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Why would humidity matter?: More humid = less dense air
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Being a br shooter, I am surprised that you have not heard of this before. Humidity changes everything from internal ballistics to external ballistics. When there is a humidity change, a lot of close range br shooters and long range shooters(myself included) will change powder charges to compensate or change powders entirely-going from say 8202 to 322 to change pressures back to their "sweet spot".
When I hunt chucks at 10,000 feet, my load is totally different than what it is for 5,000 foot gong shoots.
Externally, some guns that are on the ragged edge of stabilization in low humidity will not make it to the target or keyhole the target in high humidity. In my experience, humidity plays about as big of a role in this as air temp sometimes. /ubbthreads/images/graemlins/smile.gif