I think it depends on what and where you swabbed.
Floor approximately 2 m beyond the firing line, approximately 2 m in front of the target backstop, and approximately halfway. All samples were taken along the centerline of the range - i.e., we didn't test differences laterally, but I don't think the results would have been any different. We also tested one other horizontal surface - from the archery target butts (recall I said it was a multipurpose range) - which are stored along the sides of the range near the backstop.
I'm aware that many ranges wet down the floor, and I don't see that can hurt, but I doubt that many of those ranges have any data to show that it helps. The range I'm shooting at now doesn't sweep at all, rather uses a very expensive industrial HEPA vacuum, and has no brooms for sweeping up brass, using rubber squeegies (sp?) instead. Again, can't hurt, but I know for a fact it's not based on data.
Also, if the lead is truly from the priming compound, or from the projectiles but very finely divided, it will stay in suspension for a long time. People seem to think that because lead is very dense it will drop out of the air quickly, but the physics (Navier-Stokes equations) tell us that as the size of the particles decreases their density becomes unimportant.
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