Rick, That sounds like a great idea, especially for use in accuracy experiments. Mike and I have made two platforms over the years and they both worked very well. We built a 6' X 6' for our half scale SBR 1797 4" mortar in 1985 for an article on mortar accuracy we wrote for The Artilleryman magazine. It made a tremendous difference in both range and deflection accuracy. I'm sure you know there nothing worse than a 'Hobby Horse' mortar that digs into the dirt differently with every shot. That platform had 4 sleepers embedded into the leveled and rammed soil and staked well on the rear and sides. The rails were bolted to the sleepers also. For newbies, the sub-surface sleepers run parallel to the 'Plane of Fire' and the top rails are 90 degs. to the them.
We also made a large, 82" X 112", platform for the big Paixhans Monster Mortar we built last year. When we take it off the trailer, for firing experiments this Fall, we will mount it on six, 82" long, pressure treated sleepers which will be bolted to the rails. It's a bit of work to emplace these platforms, but the stability of a well built platform and the feeling of confidence that it inspires, can't be beat! Best wishes on your's!
Regards,
M&T