Fully automatic firearms -- meaning those that fire in bursts with a single pull of the trigger and/or fire as long as the trigger is held down -- are indeed very rare and difficult for a civilian to obtain in the United States.
- There are only about 180,000 fully-automatic weapons in the US that are legal to buy and own as a civilian. These were all manufactured before 1986. These are very expensive and require months of registration process to transfer.
- It is possible, but very illegal, to modify many semi-automatic firearms into automatic ones. Fundamentally, automatic firearms are often simpler than semi-auto, because they do not have to stop the bolt or hammer from travelling forward again while the trigger is still pressed. Most semi-auto firearms are intentionally designed to make this conversion difficult to do.
- However, gaining limited access to a fully-automatic weapon as a civilian is not particularly difficult. These types of access are the main contributors to the enthusiast sites and videos you reference.
- Fully-automatic weapons can still be manufactured and used by certain businesses, police, and the military. In many cases the businesses that develop, manufacture, and service these guns for those entities are small businesses that may also allow civilians to shoot them as part of testing
- Many gun ranges and businesses own fully-automatic weapons through one of the above means, and will rent them out for a reasonable price, but only on their own property.
- Individual citizens who own one of the automatics will often allow others to shoot them at a public or private range, often at organized events where a few (or many) owners and guests get together.
- There are many methods (with or without additional equipment) to fire a semi-automatic firearm "quickly enough" to be effectively fully automatic, usually at the cost of severely reduced accuracy and handling*. Unfortunately, those disadvantages were not very relevant in the Las Vegas shooting.
There are two major pieces of legislation that significantly affect firearms ownership nation-wide (there are more, but these tend to come up a lot and are relevant here for general understanding). I will skip over details for the sake of brevity, but the major impacts to civilians were:
- the National Firearms Act of 1934 (NFA), which in practice requires a civilian to fill out forms and follow a longer process to register their NFA-item with the ATF (a federal agency), and pay a $200 tax per item. This process generally takes around 6 months, but it varies.
- Items regulated under the NFA include automatic weapons, suppressors (silencers), rifles and shotguns with short barrels (which includes barrels of now-standard length in the military), etc.
- Today this act also severely restricts importation of weapons or items in these categories, making them more expensive in addition to the tax.
- The Gun Control Act of 1968, amended significantly in 1986 to ban the possession of automatic weapons manufactured after that date.
In other words, to get a fully automatic weapon, you need to transfer it according to the NFA rules, registration, and tax, and it must have been manufactured and civilian-owned prior to 1986. This is the fundamental truth behind someone stating that "fully automatic guns are very rare and hard to get" in the US.
Note that because either the gun's receiver (the block that the barrel, stock, and trigger all connect to), or the sear (the part that holds back the hammer until the trigger is pulled) are the regulated parts, one can often build "a whole different gun" around a registered fully-automatic receiver.
"Bump Stocks" etc.
There are lots of good resources explaining bump stocks, but fundamentally they have been legal up to this point (since their fairly recent invention) in part because they do a poor job* of turning a semi-automatic weapon into an automatic one. Up to this point there has not been a legal line drawn between "attempting to produce an automatic weapon" and simply modifying a firearm or its trigger for better performance, such as a lighter, shorter trigger pull for competition shooting (which also facilitates increased fire rates).
"Trigger crank" devices are similar in that they come with major disadvantages, but may be more effective or their "loophole" intent more clear. Unfortunately the Las Vegas shooting is an rare (or even unprecendented) instance where these "seemingly scary" but generally highly impractical devices may actually have contributed to the loss of life.
Note: *It is difficult to reference a particular source for my claims that "simulated full auto" devices and techniques fundamentally reduce accuracy and handling, because up until very recently, no one had actually used them in a "real world" matter. However, fundamentally each method (bump stock, crank, belt loop, etc) tends to change the way the gun is handled in some significant way, such that it deviates from how the gun was intended to be used or standard marksmanship practice. In the crank and belt loop cases this is obvious. In the bump stock case, consider that the gun must actually be held loosely and pulled away from the shoulder while firing to simulate automatic fire, which is precisely the opposite of proper shooting technique for multiple accurate shots. As evidence, consider the angled and vertical foregrips typically seen on modern military rifles. These exist specifically to "pull back" on the gun, not forward.*
In general, the second amendment to the US constitution can prevent states from imposing major restrictions on firearms ownership, as happened with the removal of Washington D.C.'s complete handgun ban, but the topic of what firearms the 2nd amendment protects, for whom, and to what degree, is an ongoing debate in the US. States and counties therefore tend to implement gun control as strictly as their constituencies allow, and challenges to constitutionality may or may not affect the implementation of such laws.
The waters are therefore even murkier with state firearms law when fringe topics like automatic weapons are involved. Some states require registration with county law enforcement (beyond that which is already effectively required by the NFA), but county law enforcement can potentially refuse to register an automatic firearm for any reason. Certain states allow this "may issue" behavior (thus effectively making ownership impossible in a given county), while others either don't have such a requirement or have objective rules for registration instead of discretionary ones.
This is only one of undoubtedly several different types of additional restrictions that will vary from state to state, and is based on some research about states in the southeast specifically. The short version is that yes, laws do vary from state to state, but because automatic firearms are very rare, the laws about them are neither in the public eye nor the broader gun-owning public's eye very often, so the law in a given locale can be very unclear.