While I agree with your first statement and generally am okay with YouTube's intention here, I'm willing to bet that their implementation is going to be another shitty algorithm that just blocks completely unrelated videos

Right. There will be a false positive rate, and there will be gray area cases, as there always is.

YouTube's new policy includes a ban on even showing the use of magazines or belts that hold more than thirty rounds.

Ok, maybe there are parts that are an overreach. Does that mean the whole thing is a bad idea?

Yes.

Is this a general principle, or specific just to making it harder for people to find videos depicting how to make deadly weapons?

Both. I think it's deplorable to censor something without understanding it; I also think it's deplorable to plot to disable the free exercise of peoples' right to self defense. Furthermore, in direct response to the earlier question, I think the fact that an overreaching and vague policy can be pushed out is indicative of a bigger problem in their policy-making procedure.