Gary,I can agree with the response,to a point.You actually can use a spark plug washer.(Its been done)There are laminated peel washers available.
I'll give you a tool.The pitch of a thread is 1/tpi.So,20 tpi means .050 per rev.If you divide that .050(or whatever pitch you are using)by 360,you will get the amount of washer thickness you need for 1 degree of change.From there,you can know your shim thickness.
Outfits like Carr-Lane anf Manhattan Supply,I believe,sell various case hardened,precision ground,sized by the .001 to use.I have turned some,just the right thickness.
Now,I will preview a thread I am about to write.Suppose you torque that on to 20 ft lbs to get it oriented just right.(I think I messed up a barrel).That is a good spark plug torque,2.3 white knuckles on a 6 in combination wrench.
Lets see,Pi is 3.1417,lever arm 12 in,in my head roughly 37 inches of travel for one revolution to move .050Lets call it 40 to make the math easy.I get an 800 to 1 mechanical advantage.Times 20 lbs,16,000 lbs linear force for 20 ft lbs.Now,thats not true,because there is not 100% efficiency,friction eats a lot.That poor thin walled tube that is between the minor diameter of the threads and the bore doesn't really get stretched with 8 tons of pressure.
But that stretch is at the last 1/2 in of barrel,releasing the bullet.