There are quite a few of these out there. Which are the ones for the AR, I need to get one? Thanks

Any Harris bipod out there (as far as I know) attaches to a QD stud.  Harris makes a attachment ( adapter #5 I believe ) that sandwiches between the rail and the barrel that has a QD stud that pokes out through the holes in the rail.  If you have space to install that, any harris bipod will attach to your rifle.
    - Nw -

The 6-9" model fits the best.  If you're looking to shoot off the bench with it, you're better off with some sand bags.

I like the "L" model with spring loaded spring out legss in the 6-9" lenght. Plenty long at 6" for 20 round magazines, 9" work for 30 rounders both prone and off of a bench.

Most target rifles are equipped with removable sling swivels. The swivels attach to lugs that remain with the rifle when the sling is removed. Harris bipods are designed to attach to the forward swivel lug. AR-15s do not use this type of swivel so an adapter (Harris #5) is needed. The adapter is a swivel lug that attaches to the lower handguard. The hardest part od attaching the adapter is getting the lower handguard off. The bipod attaches to the adapter and you're off to the range.

The adapter comes with a removable swivel that attaches to the underside of the bipod so you don't have to give up your sling.

I use the 'BRM-S' model. The BR (Bench Rest) is the shortest model, it's legs extend from 6 to 9 inches. It's made for shooting from a bench and allows the lowest profile when shooting prone. Other lengths include L (9-13"), H (13.5-23"), and '25' (12-25" with three piece legs). The taller models are good if you want to shoot from a sitting position.

The M indicates the legs have grooves cut in them. The leg extension locks snap in the grooves making it easier to set the legs to equal length.

The S means the bipod swivels left/right. This is handy if you're shooting off an uneven surface. You set the legs to the desired height and use the swivel to level the rifle. The non-swivel models have to be leveled by setting the legs to different heights.
If you buy the -S option you will also want something called a Pod-Loc. The bipod swivel has a thumbscrew used to tighten/loosen the swivel. The thumbscrew is small so you can't apply a lot of torque to it. The Pod-Loc is a lever that attaches in place of the thumbscrew. It lets you apply enough torque to practically 'lock' the swivel in a given position. That allows you to let go of the rifle without worrying about it swivelling to one side and falling over. The height of an AR-15 with scope is enough that tipover becomes an issue.

The Pod-Loc is made by KMW Long Range Solutions. You can order one direct from them at 318-748-8732. They run around $24 with shipping.

Been through this one.  I am 5'10" 200lbs., stocky but no big gut (yet), but have short arms, 44 short in coat size, (this aint my bio so ya'll be patient).  Have my M25 sniper clone and AR15HBAR equipped to take the Harris.  Bought the LM6-9 first.  Not a good thing for me prone (which was the whole reason for buying a Harris....bench rest shooting I use a, uh, benchrest).  The 9-13 did the trick, can use it collapsed in the 9 on a bench or extend it prone.  World of difference for me, especially shooting anything with an extended mag.  My rec: get the leg notch model, LM-9-13, if you have any size on you, and if you feel a bit wild, get the swivel base (I've been told this is the it thing, but I'm simple and don't shoot much on uneven ground).  Best bet would be to get the 6-9 AND 9-13 and return the one that doesnt work.  Let us know how it works.  Happy Thanksgiving!
Cleophis