Installing a front sight.

[Munster30]

I'm cutting down a 20 inch barrel on the 870 to 18 1/2 inches and want to install a new front sight.
That's the dilema; how to find TDC and get the sight right on?

[FAL guy]

I mark the right spot prior to cutting the barrel using the existing bead. I take a long string and place it around the existing bead, while pulling both ends back to the receiver. I pull the string taught, and twist it up until a long pointy vee is formed extending rearward from the bead sight. It helps to mark the barrel where your cut is going to be made first, so you can tell the best position to stop the point of the vee. The string is then centered on the receiver and taped into place.

I then use a scribe or punch to indicate the spot to be drilled and tapped.

works for me.