Have tried scouring the interwebz all over for a clear-cut answer to this question, and I can't seem to find anything as of yet.

Does anyone know where the dimensions differ, and exactly by how much?

Just picked up a NOS unfinished ER Shaw barrel, but the guy couldn't tell me if it was an AR-10 or LR-308 extension that was torqued onto there; there is no indexing pin present, so I guess I could try to pull it off and exchange it for an LR-308 extension if in fact it doesn't have one, but I wouldn't know how or where to measure it...

Technically it should work. But like anything else when building a rifle from parts, you need to check headspace and proper lockup.

Sorry, but this isn't what a cursory interwebz search pulls up, and there are many places that simply state that there exists a difference between the extensions, hence why each pattern's bolt is required to work with said extension in order to maintain proper headspace as you alluded to.

There is definitely a dimensional difference in tolerance or depth between the two; hopefully, if the differences can be plainly stated, it would be evidence how one could convert the larger extension to be compatible/identical with the other.

If the gas port is drilled , you're not going to want to remove the barrel extension. Drop a headspace gauge and a LR308 bolt in it and see if it'll work and go from there.

Hopefully this may help.

Attached File