Stiller 2500xr, purpose/point of two barrel/ reciever threads ?

[akajun]

Finally getting around to chambering the 2500xr I have and I see the action has a large receiver thread, then inside a smaller .750 receiver thread.
What’s the purpose of two and which one to use?

 

[linekin]

Sounds like you have a bushing. They're somewhat common in that back when 40x's, & some early custom actions. were used in rimfire benchrest. It was found that a straight contour .850-.900 barrels were easier to tune & hence the need for a bushing. There's been some debates on Benchrest Central & Rimfire Accuracy of bushings over the years if you care to dig for it.