I had some factory loads I shot threw mine. Set my die up to shoulder bump them back a few thousandths. I used the same set up on the new brass. Ive never had an issue.
Hi, I shoot competitively with these regularly. They are very common in the commonwealth countries.
If it is war time production you may have a "generous" chamber. Not a lot you can do about it.
Headspace adjustment is done by swapping out the bolt head, as there are several sizes, marked 0 to 4.
You can still find go/no go head space gauges (303 does it off the rim).
For reloading, you will probably need to size your cases after each firing. 303 has a reputation for being hard on brass, so neck sizing is recommended. I use lee dies, and although my enfield is post war production with better tolerances, I am up to 10 reloads on brass now.
Work your loads up, mine likes about 40 grains ar2208 (or varget, same stuff)with 174 gr fmj. I'm now using sierra 180 gr projectiles as it is the best BC flat base projectile I can find for the weight.
I recently got an OAL guage, and have made a significant change to my seating depth. Fist size groupings at 100 with issued iron sights is well achievable.
If you neck size, make sure you dont use your reloads in another rifle. There is normally variance in chamber size between rifles which can cause problems. If I neck size for one rifle, the bolt wont close if used in the other. Full length sizing stops this.
Enjoy your piece of history!