Don't have time to give you full feedback right now but there is quite a lot of optimisation and improvements you can make to the model, will just outline the main stuff for now but hoping others might chip in to help out with tips
First of all, what is the true tri count of this weapon, without any rounds?
Next the small tube thing on the left, underside of this weapon has too many sides for its size, and lots of parts of the model have extra edges round the cylinders they don't need all the way around them, removing them would save a lot of tris. The rounds also could use a lot of optimization.
In all you want to look at optimising this model quite a bit then you can spend the extra tris you've saved on the bits that need it the most, like that locking thing on the back, the main part of it looks pretty blocky and could do with more sides.
Finally for now, the barrel is incredibly thin right now. From what I can tell by the refs, it should be much thicker but more importantly than the refs, it needs to be a lot thicker than it is now (even if it means making it thicker than it is in r/l) otherwise your going to have serious zfighting issues with the round flickering through the walls of the barrel while loaded in the back.
Our RPG-7 for example has a much thicker wall than in r/l, also note how in this case, the inside has fewer sides than the outside, but this is something only really to consider for the 3rd person model, not 1st person, especially with a thin wall:
This mind you is our 3rd person model which needs to be thicker than the 1st person model since the zfighting is far worse the further you get away from an object so your 1st person can have a pretty thin wall and for 3rd person for your model, ideally you want to have it thicker too but could look into also making only the front and rear of the rounds visible but would mean you wouldn't have them in the reloading animation, but tbh, that might be worth it since having rounds disappear after being fired etc is a massive hassle and has draw backs like not being able to do proper LODs etc.