I went back and looked at your previous posts and see that you are an AR-10 guy, sorry for the info on the AR-15.
The extension diameter I found was 1.188" but no spec.
If it rocks in the bore your measurements are likely good.
My procedure for an AR-15 is to lap the face (Wheeler tool) and measure bore with this:
Cheap Chinese ($53) inside mic that checks very close to 0.9985" X master.
Would of course need another ring for AR-10 bore.
I checked one upper at three depths from the nose @ 0, 60,120 degrees. Got about 0.0003" under an inch.
My extension has a slight taper and
almost slides all the way in.
Now, where I do things a little differently is shimming.
I use a spray on shim (high heat engine/Bar-B-Q paint) burnished smooth. For 1 or 2 thou getting it on evenly is easy peasy. Maybe your gap is too much for this method. Steel shims would probably be better for the gap you have. Blue or green goo would be hard to end up with the assembly centered.
The receiver goes into the oven @ about 220F, barrel stands up in freezer @ about 4F. Slap together and hand tighten nut and wait.
Aluminum and steel will remain a "Thermal" fit even at pew pew pew temperatures