Use every weapon in your arsenal to fight the battle. The Navy is already blowing you off. Add pressure. Contact NHT, Congress, Navy league, Marine Corps Leaque, EPA, Sons and Daughters of the American Revolution, Grand Army of the Republic, Sons of the Confederacy, White House Council on Environmental Quality. There are also many unit organizations who will help you.
Find a cannon that was captured by a specific Military unit. Contact that Unit Veterans Organization or Association and make them aware of the loss.
You need help. But you need the right help. We can all write letters. But our letters won't carry the weight of the type organization listed above.
As a result of the American Recovery and Reinvestment Act of 2009 DoD had a change in their Financial mangement plan that requires them to account for Property, Plant and Equipment in two catagories; General PP&E and Stewardship PP&E. Under Stewardship is two subcatagories; Heritige Assets and Stewardship land.
In April all Agencies in DOD had to make a report to White House Council on Environmental Quality on what they were doing to comply National Environmental Protection Act/National Historic Preservation Act requirements.
You think the NHPA won't help you, you are very wrong. They are hot button right now.
As an old Navy guy, I understand your loyalty to the Navy, but the Navy, (Army, Air Force, Marines) need a bit of a boot to the backside and your boot alone doesn't carry much kick, but you add in the weight othose other agencies and groups, Navy will listen!
You can make a boiler plate letter send a letter to all these groups. Make the letter very basic about what the probelm is ask for help and direction. Ask for some one to contact you to help deal with Navy. Make the letter one page. Cost a sheet of paper, envelope and stamp.
And trust me from experience, Navy doesn't ignore any complaint from any source. I know from my time in Government when competing for budget dollars, one Agency does not like to get a letter of complaint or compliance from another Government agenciy.
And when you write your Congress-critter, be sure and tell them all the agencies you have contacted and how the have blown you off.
What do you have to loose?