you did not have access to all those other form dies? 405 brass is expensive, you don't want to dink it up. The step method is necessary. From the lubed .405 case you can use one close to neck to a lesser step. A .375 die sure would be nice. Maybe a .375 H&H would work, maybe. Once to a lesser diameter "neck" formed, another step down. Maybe .35 cal. like a .358 Win. die etc. (just sizing the neck down). The .405 body is smaller than the .30-06 case diameter. A like process using what you find in lesser steps, say .30 then 7mm.

The "neck" can be a little longer and the shoulder ugly, as long as it chambers. A "blank" charge or a lighter load can be used to "form" the new brass to chamber dimensions. From then on, after formed, you'd only want to neck size with a little bumping of the shoulder to make certain of easy chambering and complete action lock up.

If you had a buddy with the same caliber, to interchange ammo would require Full Length resizing of fired cases in the .280 GNR die. Don't expect very long case life if you do this, fire brass in two different guns. Used in only one chamber it usually lasts many loadings since you are not over-working the brass.

I'm not trying to confuse or complicate. IF you bought a barrel off the net and only had a two die .280 reloading set, all is not lost. With some forethought you can overcome.