88 Mau - if you shoot a light, 100gr. to 125gr. 9mm or .357" pistol bullet out of the rifle at a descent velocity to get the neck in a fired, fully expanded form, trying a 250gr. Hornady in the neck for slip fit, will show if they are safe to load and shoot. If the bullet fits easily in the fired neck, they're OK.

I suggested the light pistol bullet as they have soft jackets and will not produce excessively high pressures, even if the bullet is too big for the chamber's neck. Something like Varget should allow a case full or close to it. A pressure around 40,000cup might be necessary to get full expansion of the neck. Annealing a case neck first would allow a lighter load to do the job.