Newbie questions. Just bought the M&P and I have a Harris bipod. What would be the best mounting option? On the bottom of the gas block there is one small threaded hole and a larger non threaded hole. If I am not using the bipod, what are my sling options? I am assuming I can use the smaller threaded hole for a sling stud. Last, I wanted to get a Bushnell banner illuminated reticle scope, but I couldn't get it mounted without making it difficult to grab the charging handle?

Yea, I have that rifle too and would like to install a front mounted sling on the gasblock
I havent seen any option to do that tho. I am thinking a single point sling will have to work for me.

Shamless bump

The larger hole in the bottom of the gas block is for the quick disconnect swivel sling mounts.  I'm not 100% sure about the use for the threaded hole.

The smaller hole is threaded for a standard sling mount.

The smaller threaded hole is threaded for a set screw that centers the gas block and may help the stability of the gas block, although it is pinned in place, maybe since there is only one pin the set screw is for extra stability.  If it is threadeded the same thread as sling mounts, that could work also for your purpose. However, as a previous poster noted the larger hole accepts the quick detatch push button type sling swivel.

Any help with the bipod? I don't want to buy a whole new hand guard w/rail.

What I did with this thing,(S&W OR), at least temporaily, mounted Brownells Modular Gas Block  on my barrell just about 3/16" foreward of the gasblock and also bought the bayonet lug which is extra $ if you do decide to go this way.  I have a Versa Pod Bipod with the bayonet lug mount so it clips on & off easy.  You will also get the plus of some rails for mounting whatever if you want to attach them.  Cost around $65.00 altogether if memory serves.  Also,  the top rail of the gas block is lower than the rail hieght gas block on rifle.  Perfect for the Lasermax laser I mounted on it.  Vesa Pod has Mounts that will work with the factory plastic AR handguards.  I'm sure Harris does also.  Check Harris Bipods web site.  Check out brownell's for their modular gas block.

Hope some of this helps