The WWG triggers are great, I've installed a lot of them. Do a thorough function check after install, sometimes they lighten things up too much.
Timney just came out with a trigger and hammer set for the Marlins as well. I don't have any experience with those yet.
Powder charge does have an effect, velocity aside. Gasses do have weight. More gas equals more weight. You can calculate recoil energy. Felt recoil will vary by stock design and action type.
Guess I should've read that better!
The other thing that comes to mind, is that the charge ladders got mixed up. He may have been shooting each charge weight per group instead of the same charge per group.
A: Pull weight
B: Safety
C: Overtravel
D: Sear engagement
Set the sear engagement through the sight port on the side. You want at least .01" engagement.
Set the pull weight no lighter than 3 1/2lb.
Set the overtravel so the trigger clears the sear + a touch extra.
Set the safety while on safe...
Yes that's either an 1899 or a 99. On the left side of the receiver ring it will be stamped with the model number. Some of the early 99's were stamped with 1899, but you can run the serial and get a DOM.