The CCI valve closes to a much smaller ID about half way down the valve body. The spring then sits against the back wall of the valve.
The RTR valve has the same ID the entire length of the valve body. They have grooves around the outside that are meant to help with expansion of the CO2.
The inserts currently come in to styles the first standard one is made so that you can use standard Nelson/CCI springs in it The second is a different length and is designed for use with the popular Autococker spring kits that are easier to find valve springs for in different pencil strengths for tuning.
I can check if RTR doesn't get to it first, I'm not sure even the shorter valve insert would fit in a CCI valve and allow it to open.