The director of Otago University's National Centre for Peace and Conflict Studies, Kevin Clements, said these guns getting into criminals' hands was hugely concerning.
He said the guns had a huge lethal capacity in the hands of gangs and could cause mayhem.
"If the police are capturing larger and larger numbers of them then it means this [gangs and organised criminal groups] is where they're actually located."
Firearm owners said an amnesty or seizure of military-style guns would still leave them in the black market.
The Council of Licensed Firearms Owners said it was a problem that military-style, semi-automatic guns were in the hands of criminals but was adamant there were legitimate, safe uses for them when in the hands of licensed people.
Council vice-president Michael Dowling said buy-backs had a whole lot of issues and wouldn't even target the right people.
"So if we took access off those who haven't done anything wrong, removed the firearms from the system and say it's all fixed, we still have illegal importation and the existing stocks."
- Newstalk ZB