Each rifle is unique.
The best way to get help by way of this kind of approach is to report the specific performance of each individual rifle, including type of ammunition, chronograph data, and target examples -- also including detailed explanation of each shooter's hold, shooting posture, rest configuration, and gun fit.
Also helpful will be full chamber-casting and bore-slugging data, just as with any other unknown double rifle...

Get on it!
I eagerly await the reports.

Cheers
Tinker