I just learned that lots of materials shouldnt to be used. So i need one material. But how to seperate gun’s barrel and gun’s body in one material ? If i do metallic, everywhere is metallic. If i do polymer everywhere is polymer.I really need help in this case . Also quality is really bad . I want to avoid this low resolution too.Waiting for help guys i’ll be really happy . Thanks !
Who told you that? Why does that person hate you? 
Use as many materials as your project dictates!
Man im working on a game and i use UE4. Thats why i need that.
You’re probably going to have assign materials to vertex groups and bake it all out to a UV map, that way you have all your textures in one image file. Someone better versed in game design/UE4 probably should chime in at this point.
Yo have several solutions here.
You can paint your model with Layer Painter or Substance painter to put your different materials where you need and bake the final results to maps.
The other methode (the one i use most of the time): You create a high poly with as many materials as you need (the advantage here is that you can use fully procedural materials, meaning no seams) and then bake the metal, roughness, normal etc on a low poly mesh.
There are several addons to bake things BakeWrangler, Principled Baker or simply blender default bake methode, but you’ll have to plug metallic to emissive to get an output. Basically those addons do that for you without the assle to do it everytime
Hey! Thank you for responding.Then let me ask something.How can i get scrathes or grunges? Should i model them on high poly mesh ?
i can’t understand what you saying actually. 
If your scratches are really noticeable (so more like an impact than a scratch), you can sculpt it. Otherwise you can use nodes to add those effects. If i remember well, musgrave node works well for grunge. For scratches it’s a bit more technical.
Another solution would be to use a stencil in paint mode. You can then define a map that will blend two shaders together. Lets say you have a metal paint and a iron shader. You can say with the grunge that the paint will only cover the white parts, and so the iron the rest.
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