Online sales will be 4/1 in today's market in most retail businesses that don't rely on an indoor range for sales.
For the first and 2nd-time AR15 buyer, you will be dealing with customers who have a very sharp learning curve ahead of them, who have read a few things online, and now they're an expert. Instead of trashing them and opting for the counter-guy gunstore clerk, use this as an opportunity for sales.
That requires the employees to be knowledgeable, and so far, I have only seen maybe one retail firearms store in the entire US that had employees who knew their AR15's and AR10's like nobody's business, and that was Shooter's Supply in North Carolina on Bragg Boulevard, but it's been a while.
I would stock mostly basic, but quality guns with standard handguards, then upsale them into float tubes with service if you have competent staff. Have maybe 1/4th to 1/6th of the AR15's for your bigger spender clientele. Also, the staff can make a lot of sales by having their personal guns there, with all the doo-dads, optics, and functional, quality components, and that will help with educating and up-selling the customer. Nothing like pointing to your Chevy/Ferrari on the lot of the dealership you work at.
Vortex Optics makes one of the best line-ups for AR15's and AR10's, because you have price points across the board for all customer types, and reliable units that are backed by the best warranty service in the industry, in the rare event you need it.
You can also offer custom-order service for your regulars, and cut between margins to get them back into the store. You'll want some good brand names that have good margins, like Surefire used to. Not sure what they're like now, but they used to have excellent margins for the retailer.
I would say that customer treatment & service is more important than the product line, but you still need reputable products. If you remember customers by name, smile big, and genuinely take care of their interests, they will remember that they had a positive experience in YOUR store, regardless of what cool doo dads Joe's Big Anal Rape You A-Hole Guns has down the street. I learned to know my customers by name, their needs and wants, interests, shooting disciplines, and did everything I could to take care of them.
If your owner and employees adopt a customer-friendly human interaction posture, backed with good prices and reputable products, and the owner knows how to manage a business (only a small % actually do), you will be immensely successful.