How bout just split the upper and lower, pull the bolt, look through the center of the bore at 25 yrds (50 is what I usually do), zero the optic, reassemble, fire - if you aren't blind or retarded you will be on paper at your intial target, once confirmed move out to 100 and zero at your desired POI at 100.

Good luck

I used both the Tasco and Simmons bore sighters with spuds (or arbors if you want to call them that.) They're okay, but only okay. I have found that a lot of them have their grids off center, so I compensate for that. I try to use as little internal adjustment as possible when I mount a scope.

I'm not sure there are any "good" bore sighters out there, to be honest. I know there are no great ones. I'm not sure about the laser ones either. That's a lot of money to see coherent light spread itself over a target.

You can do what Face_In_The_Crowd does. That'll cost you nothing money wise and it is a tried and true method.

A "regular" bore site kit will not work due to the increased height of an AR sight set over the bore as opposed to the non-AR type rifle.  Offset spuds are available, but I haven't found them to be very reliable.

I currently use two types of laser bore sites.  One is the LaserLyte.  It has a tapered shaft with a set of mutli-cal tips that screw onto the end one inserts in the bore.  Select the proper size tip, attach it to the device, insert the unit in the bore, turn on the laser and start lining things up.  A word of warning, the best this unit does is get you windage close.  The laser is direct and your line of sight is direct.  If you try to set elevation, you can make the two points meet, but you still have to estimate elevation.  Most of the time, I will be on the paper at 50yds (min range at the range I shoot at).
www.laserlyte.com/

Another device is available that also uses a laser, but it is more expensive.  The AimShot laser unit is used in conjunction with different inserts, each in the specific caliber of the chamber (not just bore diameter).
www.aimshot.com/

NOTE: Bore sighting is a WAG aided by a tool.  A trip to the range is required to get things truley in line.  Good luck.

wal-mart sells a decent one, anodized red for like 30 sumthin.