Since this is about objects falling from a certain height, yes, weight is a useful metric for rough safety estimates.

A bullet does damage because it holds a lot more energy from being accelerated - a 9mm bullet will go around a mile up before falling down (source: MythBusters). Two 500g boxes dropped from a likely much lower height will have a similar effect (concussion?) when dropped on a pedestrian regardless of the contents, except for a few outliers. Even for small, heavy objects (like a 1kg camera lens) the load would be spread by its enclosure unless it separates mid-fall.

a 9mm bullet can only go so high because it leaves the barrel at a much greater speed than its terminal velocity in freefall. 9mm in particular has a fairly low terminal velocity; if it falls straight down it is unlikely to seriously hurt an adult.

The bullet, falling from the same height as a crashing drone, is at least as lethal if it strikes a human. There is no "safe weight" for a falling object... but there may be a "safe momentum".

The proposed flying zone for drones like these is between 60-120m height. Seems like a bullet must travel at least 62m/s to penetrate human skin [1], for which you need a height of at least ~200m [2]. So even dropping a bullet from one the delivery drones would not carry a high risk of fatal injuries.

That, and P=MV, momentum/energy increases proportionally to mass. A 'safe weight' can be defined if the altitude is known.

[1] https://slate.com/news-and-politics/2011/03/can-falling-bull...

[2] https://www.grc.nasa.gov/www/k-12/airplane/mofall.html

Right, but a box with a 1lb rock in it, and a box with 1lb of feathers are going to fall from the sky to your head about the same.

I mean, the 1lb or feathers will have a much larger box, and a much larger surface area for wind resistance. In addition, it will spread the impact over a larger area.

The box with the feathers could be very tall but narrow so that the air resistance won't be large.