Philadelphia's public employees union is asking gun-related companies in which it has invested pension funds to do more to keep guns out of the wrong hands.

Philadelphia joins other cities and states trying to exert pressure with their investments, with its own twist

The pension board said it will take its money out of companies that won't adopt Mayor Michael Nutter's "Sandy Hook principles," named for the horrific school shooting in December in which a gunman shot 20 children with a semi-automatic weapon.

The eight-point proposal asks companies to support federal legislation requiring background checks of every gun buyer and for them to re-evaluate their own sales practices.

The mayor's chief of staff, Everett Gillison, compares the initiative with divestment from South Africa during apartheid.

"We believe the same can happen here if we begin with one step and all journeys begin with the first step," Gillison said Friday.

Note that Philadelphia's is a small step -- emphasis on small in the world of pension investments.

In all, the pension fund has $15 million invested in gun-related companies. The largest chunk is $6.3 million in Walmart, which sells guns. Walmart did share any comment upon request.

However, proponents are counting on cumulative pressure.

Philadelphia's public employees join Chicago's municipal pension members, New York state employees and California teachers. All have started to back away from gun-related investments.

Last month, pressure from the California teachers led the capital equity firm Cerberus to sell the company that made the Bushmaster rifle used in the Connecticut shootings.

A representative of the Pennsylvania teachers union said the only gun-related holding in its pension fund had also been in Cerberus.