Assembling and matching normals maps of different meshes to UVs

So i made a flamethrower using zbrush … i got the high poly then i baked normal maps for each subtool ( because you know it`ll be a mess of i baked the whole subtools as one mesh ) i want to import the flamethrower as one model so i “merged” all of the subtools and unwrapped them as one model … so my problem is matching the normal maps of every single subtool to the uvs of the unwrapped merged subtools mesh… ive seen people use photoshop but i think that its impossible in my case and here is why :

There you go … more than 80 uv islands … i could unwrap them again to get rid of the small islands which is impossible to match with on photoshop but the seams will make every thing worse … so ? how does this work ?? what am i supposed to do ?? how do they make all that complex weapons and guns on games ???
Sorry… its my first weapon model thats why im a noob xD.

Have a look at this tutorial to start with: how-to-bake-a-flawless-normal-map-in-3ds-max

Basically you want to have your low poly uv mapped all as one thing first so you don’t have to try to match everything up. Even though this is specific to 3ds max, the technique is the same for pretty much all modelling software.

My low poly is uv mapped all as one model … but i need to make normal maps for every part of my model so they dont intersect when baking them all at once