Hopefully you got his name.
Now call the store & get in touch with the store manager. It may be that this guy was only the shift manager, or the kitchen manager, or the seating manager, or the cash register manager... IOW, someone with
no authority to be making store policy.
Alternatively, contact corporate.
Outback Steakhouse Corporate Office & Headquarters
2202 N West Shore Blvd., 5th Floor
Tampa, FL
33607
(813) 282-1225
Here's their webpage. At the bottom is a "guest survey" link and a "contact us" link.
http://www.outback.com/
If you use the "guest survey" link, I'd suggest using the bottom button, the one for general questions. That way it'll probably go to corporate. If it goes to the Missoula place directly, they might ignore or lose it.
Here's what I submitted through their "contact us" link. Feel free to borrow, but please make your email your own so they don't start ignoring form letters. (And check spelling, etc.)
Last night a friend of mine went to your Missoula location for dinner, as he has done many times before. As always, he was lawfully armed. This time, someone claiming to be a manager told him to go put his pistol in his car. Peaceful exercise of a civil right should not be a reason to turn away a customer. Please clarify that Outback's policy is to welcome all law-abiding customers, and contact the store management so they understand as well. I found out about this by his posting on a national gun-rights discussion board, and I'll let people know when you clarify Outback's position.