Aluminum and Heat
Well, the full answer depends on three things:
-The type/alloy of aluminum and any treatment it's had (tempering, etc.)
-The use of the piece (just a drop-forward? An expansion chamber? A tank?)
-And the temperatures and duration involved. (Was it tossed into a burn barrel and fished out within a few seconds, or was it recovered from the rubble of a house fire?)
When dealing with aluminum, you'll typically hear terms like "2021" or "6061" (an alloy designation) followed by a T number. "6061-T6" is very common. This means the alloy has been heat-treated (or tempered, or precipitation treated or age-hardened or whatever. Techniques vary.)
The difference between "annealed" (softened) 6061 and T6 hardened 6061 is fairly large- I seem to recall T6 being something like 30% stronger against bending over annealed.
Aluminum, like many other metals (not all) is annealed by heating it to or past a "critical point" and then letting it cool slowly. As far as most aluminum is concerned, this 'critical point' can be as low as a few hundred degrees F (paper burns at 450 degrees F.)
As a specific example, the alloy that most aluminum-bodied CO2 tanks are made of (I forget the number, it's not a "common" alloy like 6061) if heated to more than 300 degrees for more than a few minutes, can anneal to the point it loses fully 40% of it's strength.
Naturally, as far as a CO2 tank is concerned, that much loss in strength is extremely dangerous, and the tank should be junked/destroyed immediately.
The same problem exists for many markers and other accessories, like rail ASAs, bottomlines, gas-thru foregrips, "low pressure" chambers, expansion chambers, regulators... Basically anything that has to hold pressure.
And remember the gun body itself has to hold pressure too- think of the valve chamber and how thin the walls are on an Angel, or the valve chamber on a Spyder, etc.
A thicker-bodied gun, operated on lower pressures, like an unmilled 'Cocker body, could probably survive a full annealing without too much risk the body would blow out from the pressure. However, it would STILL be at extreme risk of other parts failing due to the weakened aluminum- the bolt that holds the ASA on, the threads that hold the valve in, the threads that hold the front block bolt, and so on.
Guns with thinner walls, like Spyders, or Intimidators or Angels, would almost certainly fail, probably catastrophically, upon being repressurized after fire annealing.
Nonpressurized items, like grip frames, drop-forwards, beavertails, sight rails, etc, would naturally not really be any danger. Easy to bend and ugly looking, sure, but they ain't gonna 'splode.
Bottom line: If it's been in a fire, toss it. It's not worth the risk.