Child protection organisation the Parents Television Council has claimed 19 per cent of US game retailers will sell M-rated games to underage shoppers.
The PTC sent 109 children aged 12 to 16 to 109 stores US-wide, and found that 21 of them were prepared to sell the young spies titles ESRB-rated as suitable for those aged 17 years and above.
Ratings board the ESRB has dismissed the report as having "questionable methodology", but noted that it was an improvement on a 2008 study which found 35 per cent of retailers would apparently sell to minors.
"Frankly, the latest PTC member sting operation actually verifies the effectiveness of the ESRB rating system and the ever-increasing support it receives from retailers," director of communications Eliot Mizrachi told Gamasutra.
The PTC used its findings to support next week's ruling on Schwarzenegger vs EMA, a supreme court decision on whether the US requires further restrictions on the sale and display of violent videogames.
"The industry's PR spin about how ratings empower parents is specious if unaccompanied minors are able to purchase adult-rated games," said PTC president Tim Winter.
"A California law that would simply put consequences in place for retailers who sell exceedingly violent games to minors has been fought tooth and nail by the gaming industry and will come before the U.S. Supreme Court in a matter of days.
"We urge the Court to uphold the California law and heed the calls of concerned parents by requiring retailers to check IDs."