I don't think I have ever seen muzzle blast when shooting service rifle. Now, we have all seen muzzle flash when shooting in the dark...
And what info does the ejecting case tell you? If my best M14 doesn't put the empties in a small group, I would expect those out of the group to be possible "flyers" on target. My ar always puts them in a nice pile. But I never look at the flying cases in real time. How many eyeballs do you have, anyway??
With service rifles, I see the muzzle flash as a distortion of the target caused by muzzle blast. It's visible around the front sight as its seen through the rear sight aperture.
Ejected cases from the 7.62 NATO Garands I've shot are visible in the upper right part of my field of view around the rear sight hood on Nat'l Match rear sights. However, only when the ejector spring's "tuned" to make the empties eject out between 1 and 2 o'clock relative to the sight line; the target's at 12 o'clock. Often with new ejector springs, the empy 7.62 NATO cases get flipped back over the bolt and go just above the rear sight and under the bill of your ball cap. The come to rest on the bridge of your shooting glasses frying themselved into your skin above your aiming eye. I don't like that. And others will bounce off the shooter to your right; they don't like that.
I've tuned many a Garand ejector spring; 12 in one day. Took 12 of them I'd checked out for a major command's rifle team out to a local military range with a few boxes of M118 match ammo. Load a clip full, then hold the rifle waist high and normal and fire 2 shots. Watch where the empties eject. If they don't fly out between 1 to 2 o'clock, unload the rifle, use a combination tool to remove the extractor and ejector and its spring. Use a diagonol cutter to remove half a turn of the ejector spring. Reassemble every thing, load then shoot two more rounds. Repeat until the empies go forward and to the right. Grab the next rifle and do it again.
I don't know about AR's built on M16 style of actions. They may well respond in kind to the same ejector spring shortening. M14's and M1A's do as their system's the same as M1's.