I have - or
HAD - shot the M-21 quite a little bit and find the platform WELL worthy of scoping and then returning satisfying results out to 800 meters. Some folks better. Ironically, I often shot out M-21s better than I shot our M-24s on the same day, with the same ammo. Weird, but there ya go.
Because of my experience with the M-21s in the Army - on my personal M-14s - I tend to stay with like-minded optics and as such, prefer vintage Redfields. Most particularly in the 3-9 ranges and Accu-Range. I wasn't particularly fond of the installed Leatherwoods with the *bullet-cam* but we were able to get a pile of former USMC Redfields from their M-40s and had them rebuilt. LOVED THOSE!
So, now that's where I go a lot as well as another favorite; Leupold VXII also in 3-9.
Vintage Redfields aren't too hard to find but make certain that the ranging *tombstone* tracks with adjustment. Glass, tube integrity and knobs are all easily dealt with if those are a problem on a used scope.
We also used a *fixed 6* Kahles on the M-21s and M16A2s quite a little bit. Surprisingly worthwhile for a non-adjustable scope as long as expectations were realistic as in not asking much more than 500 meters from them.
Now, to mounting. Since retiring, the single greatest *common denominator* in dissatisfied users of scoped M1As in my experience has had to do with a period of shoddy dimensions on mid-late 90's SA M1As as well as some of those early mounts made of cheese. I can't speak authoritatively to your Gen 4 as I've not dealt with one. Such is the bad taste in my mouth from their earlier mounts. I've got to think they've straightened those out by now. Mounting these is not really as intuitive as it might seem so I would certainly adhere STRICTLY to SA's instructions while watching for signs of a potentially rough interface with the receiver.
Additionally, I caution everyone, regardless the mount or receiver to be ready to possibly need to fully remove the mount to completely clean the mount and receiver points of contact (including threads) after some initial shooting. I always suggest that if the mount loosens - you do NOT merely tighten it up but rather disassemble to remove any loose finish granulation between the surfaces. Usually only need to do it once - if at all.
Todd.