In the UK, we use either PPU 174 grain or Sellier and Bellot 182 grain ammuntion for our 303's. My favourite supplier is John Bradshaw Guns up in Northhamptonshire. There is not much that Marcus and Iain dont know about guns, shooting, shotguns and ammunition. But the wonderful thing is that when they don't know something- they say so immediately and find out. They usually have PPU 303 in stock at quite reasonable prices. The best thing is to give them some time, order a few hundred rounds and then they can make sure they have it; and that the price is right. (They also have just about every other calibre under the sun).
Sometimes, I buy special match grade ammuntion from Fultons of Bisley, but in general for my standard of shooting, factory ammo is more than enough. Our friends in Australia and some of my British friends all load their own. Rather than expound on why- Its easier to ask Gunsnzen to put his theories forward directly:
To load or not to load By Gunsnzen
I reload for two reasons. The first is improved consistency/accuracy and the second is quite simply cost.
No two rifles are quite the same. Tiny imperfections in manufacturing produce differences in the barrel and chamber. These imperfections alter the harmonics of each rifle. This is a subject which can be argued ad infinitum, but in essence reloading and varying powder charge allows a rifleman to tune the load to each specific rifle. When one reloads one selects loads which produce the most consistent results because they provoke a harmonic wave which is at its shortest at the muzzle.
The Cost saving of reloading
If like me you have more spare hours than £20 notes then reloading will suit you. While it does take several hours to produce 100 decent rounds it works out about 30%-50% cheaper than buying factory ammo. It also allows me to buy material in bulk which legally most target shooters are unable to do due to low ammunition allowances. Buying in bulk ensures consistency over time.