You don't say what kind of guns your interest lies in... Most guns that are collectable, are listed in the Blue Book of Gun Values .
Many manufacturers never kept records, and, of course, serial numbers were not required before 1968. And in some cases, records were lost, or destroyed... either by intent or accident...
If one is enough of a rifle looney to care about serial #'s and values, then one should buy a copy of the BBoGV annually or at least semi-annually.
Hope this helps...
GH