I just worked up your tube fin design on RockSim 9, to see if there were any issues that V5 might have missed. Hope you don't mind the changes made. The tube fins are not performing as well as the simulated balsa fins would make you believe. This may come as a surprise, but they're too long.
What I did was start trimming the length of the tube fins until they were half the original length - 1.5" instead of 3" - and this brought the A10-3T loaded model to a flyable margin of 1.08. I don't understand the way RockSim calculates the stability margin yet, so I can't give you an explanation why.
With a 1.5" tube fin set, the loaded margin was 1.08, requiring 3/8th oz of ballast. The nose cone was changed to a balsa version (in keeping with the SEMROC components requirement). My simulation gets 308' altitude, with a Dv of 7.73 FPS, with all three motors burning. The 1/2A3-2T gets a respectable 142', with a Dv of 20 FPS. Not bad...
Have a look at the attached screenshots. I'm attaching the V9 RKT file, but you won't be able to open it up and view it with V5. What you might want to do, though, is download the V9 demo, so you can see the new file. It's still free, you just won't be able to save anything.