The law already passed in Missouri's Republican-led House and Senate, but Nixon vetoed it, saying it violated the U.S. Constitution's supremacy clause, which gives preference to federal laws over state laws, CNN reported.
The author of the bill, Rep. Doug Funderburk, told CNN he's optimistic legislators will vote to override Nixon's veto.
"This bill doesn't put one new gun on the street," he said. "It strictly says that Missouri is going to protect the Second Amendment rights of Missourians."
The law would allow state residents to own a machine gun, which is banned by federal laws. It would also nullify federal gun laws and make it a misdemeanor for federal agents to try to enforce federal gun laws, CNN said.
St. Louis Police Chief Sam Dotson said he worries what passing the law would do to crime rates in the state.
"(We are) basically saying to criminals, 'OK criminals, it's OK to come to Missouri. We won't prosecute you to the fullest extent of the law,' " he said.
If the legislature succeeds in overriding Nixon's veto, the matter would most likely end up in court, CNN reported.