Test with clone painting (blender tool) but hard to have the correct perspective for this and some parts are missing to complete.That gun is really looking great!
Furthermore parts (handrails) are also reproduced in cloning and there are texture deformations.
I'm not familiar with a "clone painting tool", but sounds very helpful to get a good starting base texture to work from.
Yes, there will be parts that need to have perspective corrected, or completely filled in (missing from original image), or erased like the hand rails. I simply use Photoshop tools (skew, perspective, or warp) to correct, and copy parts of the real image and then reshape to make new parts. It can take a lot of work to apply real images to the texture map, but it looks great and you've done a wonderful job.
I have tried to use as overlay layer.RE: the 'white' poking through your overlay camouflage layer
I begin by desature at 100% the previous texture model.
Any way to suppress the white color (i try the replace color but withour result) ? Light colors are too visible through the layers of paint camouflage.
, I can only suggest that you will need to correct the base texture image layer (the green and white camo) by Photoshop copy, paste, and blend parts of the green colour sections on top of the white to cover it.
Then again, the render images you show us above with the green and white looks good just like that (after correction of course). Is the two colour camo not correct for WWI?
You could also try to use just the base green and white camo layer, duplicate this layer, and then erase parts off that are not brown (you need brown to make it 3 colour). Then use Photoshop colour correction to make that layer brown.
Nobody said texture production was easy, it is a lot of fussy work, but when you get it right, it feels good.