Chris: You compared cylinder heads to an AR? A cylinder head has water running through it to keep it from warping due to heat, and (in your case) 16 bolts in it. Aluminum heads warp all the time (usually due to mechanical issues) and pistons don't warp, they melt.
Every analogous example will break down under a microscope; by definition you aren't comparing exactly the same two things
The point I made was that when you choose aluminum for the right reasons and engineer parts properly, they succeed. It's when you don't use the right materials that things fail- example- use steel pistons in the engines I mentioned. And yes, you could further cite the fact that I probably (actually I do) run the engines a little rich if possible to hedge my bets- but that's turning up the microscope too high again