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In some of Los Angeles’ more crowded outdoor ranges, I wouldn’t be surprised if close to half a ton of lead was getting put into the mountainside on a busy day, with who knows how much of it becoming particulate matter.
Both ranges I’m thinking of are in the hills of major watersheds.
And then the rains come…”
The metallic lead would most likely just stay where it is.
Lead from bullets is pretty inert. Not bioavailable unless somehow acted on. Acid from a condor’s stomach, where they have no way to poop it out like other animals, so it just sits and dissolves might do it.  Drinking water from a acid contaminated water supply running thru lead water pipes does it. Bottom feeding ducks that continually ingest shotgun pellets are harmed as well. Hence the ban on lead shot over water for hunting.  A typical CA rainstorm  wouldn’t cause measurable results.
Outdoor ranges just mine the lead and reuse it. For shotgun shot they don’t have to do any other process other than use a sorting machine then load it back up. Other metallic lead bullets they recycle. There are no smelters in the US that process new lead ore anymore, just recycling facilities.
Indoor ranges require good ventilation and many also require coated or plated lead bullets as someone mentions. Lead bullets, are covered with paint or copper to prevent the heated gases from the smokeless powder from turning a bit of the bullet base into a gas.
There are some new developments in lead free primers, as they put a tiny bit of more bioavailable lead in the air when fired. At an indoor range this can be an issue, likely more than the bullets. Hopefully these will become more reliable/available.