Gun dealers are converting the controversial Adler rapid-fire shotgun so it can shoot twice as many rounds, sidestepping an import ban on a previous version of the weapon.

Plans to import the seven-shot lever-action shotgun were halted last July when it was banned by former prime minister Tony Abbott for six months, later extended to 12 months, only for a modified version that instead fires five shots in rapid succession to be introduced.

It has now emerged gun dealers are using a legal loophole that allows them to significantly boost the magazine capacity of the weapon.

One gunsmith and dealer in South Australia, who openly markets the "magazine extension tubes" via an official company Facebook page, is charging $250 to convert the Adler A110 to accommodate a 10-shot magazine, and one additional cartridge in the chamber.

The modification enables the weapon to fire 11 shots in as many seconds without the need for reloading.

The same gunsmith will convert the Adler A110 to its original seven-shot capacity for $165.

"Latest shipment of barrel clamps has finally arrived so we are shipping out Adler mag extension tubes to the top names on the waitlist(sic)," a posting in February on the dealer's Facebook page states.

"If you want in, we can put you on the pre-order list for the next batch."

The development comes amid revelations more than 7000 of the five-shot version of the Turkish-made gun were imported into Australia in the past six months, and as a review considers whether the type of firearm undermines national gun laws struck after the Port Arthur massacre.

The gun control lobby argues that technological advances mean even the five-shot version - sold under the least restrictive category of license which includes air rifles and paintball guns - should be classified as a semi-automatic firearm, warning tough laws introduced by John Howard in 1996 are being eroded.

The gun lobby disputes the claims.

Samantha Lee, the chair of Gun Control Australia, said it appeared the gun lobby had also convinced Justice Minister Michael Keenan that despite a ban on the importation of a seven-shot Adler, the same firearm with two less rounds was safe enough to bring into the country.

"We know that decision to be even more absurd as we now know the five shot can be modified to become an 11-shot," Ms Lee said.

Mr Howard has also rejected arguments from the gun lobby that the Adler A110 should not face further restrictions.

"The Adler lever-action rifle is being argued to be not within the ban and it's really a weapon that doesn't have the lethal capacity of automatic and semi-automatics," he said.

"I'm pretty dubious about that."