I was told that a police dept burned lots of ammo in a pit that I used to work in. They used metal pots with lids that did not lock on because the lead would melt before the powder could be ignited. I have not tried it and don't know how true it is but if it is true, you shouldn't have a problem. 
Our hunting camp burned one year. My ammo was stored in a .50 cal ammo can. The exterior of the can looked like there was a small guy inside trying to beat his way out with a ball peen hammer. Several rounds made it through. Several rounds that weren't in an ammo can also "fired". Several 180 gr Noslers in .30-06 and a couple of 240 gr .338 win mags were found outside of the tent area. Just the lead, not the brass.
The ammo in the can did "fire" as they were packed in pretty tight. The brass cases came apart and a lot of the lead melted. So...if the ammo is not "confined" the brass will go further than the lead if the powder goes. It's a matter of physics.
If I rememer correctly....powder won't ignite at temperatures under around 450F (I think).