Casting .257 with wheel weights

You'll just have to try it and see. Wheel weights are not pure lead. They're harder. My guess is that there isn't enough chamber pressure in most airguns to seat wheel weight lead tightly against the bore. Your question might be better posed to Neilson or someone of his caliber and occupation. But don't expect to be let on to any of their trade secrets. HA HA.
 
I mainly use pure lead alloyed with 2% tin. Some DO use clip on wheel weights (COWW) successfully and although their composition varies somewhat they usually contain .25% Tin 2% Antimony and the balance is pure lead. Stick on Wheel Weights (SOWW) are mainly pure lead but all can contain small percentages of Antimony and Arsenic as well. IMHO if it is a lead alloy from any of the above and it will shoot good from your gun then shoot it. If a barrel wears out from hard lead alloys then it was not a good barrel to start with is my take on barrel wear. Most likely this is somewhat of a myth carried over from brass airgun barrel days of long ago.
A lot of information on casting can be found here: http://castboolits.gunloads.com/forum.php

Thurmond