Any suggestions for an accurate load for my new (to me) rifle?

To all intents and purposes the 303-25 is identical to the 250 Savage but the action behaves differently, so .... My powder choice is AR2208 aka Varget. I do have a selection of good powders available but would like to keep to the same powder I use for the 303 Brit (avoids confusion). I have a box of Sierra 100gr Spitzer’s. Is there a more suitable varmint bullet? Is light-weight better for longer ranges?

Being a small caliber, that is to say, a 'varmint' caliber, I would like to keep velocities as high as I can but only for the sake of trajectory, as I plan to use this rifle for long range shooting.

As suggested before, I would google [ .25 Krag ] 'cause it was once so widely popular in US long, long ago. Unimproved, it should be very close to what you have. You might be able to work from light .257 Roberts loads too. This is a wildcat and this is the part that is "wild..."

100 grain bullets in US are used more for game. Deer. Etc. 70 grain, 75 grain, 85 grain and 87 grain are the "varmint bullets" usually.

"Is lighter weight better...???" What your gun shoots well is better. What rate is the twist of the rifling. ???

Yes, velocity makes distant hits a bit easier. Good goal, but you have to work with what your gun likes. luck.

Thanks again, iiranger. I think that ballistically, the 303-25 is identical to the 250 Savage. I have a hunch that the Lee Enfield is going to be trickier than front locking actions to achieve accuracy. The problem I foresee is that suitable powders seem to burn better at higher pressures than the action is going to like.

I haven't checked the twist rate yet. (I need to get a bronze brush). I do know that 100gr bullets are used in them but the 87gr seems more popular.

What about trying some Berger Bullets?
All you need to know is your twist rate to order.
They seem to have a good selection in 25 cal.
http://stevespages.com has a selection of ADI powders for reloads for 303-25