Re: British 303
Cougar - most of the surplus ammo I've shot thru my various .303s have been disappointing for the last 20 or so years. I've had lots of hang-fires in both old WW2 British and later Pakistani ammo. I'm guessing it is a storage issue.
Although I personally prefer the .303 to the Russian 7.62R, if it comes down to buying the cheapest and most reliable mil-surp ammo, I'd have to say the 7.62R.
I'm not wild about the Mosin-Nagant's design, and I really like the much slicker action of the Enfield. I ended up pulling down .303 surplus ammo and reloading it into commerical cases. Most times the 'powder' is that straw like cordite, so that's not even useable - just the bullets.
Grouping - that is a hard call when you expect a hang-fire or mis-fire half of the time. When I reload mine with commercial cases and powder, they shoot 2 or 3 inch groups at 100 yards with the open sights. As far as the 7.62R, I only owned an M44 carbine, and that particular rifle was a 4-5 inch grouper at 100 yards with mil-surp ammo.