Dino82520 is absolutely correct ,  worst case scenario would be trying to polish loaded ammo in a vibratory tumbler or regular drum type tumbler very bad idea.  It's just not rifle ammo but pistol ammo with a much hotter powders in weaker guns.  I think maybe the best way to do that if you must carry ammo in a vehicle on a daily basis would be to rotate the ammo on a regular basis, go out and shoot it and put in fresh ammo.

Sadly I own just one Weatherby rifle, but it is the rifle of my dreams, so life is good. It is a .340 Mark V, stainless/synthetic, topped off with a Leupold long range target scope.

I'm hanging' with Oregon Jim. I have one - a lefty Mark V Accumark in 340 topped off with a Burris Veracity 4-20. I needed an elk rifle, so I got the hammer.

I don't normally leave a gun or ammo in my truck.  The waiting room at the doctor's office would be the single exception. 

Depends on where you live , we don't have the humidity here in Arizona but we sure have the heat. won't comment on humidity but the heat can and will definitely affect ammo, seen ammo breathe and smell while stored in old military ammo cans left in metal bldg's in the sun.  Generally most people here keep ammo in the house or garage or insulated cool place.  Wouldn't be a bad idea to have an under or above ground ammo bunker covered with dirt  and shade trees for insulation to keep it cool. At least here I can't leave ammo out sitting on the tailgate or any other place open to the summer sun ammo just gets too hot even to touch sometimes   I have to keep it in the shade as much as possible.  I do like to test loads when at or near max charge weights  when it's pretty warm out.  I've stuck enough bolt's thru the years to know heat and powder don't mix real well.  As temperature insensitivity as some of these newer powders are suppose to be imo there is a limit on just how hot you can get this stuff and be practical.  I keep my ammo covered out of the sun as much as possible.  I've melted plastic camera in the back seat, bic lighter explode on the dash , melted all of one of our boats plastic  electric panel switches  get soft and deform luckily our other boat has old school metal switches just can't touch them in the summer sun,  so we carry beach towels to lay on the vehicle  and boat  vinyl seats.  we literally have to crack the windows open a little in the vehicles when it's that hot.  luckily I will be hunting down in the Superstition mtn's and wilderness on Coues White tail hunt in Nov so the heat won't be a problem, rough ass country long shots are probable.  will be using a little Vanguard synthetic in 6.5 creedmoor  with first focal plane Nikon 4x16.

Putting in for spring Turkey on the north rim of the Grand Canyon,  hopefully I can get drawn, the plan is to take the wife up on a scenic  side by side drive around and look at all the northern points looking down on the canyon. getting older probably get a motel in Kanab Utah and short drive into AZ.  We have been up on the north Kaibab and strip  deer hunting several times always something interesting to see,  will stop in at Muley Crazy (the magazine) in Kanab and look at all the trophy deer  mounts, we try to stop in and visit  and get good info. 

I remember once a long, long time ago it rained here. I have pictures of it somewhere.