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They shoot a pin (usually 2 1/2" long for a 2x4 wall) through the 2x4 into the concrete (usually about one pin per bay along the wall). The pin should only penetrate about 1/3 of the slab thickness to prevent the slab from cracking. They also recommend testing the concrete with a fastener and a hammer to see if the concrete is either too hard or too crumbly to use the powder actuated method.
If you can't use this method I would just drill a 1" deep hole with a masonry bit and use regular expanding concrete anchor. Over the span their strength will be enough.
[This message has been edited by geko (edited January 25, 2002).]