The brad bit will usually go straighter, regular drill bit will work also as said, both need to be pulled out to clear waste, I will even tap stock/barrel on floor to get stuff out of the hole. But what is most important is the straightness of groove on the forestock both left and right then up and down. If it doesn’t match your layout on the side the stock, that hole is less likely to go where you want it. You also need blocks to hold the drill bit in that groove. I’ve found pushing too hard on the drill doesn’t help either, let the bit do the work. I know, it gets harder the farther you get, sometimes a little wax on the bit/extension helps. Sometimes you can do all things right but bad things still happen, the grain of the wood does crazy things.
Bob