I had some problems with Winchester shells not feeding in my 303 back about 1980 or so.
It seems they were made a bit longer than the military rounds and have rounder points. They were OK if you dropped them in from the top single shot style, but they would not feed from the magazine. Maybe they have fixed that now, it was not every shell either, but out of a box of 20 at least 4 would be like that.


You can use .308 bullets if you are loading your own shells. They are slightly smaller than 303 bullets but not enough to hurt anything.
Almost all bullets in 30 calibre are .308 diameter while .303 is just a bit larger and actually measures .311 or .312 diameter.

Sierra might still make match grade 303 bullets.
I used to use 150 grain .308 bullets instead of the .303 bullets when I was hand loading. They seemed to shoot about the same as the 303 on the range. They were certainly easier to find in the stores.


When you are buying ammo you will notice 303 is available in up to 220 grain.
You really don't need that heavy of a slug for anything you might hunt in North America, except possibly a Kodiak.
The most common weight seems to be 180, and even that is heavy compared to most other 30 calibre rifles like 308 or 30-06.
You can still get the 174 grain bullets too if you look for them, but they are usually older surplus stock.