how do i make this like to the other side like symmetry i dont how to put it but i want this part of the gun to go on the under side like extrude something like shell

Like shift drag extrude ? select the outline edges and shift drag on the axis you want to extrude.

i have over 100 edges

You can select the outline edges in one go with a special and secret command.
Look the problem is that you should at least do the 3ds max tutorials that come with 3ds max.They are very good for beginners and even i have done them when i started with max.

surely he just wants the symmetry modifier ?

I work with Maya, and I don't know anything about 3Ds Max. Though, I do know how to do that in Maya, does 3Ds max have Duplicate Special?

If it does, goto Edit > Duplicate Special, and set it to mirror, and I think you have to position it where you want it. I don't know for sure, I just somehow know that answer. xD


[Warning] This could not be the correct answer, I'm not a pro and I literally just picked up Maya, this information is worth trying if 3Ds Max has that command. [Warning]

Editable Poly, Border mode, select the border loop.
Shift-drag on X.
Add a Symmetry modifier (under the TurboSmooth).