February 20th, 2017, 12:29 PM
Here's I did the barrel change on my brother's bench gun. the receiver is glued to the stock, since i chamber on the headstock there is no way test fito the new barrel to the receiver. I took repeated measurements of the tenon; pitch diameter, tenon length, and relative measurement of the headspace and the cone. To do all the measurements I had to machine a couple of jigs. See pictures.
In your case if you have access to a lathe, machine a bushing to fit over the tenon and stop at the barrel shoulder and long enough for your depth micrometer to engage the back of the gauge when insered in the chamber. I made mine with aluminum. Take relative measurement of the old barrel, take measurevent of the varmint barrel the difference is what you need to compensate for to get the headspace right with the new barrel. This is assuming using the original recoil lug.
You have access to surface grinder, and if you have access to a lathe just make a washer type recoil lug and grind it to the thickness needed to attain headspace. Bed the rifle with the washer recoil lug. Back when I was hunting a lot I put together a switch barrel SA 700 with 308, 243 and 22-250 barrels using ground flat round washer. It is not prettiestthing with the washer sticking out above the barrelaw, but it worked great. In your case it just an interim thing, it should serve you well.
Edit. Too lazy to redo about the bench gun. I remember I made one for a 700 when I helped re-barrel a 700 for a friend. The pictures tell the tale.