Is the gun complete and original? Does it bear any markings identifing past military history?

Complete and original in decent shape in .303 British with trashed bore it is $1000 gun. A bit more with specific unit markings and good bore.

Rebarrel to .307 Winchester, it becomes $400-$500 gun.

Reline the bore keeping it .303 Brit back to $1000 gun and maybe more.

The .307 is s 52,000 psi cartridge. The problem with loading down, is the next owner might not. Making the guy who built the gun liable.

The .303 Brit is a 45,000 psi gun.

The rule of thumb is never chamber a gun for cartridge of higher pressure than the original.