I just got a really nice Rock Island 1903 and definitely want to shoot it. I know I need to use a light load so I don't blow up the receiver. Anyone got any suggestions on starting grains? I'm using IMR 4320, and for a normal load, the recommended start is 45.9 grains.

Sounds like a good idea to start low and just work up slowly till you get good groups then just stop there.
If I'm not mistaken 4895 was the powder to replicate the old military ball.

I'd run lighter bullets like 150's or 155's. (Hornady 155 AMax is very accurate in my aught six)
I am running Varget with the 155's and very good accuracy in modern 30-06 (hot load), but I have heard of folks getting good results with milder loads. 

One must be careful with these faster powders because of the low load density. It would be easy to overcharge the case. Utmost attention to loading process is a must.

Good luck and update with your results!

Thanks Heavies, I appreciate your input. My rifle was manufactured sometime in 1918, but has a 1911 barrel, which leads me to believe the original bbl was shot out and replaced. This kinda gives me the confidence that this rifle has shot lots and lots of rounds, and hasn't fell apart yet. So, I'm going to have fun with this piece. Aloha & Mahalo

Actually after re reading, IIRC, Rock Iland Arsenal below#s 285,507 may have had improper heat treatment on the receivers.
I'd proceed with caution.