old 303 ammo

[Son]

smle13, you say you want to shoot what the rifle was meant to shoot. I'm assuming here (from your holy grail post) what you have is a 1905 BSA MKI*** (more on the designation later...)

The HV marking on the barrel means the sights had been set up to shoot MkVII ball, no longer being sighted for the MKVI.

The mods to the sights to fire MkVII was introduced in June 1911 with LoC # 15638. Sometime after this would have been when your rifle was changed from MkI* to MkI***. The fact it has the three "*" shows it has been through the mod.
(To have the HV mark it must have been through the mod, and must be a MkI***)

The reference to SC is the mod to the chamber of opening the small cone diameter out by .002". This modification was introduced through the List Of Changes (LoC # 21209) sometime between Dec 1917 and June 1918 and looks to have applied to the manufacture of new barrels after that date.

To sum up, all MkI*** rifles are sighted for MkVII, and unless they have had a barrel changed after 1918, they should not have the SC marking. (will take corrections to this- backed by photos of SC marks and barrel manufacture date)
MkVII is the correct ammo for your rifle, but the MkVI won't hurt it as long as you realize it might take a bit more cleaning afterwards.

Here's a link to a great MKL entry on the Sht LE MkI series of rifles by John Thorne. On the end I've posted a complete explanation to why they are not called "No1 Mk1" by the book. The terminology I'm told, first appeared used by early milsurp importers in the US.

1907 ShtLE (Short Lee-Enfield) MkI*** Rifle - Military Surplus Collectors Forums