rimfire,
I think your chronograph should be suspect. I would purchase a couple boxes of commercial ammunition with the same lot number that you can use as a "control". Since you have what I would call an uncommon caliber (7-30 Waters) you might want to use another gun in a standard case such as .308;30-06;.270, etc. to get a base-line for the chronograph.
Establish a good repeatable routine when you set up at the range. I understand that sky conditions will produce variations in the readouts, so be aware and record weather conditions too.
Finally, I would re-read the literature that came with the chronograph and if you have any questions, call their customer service hotline for help.
Jim