I have learned that unfortunately, you pretty much have to slot the tenons for the pins (though I do not know of any old guns where this is evident... of course I have seen them with busted wood around the pinholes, and with the tenons completely pulled out of the barrel...). If the wood has any cross grain or is curly AT ALL, it will expand and contract a surprising amount with varying humidity, stretching and shrinking the fore end. If the wood is STRAIGHT and level through the fore end, then, no, technically, no slots are necessary.
I learned some time ago from a gun I was making, where I had inlet the barrel during our normal swamplike humidity, and then took the barrel out to work on another part over the next day or so. During that time, the weather got nice and the humidity went down. Two days, now. I put the barrel back in and tried to put the pins in and they wouldn't go! The fore end, which had only minimal light curl, had shrunk in length nearly an eighth of an inch! The front two tenon pins simply were not going to line up, so I had to slot them. And now I slot all my barrel tenons. :wink: