As I posted earlier, .303 British is only one of at least five cartridges that use this bullet. Homegrown US .303 Savage aside, not very common even in its home country for a very long time, the others are all imported surplus military items. Hornady likely designed the bullet to best suit whichever 0.310" user was most popular at the time of its introduction. Although 7.62X54R Nagants are very cheap and popular 'over there' nowadays, this bullet has been around for a long time, and I imagine it was the 7.65 Mauser that it had in mind. M1891 Argentinian Mausers were imported into the USA in vast numbers 40 or 50 years ago and were very popular, so much so that many American loading manuals incorrectly describe the cartridge as the '7.65mm Argentine' (if you want to be pedantic, it's the 'Belgian Mauser' as it was this country it was originally developed for.)...I'm still curious as to Hornady's purpose for the groove, and if they intended that bullet to be used for '.303 British' loads...?
This is an old bullet, and back in the days when it was introduced, most FMJs were made with a crimping groove. In fact many older Hornady expanding designs were also cannelured, a good example being the 0.284" 139gn BTSP, presumably for 7X57mm Mauser. Crimping has gone very much out of fashion in recent years, hence few modern introductions incorporating the required cannelure with the exception of those intended for automatic service rifle cartridges, 140-odd gn 7.62s and 55/62gn 5.56s.