I've spent very frustrating hours trying to catch wheeling seagulls with the 500mm reflex on my crop sensor A550. Almost impossible. It takes long enough to aim this lens at something that's standing still! I tried a version of the bead and notch kind of sight found on pistols and some other reflex lenses. Better, but you have to line up three things to aim the lens, the bead, notch, and eye. I found it easier to aim at static things, but still difficult for wheeling birds.
I thought I had discovered the hard way why BIF shooters use zooms such as 100-400mm f5.6.. They can use the wide end to catch the bird and then zoom in, removing the aiming problem, and the wider aperture brings in more AF points so the camera can do predictive focus magic. But they're much bigger and heavier than the reflex. I carry the reflex around a lot just in case. A lot of my best shots with it have been completely unexpected opportunities and I just happened to have it in my bag.
A bigger heavier lens is something I'd only carry when I was planning in advance to use it. Then I discovered these new red dot gun sights which seemed very promising. In a zero magnification version you keep both eyes open, thus seeing the entire field of view, but the eye looking through the apparent sight tube sees a red dot superimposed on the view. No focus differential problems either -- the dot looks like a projected laser spot, but is generated inside the optics of the tube. No need to lie up the eye centrally either. The dot stays in position in the view despite moving your eye around.
And if everything is lined up correctly, what the red dot is sitting on will also be sitting on the central spot AF sensor. But you can't line it up like that because of parallax. If you line it up for a specific target at a specific distance, as rifle shooters do, it will be off for other distances. So for unpredictably fast moving birds best set up with axis of sight aligned with lens axis, and offset your aim by the constant known parallax.
I tried mounting the red dot sight on the hot shoe, a popular method. But so wobbly that the sight no only has to be rezeroed every time you mount it, but every time you accidentally nudge it. So I experimentally mounted it on the lens body by gluing it to a cardboard tube of slightly less diameter, split so it could be sprung open and grab the lens body softly but firmly.
That worked very well indeed. Once zeroed in by using the calibration knobs (vertical and horizontal) provided, it keeps calibration despite being knocked and taken off and put back on. Using that was how I too the above flying seagull photos. The lens forces central spot focus. I set it to continuous focus and slow repeat. Then with shutter held down I try to keep the red dot on the birds head as it wheels around. First autofocus lock can take a while if the lens was far off focus, so it helps to do a quick focus preload on some something like a tree at the middling distance of the birds. Then autofocus locking happens much faster. And as so as it locks the shutter fires. And as the bird moves it tries to refous for the next shot, and only succeeds when you manage to get dot on the bird for long enough.
That was easily enough done that I had to use the slow repeat to avoid filling the camera with similar shots.
I'm a complete novice at BIF shots, but what was my most difficult BIF lens of all has suddenly become the easiest of all with which to catch birds in flight. Because I don't use viewfinder or live view, I simply use to the red dot to hit the bird with the central spot AF sensor. You can't worry about composition with wheeling birds. Well, I certainly can't yet! But now I have the ideal device for the shot I've dreamed of for years, and never got anywhere near with any lens -- our local herons flying up and down the small river which cuts through the back gardens of the city. Made difficult by being in a little often wooded valley, so there's plenty of obstructive shrubbery to obscure shots, and patterned backgrounds of trees or cityscape to confuse AF. An interesting challenge which no doubt will require learning skills and techniques I've as yet no idea of.
But this device of 500mmj reflex plus barrel-mounted red dot gun sight works well enough that the next steps will be reconstructing it in metal. I'll see if I can find suitable cheap lens collars, or perhaps open up a tubular lens hood in the same way as the cardboard tube.