2nd Amendment was for muskets
This argument sort of misses the whole point and intent, which the Founding Fathers wrote and debated about — which is the 2nd was about balancing power between the people and the government. So when the firepower changed, it needed to change equally for both sides (personal weapons). But ignoring the flawed premise, and lack of historical understanding is gobsmacking stupid. Let’s look at the technology, since at the founding of the country they had 8-shot revolvers, 9 shot "repeaters", 11-shot field artillery pieces, and Jefferson had a 22 shot repeating rifle. Not to mention "burst mode" automatics that fired up to 20 rounds with one pull of the trigger.
- 1 1400's gave us 2, 3, 4 Shot multi-barrel guns
- 2 1597 gave us the 8 Shot Revolvers
- 3 1600-1700 gave us 30 shot "repeaters"
- 4 1718 gave us the 11 Shot Field Artillery Piece (Puckle Gun)
- 5 1779 gave us the 22 Girandoni air rifle
- 6 1777 gave us fully-automatic weapons
- 7 Conclusion
1400's gave us 2, 3, 4 Shot multi-barrel guns
Some earlier repeaters just had multiple barrels, and you rotated the whole barrel (like a revolver) or had a barrel selector for each shot. In field artillery they were called "Volley Guns" (because they could fire a whole volley by themselves) like the one from the 1500's Ottoman Empire pictured to the right. By the late 1700's they were miniaturized into small pistols called pepper boxes, or the earlier multi-barrel turn-over pistols and rifles (2, 3, 4 shots). The volley guns go back to the 1400’s in the west, and some go back to the 1100’s in China (including exploding shells). There are other more dual-use designs like the four barrel Landsknecht mace (circa 1540), that combined a spiked club with 4 shots. The idea of multi-shot guns predates the 2nd Amendment by nearly 700 years. 
1597 gave us the 8 Shot Revolvers
You can visit Maihaugen Folk Museum in Lillehammer, and see a 400 year old German-made 8-shot flintlock revolver in perfect condition (circa 1597). But Italian 3-shot matchlock revolvers predate them by over 60 years. While flintlock or matchlock revolvers weren't common, they were well known. Certainly the Founding Fathers could have put in magazine limits to the 2nd Amendment, if they thought magazine capacity should be a reasonable restriction on liberty. The problem isn't their lack of imagination or information, it's with the gun-controllers arguments. 
1600-1700 gave us 30 shot "repeaters"
By the 1600's and 1700's we had sophisticated mechanisms that could hold balls on one side, powder on the other, and cocking the lever would add enough of each to be ready to fire in a repeated fashion (basically a lever action). The includes the 30 shot Kalthoff repeater (1600’s), the 7 shot Cookson/Lorenzoni repeater (1680) in rifle and pistol form (pictured to the right), and the Ferguson Rifle (used by the British, against America, in the Revolutionary war / 1770), and of course, the 9 shot mortimer (1790’s).
1718 gave us the 11 Shot Field Artillery Piece (Puckle Gun)
The Puckle Gun created in 1718 (73 years before the Constitution). Imagine an 11-shot field artillery piece that could fire 1.25 caliber / 32mm bullets or shot, and the founding fathers had no problem with private ownership of that! The relative firepower and survivability difference (e.g. their healthcare) between this multi-shot bazooka in those days, was far superior to a modern AR-15 in ours. Those whining about the Founding Fathers not imaging more firepower than a single-shot musket demonstrate only the breadth of their ignorance. 
1779 gave us the 22 Girandoni air rifle
One of the most impressive repeaters of the era is the .46 caliber Girandoni air rifle (circa 1779). It had a 22 round magazine (which it could deliver in under 60 seconds). It also has a speed loaders + extra reservoirs so that you could deliver 88 shots in under 5 minutes. Those pretending that muskets were 1 round per minute, are only underestimating capacity by about 18 times. And before anyone is silly enough to scoff at "air rifles", the Girandoni 700fps .46 caliber bullets is similar to ballistics of a modern 1911 .45acp, and it was used and the time to hunt bear and elk. Not only did Thomas Jefferson have one, but the rifle was most famously carried by Lewis and Clark. In fact, the reasons credited for why the Natives never attacked the expedition, is because whenever they setup camp in a new area, one of the first things they did is demonstrate the Girandoni rifle’s capacity to the indigenous tribes. Just the threat of that kind of firepower kept them at bay. 
1777 gave us fully-automatic weapons
Then there was the Belton Flintlock, which was demonstrated to Continental Congress during the Revolutionary war (1777), which stacked rounds in the gun (superimposed load), and fired them like a roman candle (either 8, 16 or 20 rounds). They would be expelled sequentially, anywhere from 20 to 5 seconds. This was a "burst mode" (automatic) weapon. Multiple rounds from a single trigger pull. But slow enough that you could control it and aim each shot. This lead to innovations like the Isaiah Jenning's repeating flintlock rifle (let them fire them one at a time). If these weapons of mass destruction were too dangerous for civilians, then why didn't the Founders pencil in restrictions against fully automatic weapons that were demonstrated to them before ratification? 
Many Founding Fathers lived well into the 19th century, not one ever thought to complain that technical advances were moving further than the 2nd Amendment (or 1st) should allow: that invention came in the second half of the 20th century, by collectivists that wanted to invent excuses take away our liberties. But that delusional construct has no basis in history, just in desire.
There is another argument that guns have gotten "more lethal over the years". This is true, but it totally ignores that medicine has gained in technology too. You are far less likely to die today from a gunshot wound, even WITH higher technology and higher velocity weapons, than you would in the 18th century. Furthermore, individuals back then were allowed to own artillery and explosives, that could do far more mass damage than our measly so called "assault weapons” could today. So the lethality argument can only appeal to the ignorant.
Remember, “The word is mightier than the sword (or gun)”, goes back to the 7th century. So by the luddites reasoning, the phone, radio, TV, Internet (twitter, Facebook, email and texting) should invalidate the first amendment too, as speech is a bigger threat than swords or guns? The gun-controllers seem to think the Constitution is a “living document” that protects tweets, emails, and texts — but the 2nd only applied to muskets? Puhlease. Of course, our individual rights are NOT dependent on the technology used — because they were declared to be the people’s right, not the right to technology. Thus we don’t only get free speech if you’re using a Franklin Printing Press, and your right to be secure in your home is not dependent on an absence of electricity, indoor plumbing or only using muzzle loaders (centuries after we had semi-automatic repeaters). We have our rights, regardless of the technology used to express them.
The 2nd Amendment was about the framers philosophy that the People could and should be trusted to be free (and to be able to secure that freedom with their own hands/guns), and Governments could not be trusted (and would try to take that away) — thus they called out specifically, multiple times, our natural and inalienable right to self defense, both in the Declaration, the Constitution, and in the debates and letters of the time. We can debate whether that’s anachronistic concept, or should we allow gun controllers to take it away, but not its original intent.
Thus the argument that lethality should change the rules, or that the founder fathers couldn’t foresee firearms other than single-shot musket rifles (or that it would matter, in any way) is based on gun-controllers' profound lack of knowledge on the topics of Guns, History, Constitutional rights (law), language, technology, and the founders original intent. Anyone that still insists on using those demonstrably wrong argument (after they’ve been corrected once), shows that they're either liars or fools. And the fact that they used these arguments before doing their basic research (and knowing what they’re talking about), hints strongly at the latter.
Written: 2016.09.07 Edited 2017