If my XD9 had an aluminum slide it would weigh about half what it does now(about 14 ouces). Anybody ever seen or heard of an aluminum slide? IS it feasable? Id like to make one

It's quite common in rimfire conversions for the 1911, and the Walther P22.

I believe Caspian made some 1911 slides in aluminum for an experimental gun a gunsmith was working on years ago, but obviously you havent seen them offered for sale publicly.

Lighten the slide up too much on a centerfire and you're looking at durability and cycling problems.

I can solve the durability part with hardened steel inserts where the most stress and friction occur. I dont know about the cycling part. Id Imagine you might have to change the recoil spring.

I have aluminum sliding on my house.Or is it SIDING?

You would have to be careful not to make the slide too light.  I read a very technical article one time about gun design.  It mostly discussed the HK roller locking system, but also talked about some other designs as well.  A fair amount of math was covered which talked about spring weight vs bolt or slide weight for a cartridge.  If you change the slide weight too much, you may need more than just a different spring to keep the piece functional.  A redesign of the locking mechanism may be necassary if your slide becomes too light...

A neat idea though, I would suggest taking care not to blow yourself up though.  Sounds like you have put some thought into this so I think you be able to avoid that...

The difference in cycling would likely be huge.  While a stiffer spring could solve it to an extent, you'd probably be looking at a spring so heavy that it'd be hard to cycle the gun by hand.  

Even when you get it running reliably, it may wind up cycling so fast that your magazine springs can't keep up, causing the slide to close on an empty chamber before it can strip a round from the mag.  I've heard of this happening in some gas-operated 1911 race guns, where nothing but Chip McCormick Powermags can keep up with the gun.

The physics behind the cycling of a locked breach handgun are quite simple.  You have two major forces resisting the motion caused by the force on the breechface (caused by gas pressure):

1.  The force caused by the spring.  This force is directly proportional to the amount it is compressed.  At the beginning of the cycle, this force is at it's lowest because the spring is at its least compression.  This makes the second component important.
2.  The force caused by the slide mass's resistance to motion.  This is the old F=MA (Force equals Mass times Acceleration) equation.  The lower the mass of the slide, the less force it takes to accelerate it.  Or, more appropriately in this situation, at the same force (chamber pressure), slide acceleration (and velocity)will be much higher if the mass is lower.

Increasing the spring weight probably won't solve all of the problems.  The XD's locked breach mechanism uses a cam on the barrel to unlock the barrel.  The angle of this cam is designed to keep the action shut until a certain point in time AFTER the bullet has actually left the barrel and chamber pressures have dropped to residual levels.  If the action unlocks while the bullet is still in the barrel and chamber pressures are still high, you will get a ruptured case head, and all kinds of problems.  

In other words, if you dramatically change the mass of the slide, the locking mechanism (cam, etc.) will probably have to be redesigned.

I'm not sure if you are seriously considering this project, but if I were you, I wouldn't even attempt it.

Hmm. Why stick with conventional locking?

Steyr GB anyone? Lock it up with the cartridges' gas..Light slide, light spring fixed barrel..

One could do this, just as one could make a frame/slide combination out of composites. The question is "why"? For a limited production piece, it's a work of experimentation, for a company it's a huge gamble.

Meplat-