I have been reloading the .303 on and off since about 1970.
I spent a bit of time trying to determine the best way to reduce or eliminate case head separations. I have been loading 50 years and the .303 is the only round that I have ever loaded with such dismal case life.
So yes I have a Wilson case gage for the .303.
However
1. The Wilson gage matches US SAAMI dimensions which matches US mfg dies and brass.
2. Over the years I have owned 1 SMLE and 2 #4 mk1 LEE ENFIELDS. I have bought and traded for once fired cases fired in literally dozens of other rifles.
3. Not a single once fired case has a shoulder location that matches that gage. The gage is useful for about 5 minutes since the longer shoulder location is easy to see.
Once you realize how far off the chamber and shoulder mismatch is the gage becomes worthless.
The Brits changed their chambers somewhere because their own ammo does not match the chambers either. Yes they work in combat but they are not so great for reloading. Not only do LEE ENFIELD chambers not match the ammo and anything standardized by SAAMI they vary tremendously among the rifles.
Some have almost a 90 degree shoulder. These are what I refer to as "clown" chambers. Whoever ground the chamber reamers for some of the rifles must have been working in 3rd world facilities.
My recommendations use the heaviest toughest brass you can find with a head close to .460 dia. Of the new brass available PPU seems to be the best by a mile. PPU brass heads measure .454 to .455. Still a bit small but better than the US brass at .450 to .452. Use mild loads and neck size only.
"It's faster to lock your Vernier calipre"
Spelling counts?
Jajaja
BTW if they are digital they are NOT verniers either.
They are only vernier calipers if they have a vernier.