What's the difference between the 2?

https://www.alliantpowder.com/products/powder/reloderts15-5.aspx

Better temp stability, somewhat slower burn rate

Hmmm...

I have a 300 grain A-Frame load for the 9.3x62mm using RL-15.

Has anyone fussed around with RL-15.5 and a 300 grain bullet for that cartridge?

Yes.

https://www.24hourcampfire.com/ubbthreads/ubbthreads.php/topics/17570716/1

Mucho thanks

0.5

Couldn't resist.

There was a handloader article I saved off somewheres that IIRC said 15.5 is double based, where 15 isn't.

That isn't the only difference.