Originally Posted by Gutpile Charlie
My question is, how high does the scope have to be mounted so as the front sight does not become an "issue?"
With a magnified optic, it generally isn't an issue except for those that Jedi mind-trick themselves into thinking it is.
The FSB is close enough to the optic that, even with fairly low-mag optics like a 1.5x, it is a blurry smear at the 6 o'clock portion of your field of view, and one that is functionally transparent. The optic can't focus on the FSB, and "burns through" it, in the same way you can see through brush with a set of binos if you're right up on said brush, yet still see clearly whatever it is in the distance you've focused on with said binos.
Primary example that pops up here all the time is with ACOGs. For example, a TA31 on the stock TA51 mount and mounted to the receiver has a height-over-bore of 2.755", and is a 4x32 optic. Fixed FSBs do not impede the usability of the scope unless you vapor-lock on the blur at the bottom instead of what you're supposed to be shooting.
Regardless, that can be enough of a mental hangnail for some folks that they are forced to choose between sucking it up, getting taller mounts, or ditching the fixed FSB. Which is fine, of course; everybody has their favored setup and should go with it if they have the means, but the blurry FSB is a mental issue, and the ones that find this troublesome are a distinct minority.
Somebody did a thread about this, having taken pictures of FSBs through various magnifications. I tried and failed to dig it up via search immediately prior to this, but it's floating around and could give you some visual cues that would allow you to decide for yourself, if somebody better at rooting around in the archives than I am can find it.