Internally adjustable probably means that the point of impact is internally adjustable. I have seen tasco‘s since the 80s and that looks earlier than anyone I have seen. If the focus adjustment is frozen, I would check the crosshair adjustment before spending any more time on it. A couple drops...
It just seems like iron sights work best on a handgun but a rifle is different. A good red dot can give you hits on a 12 inch target at 200 yards with a little practice and up close shots as fast as iron sights. I see no down side to a good red dot on a rifle.
You will only know what an optic looks like to you, after you look through it. I make it to sharpshooters pretty often. You can try one of my guns with an aimpoint T2 there if you like. Most ranges have rental guns, some have red dots on them. Good luck with your search.
Just saw you are...