Anyone with a reamer can do the conversion.
The 303 Brit round is one of the better rounds for "improving" and the conversion will give it a capacity between the 308 and 30-06, and add about 100 fps to your MV if you keep the pressures the same as before conversion. In a Lee Enfield you'd be wise to do just that.
Anyone "getting" anywhere close to 300 Win Mag velocities out of a 303 Epps is working at an exceptionally high pressure.
The 303 Brit round is one of the better rounds for "improving" and the conversion will give it a capacity between the 308 and 30-06, and add about 100 fps to your MV if you keep the pressures the same as before conversion. In a Lee Enfield you'd be wise to do just that.
Anyone "getting" anywhere close to 300 Win Mag velocities out of a 303 Epps is working at an exceptionally high pressure.