I was going to form some brass for my 257 wby from 7mm rem mag brass. I did a couple rounds as a test and they look good but they are about .040" short in total length. Is there any reason that this would cause any excess wear in a normal weatherby mark V with the normal freebore that a weatherby chamber has??

No, they're fine to shoot as is (and will stretch on the first firing, anyway.) All brass has to be shorter than the chamber, or it wouldn't fit. The fact that your formed brass is a little shorter still is irrelavent.

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.040" short in total length

I shoot a lot of fireformed cases. Many are .05 short. Matter of fact I even trim factory that much short and then don't worry about triming for several reloadings.