Can 308 cases be necked up to 358 simply using the 358 sizing die with standard decapper/neck expander? Should case necks be annealed first?

Some will say the standard expander can be used. Maybe...try it slowly with lube. An intermediate size (8mm or 338) may make it easier.

A file, a drill motor, & some fine wet/dry paper oiled may be useful for reducing the blunt end of a non tapered expander. Most, if not all, quality die makers have tapered expander balls available for a few $$. A newer Redding set has a tapered expander from the factory.

If using new brass annealing shouldn't be needed.

The Hornady elliptical expander should be good, as well as the current issue Lee tapered expander.

Thanks guys I’m going to try it. I bought Lee dies to use with my brothers press and as I recall, my 300 Savage (Lee) sizing die does have a tapered neck expander.

If...you have a drill press, I'd make an intermediate size elliptical sizer ball with a file in the drill press, finishing it to the smooth polished finish to reduce friction thus force.Yes, store boughten expanders are hardened, but a plain old grade 5 heat treated bolt shank is plenty hard for the job at hand and polishes easily. You will need an 8/32 or 10/32 tap if you want to use it in your die...or leave a good shank on it, use your drill press quill and chuck and clamp the shell holder in your drill press vise. Anneal the hell out of the neck and upper shoulder of your .308 cases. Lube and expand in two steps. You don't have buy every proper tool, every time.

I really appreciate your last sentence. We all can think a little deeper and use what we have to achieve the same result in so many ways.

I've made tons of 35 Whelen cases out of 30-06 cases just by running them into a 35 Whelen die. Can't recall that I've ever lost one. Should be the same for going to 358 from 308.


Okie John

Made some in the past and don't recall killing any brass just going from 308 to 358.

Just remember to lube the necks and it should be easy.

Yes. Using Lee dies. Haven’t annealed and no case loss several reloads in. Simple process that can get more intensive depending on how bored you are.

No, simple sounds good. Wire brushing the necks inside and lubing with a q-tip will help. It helps to know that it has worked for others before trying it. I won’t be using new brass so wasn’t sure about needing to anneal them (if they don’t already appear to have been). I will fall back to that if the necks are crushing or splitting. Thanks for the responses.

i would candle anneal the case first , then tumble the cases , then use die resizing wax lightly only on the outside of each case and be slow when you resize each case. and maybe do the 2 step system 1st a 338 resizing die then a use the 358 resizing die ?