Hello everyone! I purchased ruger 556 model 8537 with Free-Float with M-LOK handguard. I would really appreciate mounting recommendations for a Nikon P-Tactical .223 3-9x40mm Riflescope – BDC 600 Reticle scope. Thanks!
Since that model has a Picatinny rail that is at the same height as that milled on the upper receiver extending down the full length of the top of the hand guard, it would be possible to mount that scope using separate rings, one on the receiver rail and one on the hand guard rail
The conventional wisdom is that it is best not to bridge across the junction of the hand guard rail and the receiver rail since there can be a little bit of wobble in the mounting of the hand guard on the barrel nut which could cause some shift in alignment of the scope rings. But I have not tested that theory so I don't know how much accuracy might be affected, if at all.
If you want to mount a scope using only the Picatinny rail milled onto the receiver top, unless it is a very short, compact scope (which the Nikon P-Tactical 3-9x40 is not) you usually need to either use a one-piece cantilevered mount, or rings that extend forward in order to get the ocular of the scope far enough forward for proper eye relief. This is especially true if you ever plan to shoot from a prone position. If you are just shooting off a bench or standing, the scope can usually be mounted farther back.
Most all one-piece cantilever scope mounts designed specifically for the AR platform are going to mount the center of the scope tube 1.5-1.6" above the top of the Picatinny rail. If you want your scope mounted lower you might be able to find a one-piece mount with less height, and you can certainly find rings with a lower mounting height, but you won't be able to go lower than about 1.25" or so because you still need enough height for the objective bell of the scope to clear the top of the Picatinny rail on the hand guard. So unless you really need to have the scope mounted a little closer to the bore, I would recommend a one-piece cantilevered scope mount mounted on the receiver rail only. Of course, you will need a mount for a 1" diameter tube (not 30 mm) to mount your particular scope.
You can pay hundreds of dollars for such a mount and get an extremely good one in the process. But for more casual use you really don't need to. I have two one piece scope mounts made by Monstrum Tactical that have served me very well. You can find them on Amazon dot com for just under $30.
You can have a really great scope with all the bells & whistles, but it only as good as its mounting system. If your loose your zero or can't get it mounted correctly because your mount is crap then your scope is useless. If you don't want/need to spend a a lot of money on a premium mount you can go with the Burris AR-PEPR mount . It has a picatinny on top of the rings where you can mount a RMR for super close targets. You can have 800 horsepower mustang but if yours tires are crap you are not going to have the full potential.
I use American Defense mounts on all my AR's and Precision rifles. It allows me the ability to remove optics a wedge as many long guns as possible in the safe. They are pricey but have absolute return to zero when remounted.