I picked up this rifle built on a rem 722 action. It came with 220 loaded rounds.The boxes are marked 48.2 gr IMR 4350 with an unknown 100 gr bullet. So my question is. Would the ackley data be a good place to start for load development ?

I know the Roberts has been improved with at least 4 shoulders. 24, 28, 30 and 40. As I understand Vickery simply improved the case like Ackely. I'm not sure of the shoulder angle.

Provided the rest of the dimensions stay the same shoulder angle will have very little impact on capacity. So yes Ackely data should be fine. Simply start lwo and work up.

A 257 Roberts Ack.I that is 2.230" long will hold 59.46gr of H2o. The 257 RCBS I (28 deg shoulder)will hold 59.26gr H2o