Well Rhuby, I would have said the same as you, but looking at the Cambridge and the OED, *they say that the expression slap-bang is indicative of excessive and reckless speed rather than exact positioning whereas smack-dab is given in the Cambridge as ‘straight or directly’. No derivation is given for either term but the first usage of slap-bang recorded in the OED is given as 1833.

There is also reference, dated as 1785, to a slap-bang shop. A squalid eating-place where you were required to put down your money on the counter at time of placing the order. I wonder if they sold the first Big-Macs .