Does not work on Ranger Hardcore as you can not use Quick save (Manual save)
Quick method, Make sure you have 40 or more 'Chemical Resources'. This is used to clean your weapons.
Find a crafting bench.
Now if your like me I had 2 very dirty guns that needed a full clean which used up about 40 of my chemical resources.
You can get them fairly dirty by jumping in water, getting caught out in bad weather conditions etc.
After I cleaned both my guns I backed out of crafting bench and then hit quick load. This should then spawn you back in front of the work bench with all your chemical resources still un-used but with the achievement tracker progress still increasing.
So in short it goes like this
Find a crafting bench
Back out of crafting bench
Repeat the above
Here is a clip for example
You can earn this naturally if you would like but I thought I would post this for those who just want it out of the way
This also works for: