reyn 61: those groups are really pretty good. A nickle is less than an inch around, probably 3/4" at that and two of your groups are just about that, 3/4". That is really good.
As for your crappy rest - well, it sure served you well. You may be able to group slightly better from a more stable or comfortable rest but I would have no problems with those groups and it appears your new, used 300 Winchester Magnum shoots very well. Mikey.