An image at the bottom of the site's page suggests customers "pimp" their AR-15 with triggers to "step up that accuracy" and ammunition storage to ensure "easy reloading".
Christchurch Gun City's David Tipple said he didn't believe the advertisement was offensive in light of the recent mass shooting at an Orlando nightclub.
If we blamed the guns, "we're missing the boat", Mr Tipple said.
He asked people to consider "where the gun becomes the problem".
Mr Tipple said it was far more difficult to acquire a gun than it appeared on the website. "You can't just fill out a form". Acquiring a gun licence was a lengthy and difficult process.
"We're not selling them to people who would qualify in the United States."
The AR-15 rifle was generally used for sporting and pest control, he said.
Police told the Herald that the provisional number of people currently holding an E-Category endorsement to their firearms license is 7,409 as at 15 June 2016.
Currently, the standard AR-15 firearm can be owned by anyone with a Category A firearm licence.
"The process for getting an A-cat licence is stringent," a police spokesperson said.
The process requires individuals to meet the following criteria:
• They must study theory surrounding the arms safety code
• They must undergo firearms license safety training
• They must have sat and passed the test to receive the license
• They are vetted by police as fit and proper to own a firearm - this includes full checks on criminal convictions, associates, next of kin, a home visit and an inspection of the person's home security
"The AR-15 firearms may be used as a sporting rifle, as well as for pest control and by competitive shooters," the spokesperson said.
"If however the AR-15 firearm is modified with military-style attachments, it requires a Category E endorsement, which requires the firearm to be registered with police. A special reason (and permit) for importation is required if bringing any MSSA into the country."
Police had no significant concerns that the firearms were being sold on the black market.
"(We are) always working to address any trading that may be taking place on the black market but we don't have any information indicating that sales of AR-15s on the black market are a particular issue in New Zealand at the moment," the spokesperson said.
"Police encourages anyone who has concerns about someone who may have illegal or inappropriate access to firearms to make contact with us."
New Zealand firearms law specialist and barrister Nicholas Taylor said New Zealand gun laws were "the envy of the world".
"We got in very early with a system of law that distinguishes us from the rest of the world."
Mr Taylor said New Zealand had one of the highest rates of gun ownership in the world per capita, but one of the lowest assault rates with guns.
The focus had always been on the person possessing the firearm, not the firearm itself, and it had "proved to be a successful formula".
The difference in US and New Zealand gun culture stemmed from Americans' belief that it was an "absolute right" to own a gun, whereas in New Zealand, owning a firearm was "viewed as a privilege".
An incident like the Orlando shooting was unlikely to occur here, he added.
The shooter, Omar Mateen, would have "had his licence revoked a long time ago in New Zealand", as police here were "very on to it" when it came to gun licensing.
American authorities have identified the Sig Sauer MCX- a semi-automatic assault-style rifle - as the weapon used to kill 49 people and injure 51 others in the Orlando nightclub shooting. It is similar in its appearance to the AR-15.
• To be issued a gun licence in New Zealand, first time applicants need to complete a firearms safety course with the New Zealand Mountain Safety Council and undergo a background and security check.
• Applicants must be at least 16 to apply for a standard firearms licence. This will allow them to have and use unsupervised sporting type shotguns and rifles and pre-charged pneumatic air rifles.
• A special addition to licences is needed to: possess or use pistols for target shooting; collect pistols and restricted weapons or stage theatrical performances involving pistols and restricted weapons; and possess or use military-style semi-automatic rifles or shotguns.