Some years back, I stored 500 rounds of .38 Special cast bullet handloads in my car trunk. After several weeks, not one round would fire! The cast bullet lubricant had melted and contaminated the powder and primers. The lesson is that any ammunition that is lubricated and which which seats the bullet inside the case can be subject to this kind of contamination. There may be some commercially manufactured bullets that use lubricants that are resistant to melting, but I am not willing to take the chance. In summary, excessive heat and humidity do not do anything good for ammunition.