I’m just wondering if this is possible? I currently have a Larue fixed BUIS and am thinking about getting a Millett DMS or Meopta Meostar. Thanks!
Depending on eye relief and your preferred shooting position, it may not fit behind the mounted scope.
If if does, it’s liable to be in the way. With the magnified optic you aren’t going to have the leeway to move your head around like with an Aimpoint or EoTech, so you won’t really be able to look “over” the BUIS. I suppose if you had the scope elevated via a riser or tall mount, it may be well below line of sight.
I just got a Trijicon Accupoint 1.25-4x in a Larue SPR-E mount – I have not had the chance to shoot it yet, but if I flip up my Troy sight behind it, it’s definitely in the way, to me.
There is no reason to run a fixed BUIS with variable optics.
Even at 1x, the lens system of a variable optic will distort the front sight to the point that you will not be able to aim with it – because the front sight is inside the minimum focus distance of the optic, and because you’ll be getting severe parallax.
In addition, a fixed BUIS will completely interfere with your optic’s sight picture, especially if your reticle has any holdover markings.
Don’t do it…
My .03 Eurocents.
It wouldn’t be a bad idea to have BUIS sights in the event that your optic became damaged during use (no idea what your using this for) and you would then have a secondary sight system. Otherwise, what everyone else said.
I haven’t fully decided what I am going to do but I may buy a folding BUIS so I can use a variable power optic. I just didn’t want my Larue BUIS to go to waste.
I wouldn’t ever put it behind a scope for reasons mentioned, but I would however have the scope on a throw lever mount and have a low profile BUIS under the back of the scope. that would be my choice.
I meant what you just said. 
It won’t go to waste if you sell to me for a great price 
Using a low profile BUIS under the scope would be ideal. However, if it was me I would just pocket the Larue BUIS. And if you need it, take off the scope and put the Larue on the rail.
In the early days of the flat-top M-4/A4 with optics, I would keep the carry handle
in my buttpack and use it as a BUIS. If your optic goes TU, you’re most likely gonna have to remove it to deploy the flip-up anyways. Sure the flip-up is faster, I could see it has beneficial in some circumstances. Personally, if I were shooting with an optic, like an Aimpoint without a fixed rear, then the flip-up would be more beneficial. Because you can keep shooting without removing the optic. If I had to deploy the BUIS, and had to remove the optic to deploy them, I would just keep the Larue on my person and install as neccessary.