I've read good things about Sightmarks.  Any others?  Opinions? Experiences?

There are chamber type and there are the ones that go into the end of the barrel.

End of barrel
I have a LaserLyte MBS-1 unit that has a nice crisp and clear dot. Nicely made in the USA. It is my goto for all weapons except the AR15 as I use a cartridge type in there.

I have a Bushnell that is blurry and worthless. I bought it when I thought I lost my LaserLyte. Haven't used it again after I figured out where I put the LaserLyte.

Chamber
I have a no name cartridge type boresight that I prefer in my 223 as there is no margin for error in setting up. The dot is clear and precise like my LaserLyte.

I have no experience with the Sightmark units but the 223 Model SM39001 is a cartridge type.

Cabelas professional series chamber boresights are made by LaserLyte and the barrel end ones look exactly like the LaserLyte ones too.
The Sightmark is 1/2 the price of the LaserLyte units.

Have you looked here?
http://www.accurateshooter.com/gear-reviews/in-chamber-laser-bore-sighter/

Save your money.  Do it the old fashioned way.

Thank you Spunky.  That article really wasn't a review although sit did get me started.   It didn't really tell me about the muzzle mounted versions other than they are potentially unsafe and then shows what calibers are available from Cabela's, LaserLyte, SightMark and Aimshot.

I ended up getting one each of the SightMarks for each of my scoped calibers.

Oh and to answer H53 . . . . :  Because I can't look down the bore of those rifles

Why can't you look down the bore??

Remove your upper and remove BCG.  
Bore sight normally as you would any other rifle.  
The scope is what you're sighting in and it's attached to the upper.  So why can't you do it the old fashioned way?  All you're doing by bore sighting is getting on paper anyway.  

I wouldn't waste my money on a laser that you might use once or twice.  They suck anyway.  Good luck seeing the dot clearly in daylight at any kind of distance.  If you can see the laser you are probably close enough to not need it. Waste of money.  Only MY opinion, as always....YMMV.

I purchased a cheap no name muzzle type laser bore sight tool about 20 years ago and it was some of the best money spend.  I only paid about $15 for it back then so you know it is crap but it does it's job perfectly, which is to get you on paper at distance.  Without it you could waste a ton of ammo just trying to get on paper.  First of all looking down the bore to sight your gun in is not always an option as I use it on everything from pistols to rifles semi-auto to single shot.  Mine has inserts for calibers ranging from .22 to 50 cal and I have used them "all".  Some say you will use it once and never again.............not me.  I use mine all the time as I am constantly getting new optics or changing them around on my guns.  My collection is not that big, 46 guns, but making optic changes to just a few a month, which I do, made this thing pay for itself. a long time ago.  As an example I just picked up a new Burris FastFire III and mounted it on an FN FiveseveN pistol.  I paced off 25 yards in my basement and sighted it in.  Took it to the range yesterday with some other guns and when I went to finish sighting the pistol in at 25 yards, the windage was dead on and the elevation was 3 clicks off.  When I started to bore sight the pistol in I had to turn both windage and elevation so much I would never have been able to tell where the shots were going as the range has a metal bullet trap for a backstop.  Saved a bunch of expensive ammo right there.


As far at the type to get, cartridge type or muzzle type...............I would go muzzle so that you can use it on "any" weapon with a barrel longer then 4".  And as far as quality goes, I can't help you there because my piece of crap has been used often and lasted 20 years.  I am generally a fan of buying quality the first time whenever possible but I am not sure it makes a difference in this case.

JMHO,
MadDog