Related: Was this ban truely inspired by ethics? Or did the Pope have ulterior motives in calling for a ban on crossbows? Did the advent of crossbows threaten the papal state?

To add on the topic (If it derails op then please be free to remove)

• how was it enforced/punished

• was the ban as wide as the Catholic world or was the law enacted only in certain location?

• How long did it last?

I.e how serious and how widespread was this ban?

I can very briefly add to /u/MI13's fantastic comment they linked with some background to this kind of situation in a historical legal sense. I study International Humanitarian Law, so that's the brief background I have and where my answer will focus. I hope this somewhat sheds light on why the ban was only against Christians, and more broadly that these sorts of bans are very rarely for truly ethical reasons.

One of the tenets of early Christianity, as characterised in IHL anyway, was pacifism within and external to the faith. To make a very long story very short, this becomes gradually less and less tenable as Christianity grows and begins to meet tension with other faiths and groups, and the Church and religious scholars began to legitimise the use of force in certain circumstances deemed "Just" - essentially that the state declared it, the cause was a good one and your intent was "pure" (ie to restore peace). This starts as early as the 6th century AD, and continues developing for a long time. Early scholars (around the time of Canon 29) made no real effort to limit the means of war, which from my understanding was fairly normal for the time - people were more focused on Jus ad bellum, the law of what brings you to war, than Jus in Bello, the conduct of war.

In relation to missile weapons and why they were only banned for use against Christians this is important - it likely wasn't for ethical or humanitarian reasons in the sense that people would mean nowadays. If it were, the ban would be considered jus in bello, but my view is that it doesn't cleanly sit in that category. It's more of a continuation of a pacifist tradition amended to the reality of life in a world where you unfortunately can't be pacifist, attempting to minimise intra-faith conflict and harm without any real authority (/u/MI13 explained this much better than I can). I'd characterise it as more of a political statement than an ethical one, about minimising harm within a community but treating that as separate and distinct to what would be considered "just" wars at the time.

So in short: it's kind of about ethics, but only ethics in relation to conduct toward other Christians, rather than an attempt to regulate warfare and its methods on a grand scale, and definitely not about being an ethical legal ban on a certain armament. To be honest, you find very few examples of purely ethical weapon restrictions up until relatively modern times, maybe starting with the St. Petersburg Declaration in 1868. Even that isn't necessarily a purely ethical consideration, but also a tactical one about limiting future arms races (which clearly didn't work out).

Sources: I can list you a pile on IHL as a whole, but I'd start with looking at writings by St. Augustine and Thomas Aquinas for this time period.