Foolestroupe -

I've tried them. Locally they call them "dead-blow" hammers. Bill D. would tell you that you get them at "The Yard" in Wichita. $14 - $23 (US) for 18 - 32 oz (used/refurbished) last time I looked.

They work nicely when you have to do a lot of heavy hitting, as they kill the shock that's otherwise transmitted back through the handle and causes pain and suffering in mechanics, but most of them have a rather large face for breaking out auto glass. You'll get the glass out with one, but you'll probably also splatter it all over the place. And with tempered auto glass you'll probably have to hit it several times to clear a very large hole - unless you really scatter it.

They're absolutely perfect for driving tent stakes, except for their habit of going home with someone else everytime I take one camping with me.

A lighter (than the general run of dead-blows) hammer or other device that punches at a more concentrated spot will "fragment" a tempered window so that it can be pushed out quietly, and often once it's cracked lots of the glass will (sometimes) come out in "hunks" about like peeling a hard-boiled egg after you've "rolled" it on the table top to make lots of cracks in it. (Of course, with regional preferences you may not cook your eggs that hard?)

John