A square tube steel 3.5 x 3.5 x .1875 thk beam would be 120" in length, and cantilevered 60". 60" would be through bolted and adequately supported/integral to a sheer wall. What would be the maximum weight at the cantilevered end without deflection?
Not being silly but zero applied load means no deflection. With the application of any load on any structural beam there will be a deflection - small but it will occur.
What is the maximum deflection you realistically need?
You can see equations and access deflection and stress calculators here...
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