Rifle / pellet log book app for a pc or tablet

Excel. I have all the calcs I care about written, FPS/FPE tables written with graphs for chrony shot strings, all my dope sheets (although I've changed out a couple of guns and scopes so it all needs updating). The dope sheets are linked to a chart sized to be printed and taped on the action or scope. Not easy to maintain on a phone (you said app and that makes me think phone) 

I take notes along the way and update excel on my laptop (which actually lives on shooting table).

I add pics and notes of important stuff.

I'm a 30 yr finance guy though, been living in Excel or it's predecessor for most of that time.
 
I would suggest looking into a book that powder shooters use.....this is what I did. You can change some headers over or take them away all together....like Powder used....the sheets have sections for FPS #'s, bullet type, weight, brand....all this can flow pretty easy over to the fire arm forms.



After I get that all sorted and I know what pellet works in what gun and at what distances I will make a tabbed section in a school type binder.....they do make "tacticool" binders but I am a cheap bastage and will not pay for that to look cool.

Then you can make a type of shot card....in the air gun world think what a field target guy has....a note card with ranges, scope dope.....that kind of thing.....then you know what you need at what distances....and what pellet to choose.