Let me say first the light blue stuff, or the stuff you have, is the correct stuff. If you look at the bottle, it should say which size threads it works best on, but you can use it in the rails and rings too for a scope mount.
Now that said, I also use the Instabond 124. The stuff never really dries all the way, it turn into a very sticky film, holds up fairly well, and is easier to clean off than the "proper" loctite for smaller threads. The 124 is used to put the lug nuts on a Stryker, it is for larger threads (not all blue loctite is for the same size bolts). This stuff isn't necessarily better, as in stronger, but it does come off easier.
We used it in the army to mount our ACOG's as it was available in every Stryker and the other stuff wasn't (so it was this or nothing). We'd put some on the threads, some on the clamp, and some on the rail as well help keep the mount stay "pulled" to the rear of the particular rails to which it was mounted. We'd use it in the rings on sniper rifles. We also used it on every other thread we encountered because it was all we had. The bottles are big, and the stuff less permament, so you could use a little more than you normally would with the "correct" stuff for smaller threads.
I still have a bottle of this stuff left, it "expired" years ago, but still works just like it did when it was new. I still use it anytime I plan on having to take apart something, like when I mount rails or scopes on rifles that I plan on finishing in the future. But after I finish, I use the appropriate stuff for a better hold.
There is also this new stuff called Rocksett. It is a ceramic industrial adhesive that holds up under high temp. AAC uses it to locktite their flash suppressors. Anyway, that stuff comes out like glycerin and dries like superglue (which is what they compare it to). It is considered almost permanent, but I understand soaking it overnight in water makes it easy to remove. I've never tried it on a scope, plus I've never tried removing it, so I'd definitely test it out first if you try it and figure whether or not it will work for you (for instance, you wouldn't want to submerge wood or most scopes overnight, but the rifle would be okay as long as it was cleand thouroughly and oiled up good after, although I can imagine hesitancy with a nice blue job). If it was easy enough to remove and the water soak didn't pose a problem, this stuff would hold up better than red loctite I think. I think it is worth looking into, and I may try testing it out in the future for holding down rails or whatnot semi-permanently. I may try using it on an FAL scope mount that I want REALLY locked down.