Burt's shooting a .50 Cal Desert Eagle
You can change out the barrels, but from what I recall, he hasn't done that. He has his .357 revolver for everything else.
Most of what I've read only says the MER is 200M.
I'd venture to guess the Maximum effective range has to be for an area target though and not a point target. Point MER is going to be much shorter.
A marksman will be able to do things with a weapon that look superhuman to normal people.