As far as COMMERCIAL ammunition is concerned, if it is marked ".303BRITISH" it will be okay.
There is ALSO the old .303" Savage, but they are more than a bit scarce these days. Cartridge was about the size and power of a .30-30. Your rifle uses ammunition which is about 50% more powerful. Firing a .303 Savage in a Lee-Enfield will give you a bulged, split casing and possibly gas splitting out the back of the rifle and into your face.
Be careful.
As far as MILITARY ammunition is concerned, if you happen onto some of the stuff, get back to us here and ell us what's written on the bottoms of the cartridges, with photos if they look the least bit unusual. There were more than THIRTY-SIX HUNDRED variations on the .303 cartridge produced for military use between 1888 and the late 1990s. Some of these are rare and very expensive and you would be an idiot to shoot them off (commercial ammo is fresh and about $1 a shot; Cordite Mark III Dum-dums go for better than $100 a shot). Others are dangerous to use (Explosive type), some are illegal even to possess (Buckingham incendiaries are one example). And there are some very interesting ones out there, too: I once ran into a bag of B Mark IV Z: very special stuff for torching the armoured gas tanks on the ME-109.
Real, honest-to-goodness Battle of Britain stuff. Fortunately, I had got rid of them before they were made illegal...... 50 years after they had been dumped on the civilian market by the same Government that later banned them.
Oh, it's fun!
But somebody here will help with the military stuff. Commercial is hunting ammo and you can get it anywhere. You might think about loading your own. It's fun, your ammo costs half as much AND it's very often more accurate.
Hope this helps.
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