The British marked both rifles and smoothbores in gauge prior to the new rules of 1887. Thus, a pre-1887 .500 BPE is marked "39". This is, of course, land diameter, not groove. This simply meant that a 39 gauge ball (.492") would drop through the bore, but a 38 gauge ball (.497") would not.

Under the 1887 rules, the bore diameter mark of rifles was changed to decimals of an inch, and is primarily a reference to the name of the cartridge, not necessarily actual diameter. Thus, a post-1887 .500 BPE is marked "500EX".