Bennet won his senate seat in 2010 over Ken Buck, 47.7 % to Buck's 46.8%. Buck's campaign was some what lack luster if I remember correctly. There are 9 declared GOP candidates running against him, Neville is among the nine. There are also two 3rd party candidates, but they never really make a difference. The primary is June 28th, bust since I am a declared Independent I will not be allowed to vote in the primary. Primaries are closed here in CO. But I can change that any time to GOP and vote in the primary. Here's a link to a site that gives a list of declared candidates. https://ballotpedia.org/United_States_Senate_election_in_Colorado,_2016
I have not seen anything nor have heard anything to indicate gun rights is a big issue this election. The state legislature battled that issue a few years ago, banning large capacity magazines and changing some of the requirements for back ground checks for private gun sales. It's really not much of an issue lately.
I suspect that illegal immigration, Obozocare and taxes will be in the fore front this election cycle.