I keep a spreadsheet with separate tabs for guns, C&R only, ammo, valuable accessories, and also anything else particularly valuable. Useful for insurance, theft, loss, just general (who did I sell that to; what condition did I grade that; what issues did that gun have; how accurate was that gun; what accessories did I have with that gun, when did I zero that gun; etc....) information. Just like JR above, I note if I lent someone a gun. I lent someone a handgun for protection for about 6 months and noted it and then 'struck through' the note. It's always there, in case I need to remember the event. Top of the spreadsheet has my insurance agents name, number, policy number, and the renewal date as a reminder.
As for ammo, I just keep track of full, 1/2 full, etc. ammo cans, spam cans, and/or boxes. I know about how many rounds each box/can holds. Or, you could just weigh each can (35lbs 5.56 ammo...). Quick and easy to count ammo cans and spam cans. Then I estimate/approximate total count. I'm surely within 10% at all times. Given the price/value of ammo, it certainly is worth the effort to at least get an idea of your ammo count and record it. A few thousand rounds is quite valuable ...
I keep a separate digital copy to prevent loss.
Makes the process of inventory and records keeping quite simple.