Quoted: I've just removed my SIR 46, and am mounting my aimpoint with the arms mount directly to the upper receiver.  I added the spacer to raise the optic to the same level as it was with the SIR. It looks like it is more susceptible to getting knocked off line.  I bumped it firmly with the heel of my hand a couple times, and it seems that the reticle did indeed move a bit in relation to the iron sights, with which they are co-witnessed. Any opinions or experience with this setup?  Thanks.

The ARMS #22M68 with full spacer is a stable platform. The ARMS mount can shift on the rail a slight bit...



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Quoted: Is the spacer setup less stable than the SIR setup, with just the mount?

Well, to be honest I am not a fan of screw on risers and believe that mounting the #22M68 onto a SIR rail or a #38 is more solid setup as far as I am concerned....

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While in theory a solid mount is probably more solid than a mount with spacers that are screwed in, I have never had a problem with the screwes on any of my ARMS mounts coming loose. It wont make a difference really if you use just the #22M68 mount alone on a rail like the SIR, or the #22M68 in spacer. Either way them ount uses screws. With just the mount, two short screws fasten the saddle to the mount. With the spacer, two longer screws go in to the bottom of the mount, through the spacer, and into the saddle. Same amount of screws either way. One way or the other, I have never had a problem with screws coming loose on any of the mounts I use (ARMS and others) or have used to build others guns. I always use a little bit of lock-tite and make sure they are snug before mounting them. In addition, almost any ARMS mount I can think of which requires screws comes with a small bottle of lock-tite as well.