Hey Balf, long time no see
As well as the sear spring it can be the trigger on the Inferno that causes it to run in 'full auto' - the corner of the trigger (looking from the rear) gets a bit rounded where it rests on top of the safety catch. Put the safety on, hold the trigger in and the sear is dropped just enough to let the hammer go. Then it slips back off the sear again each time it returns, all the time the trigger is held in.
I would agree 15 is entirely possible
Ok all you Inferno users...go to the toolbox and fetch a file