I had the same thing happen to me with a TC White Mountain Carbine that I bought off a guy several years back. Before shooting it, I replaced the nipple with a brand new 1/4 x 28 Tres Ampco nipple. When I shot for the first time the nipple flew back thru my hair missing my scalp.(thank God) The hammer flew back denting my stock as well.
I ordered a .255 tap and a .260 tap from TOW. I also ordered some new .260 nipples. I used the .255 tap first, followed up with the .260 tap. The new .260 nipples fit great and my WMC shoots great.