Interesting, Where do you put that shim?
Under the hook or over it?
A problem I've seen with drop in barrels is sometimes there isn't enough wood removed under the tang where the barrel hook goes into the tang,
not under the tang itself, but where the hook goes through the tang.
It could be that someone in the past tried a drop in barrel and made some modifications to the tang.
Those drop in's usually need some fitting, usually the at the hook. If someone removed metal from the tang's slot instead of the offending barrel that'll mess things up.
Those Hawkens with a 1" channel are an older model, they only had them for a few years. When you remove the tang, is it held in with two wood screws or one wood screw and a bolt?
It's a curious thing as I haven't seen too many factory fitted TC barrels have that problem :idunno: