I had my front sight base off and I noticed that the two taper pin holes are almost 1 1/2 inches apart which is exactly how much to long my 16inch barrel is to put on a bayonet. So I slid the sight base up 1 1/2 inches and installed the front taper pin through the rear hole of the SB. I took the gas tube off and ran to the range with my new mid lenght single shot. Windage was real close. Now I ordered a smith gas block that I am going to try to cut down and install with just the rear pin (acctualy I think it uses set screws)

Then I'll need to drill a new hole for the front sight base pin. This is where I need the help.  I can't get my mic on it to tell if the hole is tapered?  If not what size hole is it? If so, where can I get the correct tappered bit?

I was going to install a Daniel Defense FF tube anyway, now I can just order a midlenght (I think I'll have to cut it down a little) and my project will be done, and for what ever it is worth I'll be able to attach a bayonet to my 16" gun.

The tapered holes were probably cut by a tapered reamer after first drilling a pilot hole. (Colt holds the sight base in alignment on the barrel, drills through from one side, then runs a tapered reamer through from the other.) I do not know of a source for the reamer and even still you would have the problem of cutting the barrel notch without enlarging the sight base holes. Here is one way tou could do it:
Pin the sight on with the rear pin. Working through the front hole, use a miniature round file to begin cutting the new notch. Once the notch is accurately located, remove the sight and finish with a file the same diameter as the pin. The original front notch is what you are trying to duplicate, so you can use it as a reference. Work slowly. When the pin fits in to it's original depth you are done. As you approach the final depth, you can use a marking pen to coat the cut surface. Drive the pin in (with the rear pin already in), And it will wipe off the marker from whatever high spot still needs work.
Just a note: Standard round (rat-tail) files are tapered. Chain saw files are straight.

Thanks.  That sounds like a lot of work.  I don't know if I'm up for it.  I may try it but what about tapping the front hole for a set screw or something like that?

RJL357

I got to thinking about what you said.  Your talking about just filing a notch not filing a hole.  Mine is an HBar (.75 inches at the gas port) so the taper pin holes are just that -- holes with material all around them.  If the barel were narrower I see how they would be more like notches in the barrel.  Anyway I think I could still just file a deep notch.  When I thought you were talking about making a tappered hole with I file I had visions of my last root canal.

I still don't see why I could not drill a pilot hole with the sight base on and then pull it off and finish with a tapered reamer.  Except that I don't know where to get one.

Yes, that would be a hole and not a notch. I should have asked about the barrel first.
I have a Colt pin that I put the calipers on. The small end is .127; the large is .141. The original pilot hole was probably .125 (1/8"). You could drill through from the small side, although the curvature of the barrel is going to push the bit off center and elongate the sight base hole somewhat (depends on how long and sharp the bit is). A 1/8" end mill would be better but should be done with a mill. I have used a drill press for simple plunge milling like this but the precautions are many. And then you still need to find the reamer. You may be able to file the taper in with a miniature file.
If you want to drill and tap, a 10-32 thread will require a #21 (.159") drill bit. If you want a smaller screw, an 8-32 would require a #29 (.136") bit. This will  give you full thread over about 2/3 of the hole length (small hole side of sight and barrel) and a large percentage of thread on the large hole side of the sight. I would go with the 8-32. You will have to look at your sight and decide if you will have enough metal around the new hole. especially if you want a recessed head of some type. This all assumes a Colt size taper pin.

I think I'll try filing a notch like you said orriginally.  I think it will give me the best end product with the limmited tools I have.  The first thing I'm going to do, after the Smith GB arrives, is make sure I can cut and fit it.  The I'll make sure it functions before I go cuttin on my barrel.

Thanks again for all your help.

Drill out the front hole of your site base and tap it.  File down some pins the same, or slightly smaller in diameter than the hole, at an angle coincident with the curve of your barrel, at that point, and install these ahead of your set screws.  Many match rifles have their site bases installed this way.

Damn, something else to think about. I thought if I used set screws I'd have to mill some flats on th barrel.  I see how ground pins would act like wedges and tighten every thing down real nice.

Good info guys
Thanks