I've been converting a school bus for several months, and I've driven thousands of 3/4" sheet metal screws with no issue. However, when I try to drive longer screws (3") through a couple inches of wood and into 14ga steel on the other side, their heads invariably snap off barely after seating in the metal.
I'm using #12x3" sheet metal screws with 5/32" pilot holes. What could the problem be?
Torsion strain exceeding screw strength. It's likely that 3" of wood is creating some friction, even though your pilot hole is nominally larger in diameter. You might try a 3/16"ø pilot hole and see if that reduces the amount of torque needed. Deep holes of narrow diameters in soft materials are never perfectly straight so the extra room of a wider pilot hole is a fast and cheap solution. I remember being warned early in my career to pay attention to the screw head type and do not expect the same results when you use a screw gun on a screwhead designed to be turned by hand (as traditional panhead and button head slotted screwheads are). You might also trying lubricating the screws to reduce the torsion load. Beeswax being typical and safe here for both wood and metals. You could expect that even though some heads didn't snap off, that they may do so in service - being under the same stress. You can also use a higher strength screw preferably with a head designed to be power driven.
you can use thicker screws and/or coated screws .. here in the EU we have SPAX .. heh, tough as nails they are! The screws with Torx heads are good as you can drive them hard .. if you use a hammer drill to drive them in you have the advantage of the tapping effect. last point .. If you have access to Robertson head screws .. the ones with the square socket .. you can drive them as hard as a Torx head and they tend to be cheaper