How they are trained is regulated by the states, GURPS. That's why maryland is the Old Line State. Because our state legislature (or at the time revolutionary legislature I suppose) set down fairly strict training requirements in our militias and things like uniforms and quality of equipment.
Here's where the consideration gets a little bit complicated; perhaps better said it's were specific intent isn't quite as clear.
When the Second Amendment was passed it only affected - it was only meant to affect - what the federal government could do. But when it came to that federal government it was supposed to be a strict prohibition: The federal government did not get to say who could have arms or what arms they could have or prevent people from bearing arms. The federal government was to have no authority - none, zero, zilch - to regulate the private possession of arms. Full stop.
The states remained free, based on their (relevantly unencumbered) plenary powers, to regulate arms possession and related things. To the extent there needed to be regulation regarding arms, the states would be able to impose it. (Here I'm using the term regulation with a somewhat different meaning than it had as used in the Second Amendment.)
But then the 14th Amendment was passed. It was intended, among other things, to wholesale apply the restrictions existing as against the federal government in the Bill of Rights as against the states. But when it came to particular restrictions found in the Bill of Rights, at least when it came to some of them, they weren't given much individual consideration. They weren't necessarily specifically considered and debated at length. When it came to adopting the Bill of Rights in the late 18th Century, each Amendment and each rights protection was to some degree considered in itself. That's how each provision got in. There were understandings (thought to some degree they differed among early Americans) of what each provision would mean, each had considered intents (though, again, sometimes those intents differed among various people).
When it comes to those same provisions applying as against states, they're all wrapped up in the words "[n]o state shall make or enforce any law which shall abridge the privileges or immunities of citizens of the United States." (At least, they were supposed to be; as it is now they're supposedly wrapped up in the words "nor shall any state deprive any person of life, liberty, or property, without due process of law.") The clear intents of the various rights protections aren't as well fleshed out; they weren't as well considered individually. So it's hard to say what in specific most people who played a role in enacting the 14th Amendment intended for it to mean when it came to each of the respective rights protections that it meant to encompass. So how are we supposed to know for sure what they specifically meant? How strict was the prohibition against infringing on individuals' rights to keep an bear arms - as applied to states, not as applied to the federal government - meant to be understood?
When it comes to the federal government, I think we can know that. We can know what particular concerns caused the Founders to enact the Second Amendment and based on those concerns it's fair to conclude that the federal government was not to be allowed any leeway whatsoever to regulate arms possession. When it comes the the states however, the concerns that lead to the enactment of the 14th Amendment were different - and they were more general as applying to a big swath of rights protections rather than being really clear as related to a particular rights protection. So, based on those different concerns, what's the proper understanding of how inviolable the right to keep and bear arms should be?
I think the best way to deal with such lack of specific clarity is to take this position: Well, they enacted the 14th Amendment to, among other things, apply the Bill of Rights to states. And to the extent they didn't qualify how any of the rights referred to in it may apply differently to states than they do to the federal government, we have to assume that they are to apply exactly the same way - or as near as possible to exactly the same way. If that creates problems - e.g., because of how state and local governments function differently than the federal government or because it leaves no government (federal, state or local) with, e.g., the authority to do anything at all to regulate arms, to even prohibit the private possession of rocket launchers - then there's an amendment process which we can use to address those problems. But we shouldn't just assume that, in enacting the 14th Amendment, they meant to - for practical reasons - have certain rights treated differently when it comes to state government actions rather than federal government actions. If they had meant that to be the case, they should have so qualified certain rights which were among those they were wholesale applying as against states. If we were to know try to figure out what they might have wanted to be the case, we'd basically just be guessing - or being more honest, just deciding what we today want the rules to be. And that goes against the point of having a Constitution and the kinds of provisions found in the Bill of Rights. We don't get to decide based on the circumstances of today what is meant by the rules created by the Constitution. We only get to apply those rules to the circumstances of today. To the extent we want to be able to decide today what the rules should be - what they should mean - we have to pass amendments to do so. And that's for good reason.
But few people that are in favor of so-called reasonable gun regulations want to admit that even they - even those reasonable gun regulations - are prohibited by our Constitution. They don't want to admit that an amendment is needed in order to allow any arms regulations to be properly enacted because they realize that although there'd be enough support to pass an amendment that would allow some regulations (e.g., such that private possession of rocket launchers could be banned or such that someone convicted of murder could be prohibited from owning guns if they ever got out of jail), there probably wouldn't be enough support to pass an amendment that allowed many of the other regulations that they want to have (e.g., such that private possession of so-called assault rifles could be banned).
There are some gun regulations that we probably should have (though for my part I think they'd be considerably fewer than what we currently have). But as it is I don't think any of them are constitutional. When enough of us agree on that, and that we need to pass an amendment to allow for reasonable gun regulations before any of them would be proper, then we can move on to having a conversation about what kinds of regulations are reasonable and needed.
