.338-303 for Lank
In response to Lank who would like some input on an addition to his collection. Rifle is an SMLE in .338-303.
If you have a No.4 Mk 1 (yours might be No.4 Mk1 long branch Ontario jobby), and its chamber design is simply the .303 case necked up to .338, you can use .338 Federal starting loads and work up as normal. Max velocities will be within 100fps of the Federal. The .308 case is more efficient than the .303 but when both are necked up to .338, the differences will be lessoned.
As for dies, if you don't have any, you might simply have to put a cut off wheel on an angle grinder (or get a machinist to part it off) and remove some of the body length from a cheap second hand .338 Win mag die set- both the FL and setaing dies, leaving you with a neck die set. Neck up the parent .303 brass and fireform with your starting loads. Accuracy will not be affected by the rough, home made die.
As for giving it an extended range test. You will need to play with the bedding and floating etc, do what you can where you can. Some rifles shot better clamped via a barrel band, some shot better free floated. If the barrel is clamped, the fore stock should be bedded against the barrel if you have the time to do this. Its one of those things you have to play with and you could certainly get carried away. For example, a suppressor would help tune the SMLE up considerably.