The horror in Las Vegas has forced us all to learn about the bump stock's role in the mass slaughter.

Now there's a petition before the Phoenix City Council to ban the sale of bump stocks. Tucson's City Council will consider a similar measure in two weeks.

But Arizona law stands in the way.

The bump-fire stock -- a bump stock, for short -- is an accessory for a semi-automatic weapon.

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Police say the Las Vegas shooter used a bump stock to spray his victims with bullets, much like a machine gun.

Phoenix and Tucson might soon be deciding whether to ban the bump stock. But the State of Arizona, with some of the loosest gun laws in the country, stands in the way.

Here's what Arizona law says:

"A political subdivision of this state shall not ... prohibit the ownership, purchase, sale or transfer of firearms or ammunition or any firearm or ammunition components, or related accessories."

The bump stock would be one of those "related accesssories" cities can't ban.

"Arizona has the most gun-friendly laws in the country," said Russ Richelsoph, an attorney at Davis Miles McGuire Gardner in Tempe.

"This statute is to make sure the cities don't prohibit firearms. There are a lot of restrictions on cities in this statute."

Two months ago, the Arizona Supreme Court affirmed the state's supremacy on gun laws, ruling that Tucson couldn't ignore state law on gun buybacks.

The Supreme Court case shows cities challenge state law at their peril.

The case against Tucson was brought under a year-old statute that allows the state to withhold revenue from a local government that defies state laws.