For the heck of it, I went down to Wally World and got two of the under $30 BSA [30mm?]. A year or two ago some people here were evaluating them. The consensus was that they were OK for the money. They are reputed to 'hold zero' --as about as much that you can expect a 5MOA dot to be said to be a zero in the first place. And they functioned decent enough in cold weather. The consensus was that for the money they were a pretty good and somewhat reliable cheap little RD.
I mounted the first on a 10.25" A2 carry handled M16 with a snap-on cheek weld. It did indeed do about 5 1/2" MOA at 100 yards and held zero. I was very impressed. The 2nd was mounted onto a pair of M4 handguards [with a bolt on Weaver rail adapter]. This is a 16" A2 Superlight and standard M4 handguards. This BSA RD is configured and 'zeroed' to co-witness with the iron sites on the carry handle and FSB. If your irons are zeroed, then you can co-witness your RD to set atop the FS right there in your living room and not even need to fire a shot. It works. It works except for one thing: the M4 handguards have 'play' in them. The handguards feel pretty tight, until you put a bolt-on rail and red dot on them, then they move about 12-19MOA at 100 yards!! I can move the RD dot L to R and even a little up and down by manipulating the handguards. So, you are best just to turn the thing off, and use your irons for day time aimed firing if you are firing at something more than 60 yards during the day IF you have it mounted on regular M4 handguards. However, if you are engaged in NIGHTTIME CQB, it might be an advantage in the Dark.
BTW, I have Trijicon Iron sights that have tritium vialed iron sights on the 16" Superlight. So, at night, the red dot 'sits above and on' the illuminated green vial post embedded in the FS post, and the dot is also positioned between the two little 'blurred' green dot vials on the large peep CQB rear site. These Trijicon irons help deal with the parallax problem in an inexpensive scope like this type. I would feel comfortable about such a set-up if I had to shoot at night.
I am not so sure about the carry-handled mounted RD at night, because even with a cheek weld piece, the parallax problem becomes more pronounced at night, because you cannot be as sure that you have got the dot positioned at near center in an inexpensive RD like this one.
One last thing: The dot is dim, almost non-existent in sunshine. In fact, in outside sunlight at day, it disappears! OK for overcast, but no good for mid-day sun.
Otherwise, I was surprised and pleased for such a low-priced RD. Also dropped the thing onto a concrete floor, and it still functions and keeps zero.
If you have never had a red dot before, and you do not wish to spend much in order to see if you would like it, go down to wally world and pick one up. You will need to get a handle adapter for an A1 or A2, along with a cheek weld piece [best cheap ones are for Carbines]. It might be too low for a regular flat-top, you will need a riser. It will be too low to be of any good if you put a rail on regular CAR handguards, and you would need a riser for those too.