I have a pre-ban Colt, it has a threaded barrel and birdcage suppressor, but it had a sear block, (which has been removed), and no bayonet lug. I want to replace this front sight with one that has the bayonet lug on it. I have a DPMS front sight I bought, but its not drilled for the pins, I really dont want to put screws in it, I would rather have pins. Anybody have any suggestions on do's and dont's before I jump in to this with both feet? The factory sight has tapered pins, should I go to straight pins or roll pins?

As Tweak is hinting at, swapping out a FSB isn't a parts change out job.  It requires machining skills.

I'am a machinist, been doing it for almost 20 years. And have my own shop behind my house for personal use, and I do have a mill.

The holes for the pins dont bother me, I can put them in there. I was hoping someone might have some suggestions on indexing this thing to make sure I get the gas port in the right spot, and get the sight straight. I thought about using a piece of wire through the gas tube hole in the sight that I could use to locate the alignment with the port. And making a small index mark on the barrel to mark the center of the old sight that I could use to line the new sight up centered.

The measurements of these sights are close enough that I think I can put it back in the same spot as the old one. I figured I would need to mark the gas tube right where it goes through the barrel nut while installed in the old sight, so I could check the length once its installed in the new one.

Am I forgetting anything? Any other measurements I should worry about? How about any other ideas on getting the gas port lined up?

Quoted: I'am a machinist, been doing it for almost 20 years. And have my own shop behind my house for personal use, and I do have a mill. The holes for the pins dont bother me, I can put them in there. I was hoping someone might have some suggestions on indexing this thing to make sure I get the gas port in the right spot, and get the sight straight. I thought about using a piece of wire through the gas tube hole in the sight that I could use to locate the alignment with the port. And making a small index mark on the barrel to mark the center of the old sight that I could use to line the new sight up centered. The measurements of these sights are close enough that I think I can put it back in the same spot as the old one. I figured I would need to mark the gas tube right where it goes through the barrel nut while installed in the old sight, so I could check the length once its installed in the new one. Am I forgetting anything? Any other measurements I should worry about? How about any other ideas on getting the gas port lined up?

You're good to go.  Make sure to index your front sight post to your barrel extension indexing pin.  That shouldn't be too hard.  If you have a mill with a DRO, find the center of each pin on the old FSB, mark that zero, then install the new FSB and drill and ream there.  Repeat for the second pin.  You will need a 2-0 taper reamer and the appropriate drill for that.

As far as getting the gas port lined up.  Set up your barrel in a vice with the barrel extension facing up and set that as plumb, straight up and down.  Then rotate your barrel 90 degrees and your barrel extension pin and gas port should be perfectly horizontal.  Then index the front sight post horizontal and you should be good.  These are the critical dimensions and you should be careful.

Don't forget to account for the gap between the barrel shoulder and FSB, where the handguard cap fits.

There will be old holes in the barrel.  Personally, I'd fill them in with some type of high heat, metal infused epoxy and remachine the surface before I proceded with the new FSB install.

I'm probably missing a ton of details and mixing machining terms with other terms, but I could get it done with that basic description, I think.  Good luck.

I dont have a taper reamer small enough. Ive heard some manufacturers use a straight pin instead of a tapered one. Think a straight pin or roll pin would work? Or should I order a reamer?

Quoted: I dont have a taper reamer small enough. Ive heard some manufacturers use a straight pin instead of a tapered one. Think a straight pin or roll pin would work? Or should I order a reamer?

MSC Direct - 2/0 taper reamer


Your original Colt barrel has taper pins, I'd go with the same.

THX Mongo, I'll let ya know how it turns out. I'll dig around at work tonight and see if I can find a taper reamer that small. We dont use to many taper reamers, but I remember seeing a bunch in the tool room, maybe I'll get lucky! THX again.

Mongo, THX for all your suggestions, got it on there today! I found a reamer at work that worked perfect. I spent an hour measuring and marking and indexing, and did it all twice just to make sure. I used a piece of wire down through the gas tube hole, and looked through the barrel to make sure I was in the port. And took all my measurements, as far as distance from the barrel nut, distance to the muzzle. I checked the measurements I had taken from the muzzle to center of the old slots cut in the barrel where the old pins went through, and marked the location for the new pin holes on the FSB. Everything lined up great, checked and rechecked everything twice, drilled one hole and removed it from the mill. Reamed that hole by hand and put a pin in it. Installed the gas tube and checked all my measurements again, looked at it from all angles, even put it on the lower and put it to my shoulder. Once I was convinced it was straight and in the perfect position, I took it back apart and did the front hole with the back pin installed. During final assembly I put one drop of #620 Loc-Tite on each ring of the FSB. Took it out and shot it this afternoon and works perfect. I dont think I would try this on someone elses rifle, one wrong measurement could get in to some money correcting it. Great experience for me though, THX again.

Quoted: Mongo, THX for all your suggestions, got it on there today! I found a reamer at work that worked perfect. I spent an hour measuring and marking and indexing, and did it all twice just to make sure. I used a piece of wire down through the gas tube hole, and looked through the barrel to make sure I was in the port. And took all my measurements, as far as distance from the barrel nut, distance to the muzzle. I checked the measurements I had taken from the muzzle to center of the old slots cut in the barrel where the old pins went through, and marked the location for the new pin holes on the FSB. Everything lined up great, checked and rechecked everything twice, drilled one hole and removed it from the mill. Reamed that hole by hand and put a pin in it. Installed the gas tube and checked all my measurements again, looked at it from all angles, even put it on the lower and put it to my shoulder. Once I was convinced it was straight and in the perfect position, I took it back apart and did the front hole with the back pin installed. During final assembly I put one drop of #620 Loc-Tite on each ring of the FSB. Took it out and shot it this afternoon and works perfect. I dont think I would try this on someone elses rifle, one wrong measurement could get in to some money correcting it. Great experience for me though, THX again.

Sweet.  Glad to hear that.