On the one hand, this being a private enterprise, they are certainly free to ban weapons in their show. The second amendment only applies to governments, not to private companies.
On the other hand, companies insult their customer base at their own risk. If you're putting on a trade show for hunting equipment, and you forbid guns (or even a particular type of gun), you're not going to get a lot of business. If you do it to satisfy some out-of-touch politician's pet cause, you're going to get backlash. Backlash costs money in the corporate world. Lots of it.