I boughta sako a3 300win mag used. It has an aftermarket stock it seems to be pretty decent. I was always told you should have a floating barrel, this stock appears to have a shim at the end pressing against the barrel. I have never seen this done and would like to know if anyone has and whats the reason. Thanks

Barrel harmonics are different with all rifles. Some actually benefit from fore end tension.
If you were to put that rifle into other stock it might need fore end tension, or might need float.