I've seen a few threads about this, it has been done. The cheapest way I have seen is to use an epoxy to build up and shape the existing knob, or you can have it threaded. Try the search function.
+1 for S.A.C Mark does excellent work. Kinda spendy for a rimfire bolt though. You might try building it up first. I did this with some quick steel and had good results.
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: 300winman</div><div class="ubbcode-body">You might try building it up first. I did this with some quick steel and had good results. </div></div>
+1
I'm planning on do the quick steel knob when I get the time, looks like an easy project and a lot easier on the wallet than a tac knob.