I use myGuns app on my iPhone...
List the important things like caliber, model, serial number, date of purchase, etc... then modifications I've done to it and certain accessories (holsters, bayonets, slings). Also have pictures of all of the guns, especially if they can be converted (ARs with different uppers).
While I understand the whole idea that anything can be hacked, I really don't care about this aspect. I really don't care who finds out what I own. It's all legal, so post it across the internet for all I care.
I keep it up to date mainly in case I ever have to report a gun lost/stolen. Available easily, and can forward pictures in case it is needed. Have serial numbers for scopes, so easy to say exactly what is gone, and have a very definitive way to identify it.
However, I do use it for remembering specifics about guns. If I have a certain modification, I try to be as specific as possible. As mentioned, if there is a serial number, I don't have to go find the firearm/accessory to get it. I also started putting model numbers in there for holsters... as having five different versions of the same Galco ankle holster makes it hard to figure out what gun fits it without noting the model number.
I do similar with the AmmoBase app. Have a complete list of all my ammo, purchase info, and exactly which container it is in (have ten Cabela's ammo boxes, so knowing all my .40 is in Box 7 makes it easier to grab. When you have 17,082 rounds on hand, across 24 calibers, every little bit helps. Since I started logging that, I've also kept better notes on round counts for my firearms. I used to do it in a notebook... until it met a half bottle of solvent. AmmoBase backs up to Dropbox (and again, ammo amounts isn't something I view as highly sensitive), so not going to be an issue in the future.
Now, if someone did an app for magazines and clips (I shoot a lot of surplus rifles)... I'd be happy. That is one thing I cannot find an easy way to import into either of the apps I'm currently using.