Re: Switch- Barrel headspace requirements
It sounds to me like you have something less than a true switch barrel gun and the other folks trying to help you have, at best, limited experiance building a true switch barrel rifle.
I believe what you probably, actually wanted, was a take down rifle with an extra barrel. Am I right? You wanted a rifle that you could if desired, change calibers during a hunt.
I built such a rifle and helped build a second rifle that was never finished due to the death of the master gunsmith. The first rifle is built on a M-70 Classic Action in 338 Win Mag / 7mm Rem Mag. The second gun was built on a M-700 Rem action with a thick locking lug welded to the front of the action ala M-70. That 700 is in 280 AI and the second barrel in 22-250. It is in the estate.
This started as a problem put to me by a master gunsmith, who's shop I often worked in as a hobby. Can a switch barrel rifle be built that would be a marketable and a viable product (expense, time, sale price, etc) to offer from a small shop.
When I explain this to you, you will have all of your answers.
The problem with your "switch barrel" setup is that you do not have a way to stop the rotation of the threads at the same point every time and lock up the barrel in the action. The way you correct the problem is to make a spring loaded, tapered plunger, that goes down through the recoil lug, into the bottom of the barrel shank and is round at the tip. The hole in the shank matches the plunger. Once the plunger drops into place, a set screw is screwed down tight onto the plunger's end, locking it into the barrel and the barrel into the action. The barrel is at the same time matched to two witness marks, 1/8 turn out, which your eye will align every time.
The setup requires a spanner wrench with a tit welded on one jaw to match another hole in the bottom of the barrel. Sometimes it requires two people, one holding the action and the other using the wrench to unscrew and screw in the barrel.
How does it work? .56 inch 338 Win Mag and .70 inch with the 7mm Rem Mag.
I think you might already understand from your experiance that as long as the barrel is only tightened to point x every time, there is no headspace difference and the wear with a good greese is negligable. Will it wear, yes, because all swithch barrel guns do eventually wear, but as has been mentioned here, it is most probable the bore will wear out first. Th switch barrel guns will also not take a lot of abuse.
Those who do not understand the process have not thought out the string as to what two scopes with matching QD rings mean to such a gun. Mine returns to zero every time for both barrels.
Those that beat their guns into plows will plow for those that do not.