I guess what gets me in the entire discussion, is why do some of you think that your right to carry a gun (which I happen to agree with) supersedes the right of the property owner to determine what goes on at their property?
All talk of 13 years, it is a good job, jobs are scarce etc, does nothing in my mind about the fact that it is "their" property that you are on.
You have a choice, just like most of us did this morning, working in the civillian marketplace, as to if you are going to go too work this morning.
As I told my crew two weeks ago, you hold out your hands, and one fill's with the crud parts of the job, the hours, being somewhere when you would rather be fishing, not being allowed to carry your gun, working with your co-workers whatever, and you weigh that against the other hand which fills with the good, the pay is usually central, then maybe benefits etc.
As long as the good keeps outweighing the bad, you get up, get dressed, and go to work. And when you do, you follow the rules that they have established. You wear the clothes they tell you to wear, etc. etc.
When they change something, then too me, you sit down and re-weigh the scales. For you, they just changed that gun carry deal, for me they just cut my (and every other Crew leads) pay $2 an hour.
For me, I weighed it out and came to work. I need the job and have 16+ years tied in. Sounds like you will do much the same.
That said, I will ask about the pay, the wheres the whys, and at the end, if they won't budge, I will just have to reweigh my decision and see where it leaves me.
I have been offered several jobs paying over 200 a year, but I weigh out the risks, and the benefits and to not find them worthwhile. (and at those jobs they would provide my gun

) and so far, I have walked from them. That said, several of my guys here lately have decided that it was worth the risk.
Well, I wander too much.
Weigh it out, and make your decision. If you want to stay, then follow the rules and speak with HR about getting them modified, or possibly them giving you relief from that rule.
Good luck, hope it works out positively.