It won't look shiny...they use vacuum metallization and the rattle cans cannot achieve that finish. Infact the can lids on them are even done using that process, which in my opinion is false advertising of what comes out of the can. The only system that would work as a spray is this... but due to cost and difficulty of application its just better to buy a repro helmet.
http://m.homedepot.com/p/Alsa-Refinish-Killer-Chrome-Kit-KC-KCH-Kit/204171696?cm_mmc=shopping-_-googleads-_-pla-_-204171696&ci_sku=204171696&ci_gpa=pla&ci_src=17588969&gclid=CJuswcWtm8wCFQ8vaQodhvsI8Q&gclsrc=aw.ds
If you spray it with krylon or something you will get a frosty matte silver at best...