Sounds like you made your way to the Delta "Contract of Carriage". The Contract of Carriage is the rules airlines and passengers must follow. Each airline is has one and is held to that document. I've found copies online in each airline I've checked, but I haven't checked for them all.
Delta's is here;
http://www.delta.com/legal/contract_of_carriage/index.jspOne of the relevant sections is on page 40, Rule 190 Baggage, H. Acceptance of Other Special Items, 9. Shooting Equipment (Sporting Firearms), sections a, b and c have been touched upon previously, but no one seems to have found "d" to be relevant to the OP's question;
d) Charges.
All standard baggage charges apply.
What are "standard baggage charges?
That would be found on pages 31
E. Checked Baggage,
5. Size and Weight Limits for Checked Baggage
Except as otherwise provided in this rule, checked baggage may not exceed a maximum outside linear dimension of 62 inches or a maximum weight of 50 lbs. Delta may in its discretion accept larger or heavier baggage, but may require payment of an excess baggage fee, as set forth below.
and on page 32;
b) Oversize Baggage
Delta may accept baggage with a maximum outside linear dimensions exceeding 62 inches, but not exceeding 80 inches, as oversize baggage. Each item of such baggage, if accepted, is subject to an oversize baggage fee of USD$175*. Except as otherwise set forth in this rule, baggage with a maximum outside linear dimension exceeding 80 inches will not be accepted as checked baggage.
I searched several different ways for a potential exemption for firearms, but found none. Section (d) seems to apply. And thus by my read of the Delta Contract of Carriage, Mr Barness got away with one, but should have been charged. The caveat is that each airline has slightly different rules outlined in their Contract of Carriage and I did not read his firsthand account, and thus don't know what airline his experience pertained to. If I missed the exemption he applied, I'd love to stand corrected.
I fly fairly frequently with firearms and have had to get familiar with the process and my rights. If I had to offer any further advice, I would suggest to behave professionally and politely if a disagreement in interpretation arises (your fate or that of your pocketbook afterall potentially lays with the person in front of you!), but don't be afraid to ask for a supervisor or station chief to review their contract of carriage with you.