went to gander mtn today, they have some guns that you can hold, but they are on a alarm cord

There is no Federal law or ATF regulation that says guns must be behind glass.

I really wouldn't do that. I've worked in gun stores since 1994 and there are two issues I foresee.

First is theft. Even if you removed the firing pin or the sear or some other important part, parts as easily obtained and not especially expensive. The person who would steal the gun is likely to be an ineligible person anyway, so the cost of the missing part isn't a factor. They want the gun. The typical means of securing guns can easily be defeated by a competent thief. He needs a couple of seconds of distraction and a small cutter to break an armored cable.

Your second issue is mishandling the guns. By this I mean morons who point the guns at other people and pull the triggers. Even though the gun is unloaded and possibly disabled, the other customers don't care. When people do stupid things with the guns at the gun store, other people leave. If I think the person is an idiot, the gun doesn't come out of the case. I've still had people point them at each other, pull the trigger as fast as they can, and say, "What? It's not loaded," when they are told to stop.

There was an issue with a gunstore in our area that had (some used) rifles wandering from their accessible racks. Their insurance carrier made them put everything behind the counter after that. Though more common for jewelry and coin stores, I can think of at least one gun store that has a 1 at a time examination policy as well (to prevent theft-losses).

It would also be imperative to check state/ local regulations. There are some locales where they do have specific legislation regarding how firearms are displayed in retail settings.