Can I tell when to trim my OA case length, by chambering a resized case in my gun?
Or how can I determine what is Max. case length for my gun? thanks Link

No as rechambering means nothing...You have to measure the case...MOst handloading books give max lenth and trim lenth..or a case lenth gage can be had for a couple of bucks. this trimming business is a very important function of reloading as a case too long can run pressure sky high plenty fast....

Thanks Ray. Guess I will stick to the book. Link

If the pressure thing doesn't phase you, I'll relate an experience with my 6PPC and lack of trimming. Having the neck just a little long can take a load that is always looking for an excuse to break .3" into a 3" load.

It's also one of the "75 common accuracy gremlins". FWIW, Dutch.

LINK,
Sinclair sells a case length gauge for about $5.00 that works great. It is necessary to to ruin a case, but the case can be used forever to determine chamber length. I warn you, the actual length of a chamber has no bearing on S.A.A.M.I. specs. It will be a eye opener. I hope that this will be of some help. Good luck.