I had a very bad experience with a sporterized Lithgow No 1 Mk III 1913 enfield in .303 british when I was just a kid. I was shooting 180 grian Core Lok't Remington ammo. I shot about 10 shots on paper, went out to the woods, and missed a deer from 50 yards, and it WAS the gun! I went out the next day with my single barrel shotgun and the rifle too. I shot a nice fat doe at 50 yards and dropped her there, but she was still kicking, so I took a shot from the blind with the rifle and saw dirt kick up about 6 feet in front of it. Later, I took it to the range. i shot at 25 yards. There was about a 36 inch pattern from 4 shots on a piece of newspaper! I had no idea what went wrong, so I got a 30-30 the next year and didn't shoot it for a while.
Then, just last year, I found surplus ammo really cheap. I was looking for something to leave out at my deer camp just to make noise with and to have something in the very rare possibility a bear should invite himself in, but noting I would cry over if it were stolen. First thing I did was take that cheesy scope off, then I cleaned it.
Before I shot the rifle I took a cleaning brush and a bucket of hot water and scrubbed the bore out, then ran patches through with Kano Kroil on them, then an oiled patch. That surplus ammo will group about 2" at 50 yards for about 10 shots, then the accuracy will start slipping, then after a lot of shooting it goes all to heck again. I'm thinking that once the bore warms up and gets dirty the rifling looses its grip on the bullet. Anyways, i have about 50 rounds of pakistain FMJ ammo and mabey 5 hunting rounds out at my camp. If my deer rifle were to go kaput out there I wouldn't hesitate to run a brush and patch down the ol' 303 and load it up again.
Balistically I've been told it is about like a .308, mabey a little better. I hear they use them a lot on moose up in canada. For a while it was one of the biggest baddest smokeless rounds around, Winchester even chambered their M95 lever action in it.
If you get any cheap surplus ammo be sure to clean the bore with water to get the salts from the primer out. otherwise, modern ammo is no diferent from any other cartrige. I hear tell the chambers in No 4s are intentionally a little loose, so you might get some stiff operating with hot commercial ammo. Good luck!