Alwims,
Leftover beat me to the punch.
The melting point of aluminum is roughly 1,250 degrees F.
the melting point of lead, depending on alloying metals, is substantially below that, but the casting temperature is in the range of 700 to 800 deg. F.
The actual temperature on the inner surface of the pot can be substantially higher than that. As you can see, you're starting to get into a very narrow margin of safety,
Consider, too, that the closer a metal gets to its melting point the weaker it becomes.
Here's a link to a page where one person used a lead pot... I hope he wasn't hurt.
http://www.cisatlantic.com/trimix/vweight/themelt.htm
Here's another link with no photo, but a similar warning:
http://members.aol.com/illinewek/faqs/casting.htm
Unfortunately, the first time that you get definitive proof that aluminum pots are not suitable for melting bullets may be when you're showing the doctor your third degree burns.
Retire the aluminum pot.