I'm considering nabbing one of the new Ruger PC9s at some point, and since they come standard (for us in free states) with a threaded muzzle (1/2-28) I think I'd like to stick on a 3 prong (early m16A1) flash hider. Obviously the bore would need to be enlarged to accommodate the larger projectile.

I'm no gunsmith or metalworker, but I would think it would be as easy as holding it in a vice and boring it out with the right size drill bit; maybe a reamer if someone wanted to get real special with it. Am I way off base or what? If it helps any, I'd only be threading it on for looks more than function.

I drilled out an A1 birdcage on a lathe one time for a FAL project. If you were careful I don't see any reason you couldn't. I would probably drill from the threaded side of a 3 prong though. It would likely be easier to get and keep everything lined up.

That's the direction I planned on going from, being careful not to nick the threads of course, that and no sudden corners on the prongs to catch on the bit and ruin the part.

You could use a section of shot out barrel, say a couple inches or so, and thread it into FH, to use as a guide. Chuck it up in the lathe and drill through both. Or indicate it into you drill press 3 axis vise and go from there. Unthread the finished part and attach to the PC9. Just a thouught........

I installed one of those 3 prong (long) flash suppressors on my "shooter" M1 Garand about 10 years ago.  Took it to the range.  Shot about 8" high at 25 yds.  Cranked the rear sight down.  Still shot about 8" high.  Cranked it down again.  Still shot about 18" high.  Got to looking at the flash suppressor and you could see the copper on the inside of the prongs.  I was shooting with 2 other guys and none of us could believe it wasn't screwing up the groups.

I brought it home, took the flash suppressor off the barrel, laid it on the steps going off the back of the deck and used a 5/8", maybe larger, drill to drill it out.  Touched up the inside with some flat black spray paint, put it back on the M1 and the next time I took it to the range it shot really nice groups.  I used to take it to Ramseur to the field shoots.  No issue hitting the head/shoulder shot pop ups sitting in the holes behind the berms at 300 and 400 yds.

I must have gotten lucky and drilled that flash suppressor out "just right."

Of course, I always wondered about those A2 flash suppressors with no slot on the bottom (so it doesn't kick up so much dust out front of you when shooting prone) as they seem to not affect the groups.

I don't think it's going to be that much of a problem. First, the drill (and reamer should he choose to use one) are going to want to follow the existing hole, so it's going to be pretty close as it is, so long as a little care it taken setting it up. Second, it's a muzzle attachment, not the muzzle itself, so a little bit off(if it were to be) isn't going to make that much difference since it's not going to have actual contact with the bullet like the crown would. And there's no guarantee that the threads are 100% concentric to the bore anyway, so even if the device were absolutely concentric, it might not be with the barrel bore. Third, we're talking about a 9mm, so I don't think the OP was expecting sub-MOA anyway. If we were talking about a match grade rifle, then yes the extra precaution would probably be needed.

As for the prongs, I don't see a problem there either. Almost all of the flash suppressors I've bothered to measure have a larger ID at the front end, they kind of "bell" out. Granted, I've almost exclusively used A1s, A2s, and older style 3 prongs. When I drilled out the A1 flash suppressor for a project I did, the drill didn't even touch the front end. If the drill is still cutting by the time it gets to the end of the prongs, it's not going to be taking very much. So I don't think it'd be that much of a problem.

The other thing to keep an eye on is the material, and the "prongs". I have heard of people trying to drill / ream out the 3-prong style ones, and having the prongs "flex" out while cutting.

Check the material as well. Some companies heat-treat their muzzle devices, which will make it more difficult to drill out without the proper tools. Smith Enterprises is one company that claims they heat treat theirs.

I would be very worried about this.

Thin sections have a nasty habits of moving under machining loads.

I got a Carlson mini comp. it’s shorter than an A1 and ready to use for 9mm.