Directly from Delta's website:

 Quote:
Shooting equipment is allowed as checked baggage only. It must fit within the very specific criteria that we outline below.
Declare to the Delta representative that you are checking a firearm.
Declare the existence of a firearm to security personnel if there's a security checkpoint before the Delta counter.
All firearms must be declared by the passenger to a Delta representative at the main ticket counter.
Present firearm(s) unloaded and sign a "Firearms Unloaded" declaration.
Firearms must be packed in a locked manufacturer's hard-sided container specifically designed for the firearm, a locked hard–sided gun case or a locked hard-sided piece of luggage. Handguns may be packed in a locked hard-sided gun case, and then packed inside an unlocked soft-sided piece of luggage. However, a Conditional Acceptance Tag must be used in this case.
Maintain entry permits in your possession for the country or countries of destination or transit.
Ensure small arms ammunition is packed in the manufacturer's original package or securely packed in fiber, wood, plastic or metal boxes and provide separation for cartridges.
You are responsible for knowledge of and compliance with all Federal, State or local laws regarding the possession and transportation of firearms. For more information about this regulation you can visit the TSA website.
If you are transporting a firearm to the United Kingdom, a permit from the United Kingdom is specifically required. You must contact the United Kingdom for more information about securing this permit.
Ammunition in excess of 11 lbs. per passenger or that contains potential projectiles is not allowed.

Pistols and accessories must be included in one case and contain:
Pistol telescopes
Noise suppressors
A small pistol tool case
No more than 11 lbs. (5 kg) of ammunition

Rifles and shotguns must be packaged as follows:
One hard sided case containing up to four rifles, shotguns, shooting materials, tools
One hard sided case containing up to five handguns, one scope, tools
One bow and quiver of arrows and maintenance kit enclosed in a case or container strong enough to protect the bow and quiver from accidental damage
No more than 11 lbs. (5 kg) of ammunition

An excess baggage fee will apply if checking more than one gun case.


I don't see any verbage about all firearms cases being charged as oversize.
The only guy I know that has gotten dinged for extra charges had a big metal case that just weighed a ton. They got him for overweight but not oversize

For you spotting scope, take the smallest case you can find or wrap it in a sweatshirt and put a rubber band around it..