Rolf, Happy New Year. Wonderful to hear from you. Was thinking about sending a HNY email.

I loaded for my brother in law's .22/250. He didn't and doesn't use it much. I think it has a 600mm 24 inch barrel. A 25 to 26 inch barrel seems adequate.

I don't know about the twist rate.

The standard .22/250 case is fine in my opinion.

What do you hope to use it on?

Here, the .22/250 is a fine kangaroo choice, feral goats and the usual sized Southern feral pigs. Imo too damaging on Fox furs but good for extermination. Also far too damaging for hares and rabbits if the meat is desired.

With a slower twist and heavier bullets would work reasonably well on lighter deer.