Sounds like a good plan. Could you make two extensions to pin on instead of threading? What is the diameter of the tenon? on the barrel? Hopefully it is a size that can be copied easily. If you are making an extension I think you will need a reamer the right size.
I wonder if the losing velocity thing would be lessened or done away with by using powder coated bullets. Since they are slicker than lubed lead it seems logical that they would not have as much of that affect. I just bought a new Chronograph. It is in the mail now. When it comes and when I can get everything together, I will compare everything identical except the powder coat to see if there is a difference from my 514.
I used an old die that I had set aside to make the 22cal bullets to crimp onto the .25" power loads. Just realized that it was discarded because it makes .228" bullets. Do you think it would be dangerous to drive those big bullets down a 22lr barrel with a #4 power load?