I'd worry far more about 150+ degree ammo than plastic/whatever. Heat greatly accelerates propellant breakdown, which can lead to duds/squibs/overpressure situations, and heat also makes things react faster, so even good ammo will have higher peak pressures than typical. Combine powder that's degraded into something that reacts faster, heat bumping the kinetics of the reaction up some more, a dash of bullet set-back in a chamber that doesn't fully support the case-head and
WHAMMO! 
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My car sits outside in the TX heat all day every day, and I have yet to find any of the plastic interior pieces melted (or even deformed).
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How many summers? My late-model Dodge is already getting a tad of "frosting" on the black plastic trim pieces, and everyone's tires get nuked over the summer months. The materials are a ton better than the days of cracked vinyl seats/dashboards, but it's still a material made of light hydrocarbons we're talking about.
FWIW, there was that story of the lady in California who's Civic melted/curled up after several days in a public lot (which happened to be a focal point for sunlight reflecting off a beautifully curvy building with those energy-efficient shiny windows at 4:00pm)
I believe temps were clocked there in the upper 100's--better put on the
TCB