Hi guys,
Thanks for all the help. I apologize for using the term "best". I should have used "most flexible". After researching this problem with your help, I don't think a gage is manufactured so as to check throat erosion in a dismounted spare barrel. As Bill Ricca stated, each T/E gage is firearm-specific. But with NOS/NIW replacement barrels becoming increasingly hard to find, I will have to be a little creative when gaging a dismounted used spare barrel or a BAR. As far as I can tell, no gage listed here is designed to check T/E in a BAR at all. All you can check on a BAR or BAR barrel is Muzzle Erosion (Muzzle Wear).
It looks like the T/E gage Brownell's sells, the gage from Scott Duff, and the T/E gage Bill Ricca referred to are all the same ARA-MOR gage.
Thanks again,
Carey