I am building a 15 wt blank and plan to a PAC Bay full wells grip. This will require that I more than double the size of the exisiting opening. On smaller blanks, in the past, I have a used a round rat tail file and slowly opened the hole to an appropriate size. I fear that this will take several hours since the butt of this blank is huge. Anyone have a faster way?

Many options. the fastest one I can think of is buying the cork reamers from mudhole or cabela's. you can make 'em yourself too. I use the round rat file also for very small reaming jobs by putting the file into my drill. Then I graduate sizes of cork reamers. After reaming holes that big, I always go back and fine finish the edges with fine sand paper on a solid flat surface.

If you use the file in your drill, be sure to run the drill backwards. If not, it will drive itself into the grip, and maybe split it. I have some first hand experience with this.
I use a lot of the premade grips, and have really gotten quick at reaming them out. First, the file in the drill backwards deal, then go to the reamers like Mudhole sells.

If you're using reamers that are built on rod blank pieces, you can Rod Bond and old drill into the butt and chuck it into a drill.

Mike

I finally got tired of the drill-and-file thing. Reamers are really cheap. They are much quicker than a file. And, using reamers, it's much easier to get the whole centered.

I guess I'm not the only one that epoxys old drill bitts into reamers. This does work great, but you must graduate size so you don't get too large of a bite and crack the cork. Some hand reaming may be needed. Running drill bits backwards is also a good solution. I only use the reamers for large holes over 7/8.