How do you install a front sight base and ensure it is straight?
Is there a jig that indexes off the barrel pin?
Seems like you could make a "ring/plug" that fits over the barrel extension, and indexes off the barrel pin?
Of do you just set it up by eye?
Do you clamp the FXB on, then drill it and pin it?

I understand front sight bases are not really universally drilled.
So how do you put a new front sight base on?
Do you just drill new holes?

What types of things can go wrong?
What do you need to watch out for?

Quoted: How do you install a front sight base and ensure it is straight? Is there a jig that indexes off the barrel pin? Seems like you could make a "ring/plug" that fits over the barrel extension, and indexes off the barrel pin? Of do you just set it up by eye? Do you clamp the FXB on, then drill it and pin it? I understand front sight bases are not really universally drilled. So how do you put a new front sight base on? Do you just drill new holes? What types of things can go wrong? What do you need to watch out for?

I'm building a jig right now to do just this, but I have a feeling that I will have some prototype/refitting to do before it is a done deal.  I've got a working drawing and a model sketched out on a piece of wood.  Off to the machine shop soon to try my first prototype.

The barrel and FSB must be held in perfect alignment and drilled as a unit.  If you don't do it that way, your chances of success are ZERO.

The holes are first drilled slightly under sized, then taper reamed to the proper fit of the taper pins.

What can go wrong:

1.  Your FSB can be canted so far to the left or right that your rear sight will run out of adjustment before you have it zero'd

2.  You can block your gas port some, hindering the operation of your rifle.

Here's the process I would use to transfer one FSB to another barrel.  I would start with an new, undrilled FSB for this process:

We use Devcon "A" metal at the power plant where I work to do repair on valves, pumps, etc.  This is an epoxy infused with steel.  It sets up nearly as hard as steel and is machinable.  Since the area on the barrel is nonpressure containing and is fully supported by the FSB, I have no doubts that this material can hold up on a rifle barrel.  We use it in systems that see 1000PSIG and temps of around 550F.  I would fill the old holes/slots in the barrel with this material, let it cure then machine the gas block back to smooth.  Then I would install the barrel and FSB in the jig, properly aligned and clamp both of them down so that they cannot move.  I would then drill/mill the first undersized hole through both the barrel and FSB.  I would then followup each hole with a 2-0 taper reamer and test fit the taper pins as I go, to ensure full engagement through the barrel and FSB.

To get this correct, you really can't take any shortcuts.  Not an easy process and machine tools are required to get it done.  Not a WECSOGer project.

So you don't think with a jig, someone with mechanical intelligence, someone could do it with a drill press?

Quoted: So you don't think with a jig, someone with mechanical intelligence, someone could do it with a drill press?

The only way I would do it with a drill press is if, first, the drill press is pretty stout and, second, I'd shorten up on the whole process, meaning, the drill bit is as short as possible, your table is as high as possible and your clamping fixture has to be very rigid and stable.  

I would recommend an indexable table vise also, so that you could drill one taper pin hole, then move your barrel/FSB/jig with the indexable vise and drill the next hole, that way you know that both taper pin holes are at the same plane along the axis of the barrel, assuming you set it up correctly to start with.  

This isn't a process that should be taken lightly.  You screw up a barrel and you severly limit the repairs that will fix the problem, especially if you had your heart set on a standard FSB for that barrel.

Thanks guys.
Just trying to understand the process.

Why are you replacing the FSB?  Just to have a BAYONET LUG?

If not, why not just go with one of these?

www.armalite.com   Downloat the catalog.

P. 32, M15A2 Clamp-on Front Sight Base.

You can do it with a drill press as long as you can set the index pin so it's 90 degrees to the drilling direction and a way to prevent the sight from shifting.  A long brass rod that is a tight fit in the barrel can be used if you don't have a degree wheel or angle blocks.  If using an already drilled front sight base, just TIG or plug the barrel holes and then use oversize taper pins (#4).  Drill, ream, and pin one before doing the other.