Ruger says to use Weaver #12 bases, so I ordered a set and moved the rings around on one of my scopes to fit.  I was surprised to find that after doing so, the spacing of the rings no longer allowed me to mount the scope on my flat top AR.  I find it hard to believe that Ruger would drill the receiver in a way that the spacing of mounted bases would not be compatible with picatinny/flat top spacing.  Any thoughts?

Weaver bases have been around since 1930. The drill holes in the receiver match the optimum positioning of scope rings for the scope. The Picatinny rail system came much later. It is based off the Weaver system, but not always compatible when it comes to ring spacing on various receivers, as receivers come in all sizes. 

The grooves on Picatinny and Weaver bases are different width and spacing.  Similar but different system.  If you need to be able to move the scope back and forth, you can get adaptors that let you mount a Picatinny rail on top of a Weaver rail basically coverting the .22 to Picatinny.

I know I can mount a Picatinny rail using the rimfire rails.  I am dumbfounded that Ruger did not tap the receiver at a distance that was a standard Picatinny distance.  It's only about 3/32" off.  

So I've tried a couple different options for mounting a picatinny rail atop the rimfire rails and none have worked so far.  Not much choice here other than to have a scope dedicated for this rifle or have the receiver re-drilled and tapped.  Gotta give a thumbs down to Ruger on the design in this aspect.

Have found a set of bases that will work, but I am choosing not to use them.  They are the Weaver T-22 mounts.  They mount to the rimfire rail as opposed to using the tapped receiver for mounting bases.  They allow flexibility in mounting adjustment width of the rings.  There are downsides, however.  One, they are an extra 1/2" in height.  If you use the high comb stock with some type of pad in combination with low rings, this would work.  However, I am using the low comb stock with a pad so I can still use the irons.  Which brings up the 2nd point of you can't use the irons with these bases installed as they are too high.  Finally, I found it difficult to true up the bases to the centerline of the receiver as there is an adjustment screw on each side to attach the base to the receiver. 

 

I do expect someone (Weaver?) will have a base that mounts to the receiver that will provide Picatinny spacing sometime in the future.  Until then, I picked up a Vortex Crossfire to use as a dedicated scope.