I've had good luck with R15 in two 250 Savage 99's (G and EG Models) and a Ruger 77, also in two 300 Savage Remington 81's and a Savage 99R (Pre War). Use Speer 150 grain SP's and cast 150 grain bullets in the 300 Savage's and 75 grain Hornady BT and Speer 87 grain SP's in the 250 Savages and Ruger. Just getting into 87 grain cast bullets in the 250's. Have good luck with IMR 3031 and Varget also in both calibers. Bought some R16 not to long after it came out figuring to try a slower burn powder for both calibers, but loading data was hard to find. Called Alliant and was told they really didn't have any load data at the time for 250 and 300 Savage calibers and it was more intune to the .243-.308-30/06 line of cartridges. Traded it back in to the dealer for some 3031. Have to check if Alliant or any of the other load manual outfits have anything for the 15.5, but unfortunately the old war horses (250 and 300 Savage) aren't given much time anymore due to all the designer calibers.