Sgt. Mike
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Harter, I just got a Winchester 121 donated for a project such as this, of course the idea is actually yours.
What you are pointing out with the parent case is very true
The Winchester 121 uses bolt lugs, a flat bolt face with a very small lip that could be dress flat with a file. that coupled with twin extractors, a single shot design
feeding should be a simple issue.

While waiting for your response, I contemplated how to do this.
I was thinking of swaging the .25 ACP case down and leaving the rim at the 300″ish would have to be done in steps. But go the straight wall case versus a bottle neck. I think your choice of cases is a good one once I read it. The bottle neck is definitely a way to do it and allows for a bit more boiler room.

Case forming:
Right now not sure if outside turning or inside turning would be best to get a .010″ wall thickness after swaging down, I’m thinking that the .010″ would be thick enough to not have a frail neck area yet allow for the case to expand and seal in normal 22LR pressure range. The neck thickness of the cases I checked so far was around .013″ without any swaging down. Which should increase a little bit as the case diameter reduces. Which is why I’m thinking of reducing the wall thickness.

Currently I’m thinking of exactly how to set up the dies first, I think I have a idea that would be pretty easy to do it.
I could use something like the RCBS / Redding type neck sizing bushings in a holder to resize the case down as those pieces are hardened already without having to do any extra work. Just have to figure how much reduction for each step.
This would be the first steps before even thinking about chambering the rifle.

The picture I attached is just a representative stock photo of the rifle.

While I have outlined my thoughts I would love to see your progress with the bottle neck that you are talking about. which should be easier in some respect than what I’m thinking of.

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