Age is rapidly catching up to my old MSR-338. Over the years, my loading tube has cracked in three places, and is now held together with superglue (works great still by the way), but recently it became necessary for me to replace the hop up arm. The fangs on my old arm started warping, which meant the hop up nub was able to shift around. Needless to say it ruined the consistency of the rifle. To remedy this, I whittled up a quick 3d print replacement with an integrated concave nub. The STL is attached if anyone finds themselves in the same position as me.
As a side note, the arm is designed for use with a standard TM vsr bucking. The concave portion of the nub does protrude a bit far, so this might not be a good solution for rhop/maple leaf users as it will likely cause overhop. Minor modifications should remedy this. I can provide the Fusion 360 file for this upon request. I'd attach it but the forum says the file is too large :shrug:
As another side note, this arm might work for the Ares styker/MCM700x as well. I did see an arm that looks very similar in shape over on thingiverse (designed for the stryker), but I didn't try printing it and I am unsure if the dimensions match.