The 22LR and 22 mag are listed at 24,000 psi. The 17 HMR is loaded to the same pressure as the 22 mag. I assume the 22 LR is of the high velocity type. So it looks like both 22s and 17s should work in a #4 rolling block. The larger case of the HMR and mag has room for more powder but probably loaded with a bit slower burn to keep the pressure the same, just takes longer to burn so the pressure is there over a longer time, giving a higher velocity.

I do have a Lyman super target but I'm thinking I might use it myself. Many years back I traded a 30 power Lyman for a large can of rolling block parts containing two complete actions and almost enough to put two more together. Don't know who got the better part of that deal but we were both happy and that's what counts.

Saw one on EBay last night.  Look under "target scope".

David

I would be using a TJ liner in the original barrel,

I have acouple of 3/4 mossie scopes,  just not with external mounts,
would like to keep this project looking close to period correct

but your right,  the mossies seem to sell for less, and there are 1 or 2 on fleabay right now