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I've used similar low power devices with low gain antennas over a mile with direct line of sight (as in; I can see them in the distance). Mountaintop to mountaintop, or cliff to beach, etc. In rural areas there is little other noise for interference but some obstructions, like tree's and bushes, will reduce range to just a few hundred yards. Maybe less. UHF generally does not penetrate tree's, hills, reinforced walls or metal.
In urban environments range will be more limited by noise and heavy obstructions.
I've been in many situations where I can see the person, or I know they are on the opposite side of a sheet rock wall, or within shouting distance in the woods and we can't communicate due to poor cell service and/or no password for the WiFi.
I'll try FireChat, but I know I would rather have something that is private. Something like Hamnet for Android is what I really want.