Let me answer by saying that it doesn’t matter at all what the most advanced weapons is existence then were, but that the Founders knew full well the weapons in existence then did not represent the ultimate level of advancement. It is childish to limit the state of arms to the date the Constitution was signed, for the Founders were alive before that date, weren’t they? Specifically in 1722, the British Army adopted the British Land Pattern Rifle–the Brown Bess. It was the first mass-produced rifle using standardized parts, and was in use around the globe for more than 140 years in some iteration or other, including adaptation to percussion-ignition rather than flintlock. The Brown Bess was a musket, or smooth-bore rifle. It wasn’t very accurate past 75 yards. The sheer inaccuracy of musketry is why the European battlefield devolved into the insanity of mass ranked infantry firing into each others’ positions on open fields. During the Revolutionary War, the outnumbered and out-disciplined American soldiers began the practice of wide-spread sniping, using the Kentucky Long Rifle, which was an actual rifled-barrel rifle, and not a musket. American snipers adopted the hide-strike-hide guerilla tactics to remove British officers from the fight at ranges beyond that of the Brown Bess, thus rendering the fiercely-disciplined British infantry squads and ranks impotent–they were trained above all else to do nothing without an officer’s order (a tactic later adopted by the Soviets). Many British units simply stood in rank and were slaughtered piece-meal after their Officer’s Corps and Sergeants were killed in the first fusillade.
By the time of the 1780’s, firearms had gone from being hand-made at foundries with only cursory attention to uniformity, safety and accuracy, to mass-produced, robust and accurate long-range weapons. Additionally, astounding concurrent advances were made in military artillery pieces and projectiles. To suggest that the Founding Fathers knew only of musketry and therefore considered it the penultimate in firearm design is ludicrous. It is to suggest that these educated men had no foresight, or appreciation of their own lifetimes’ history–when the Revolution was a direct result of suffering through that recent history. In fact, it could be argued that precisely because they had themselves in their own lives and wars witnessed weaponry advancement, they intentionally only wrote “arms” in stead of “muskets” into the 2A, allowing for as then undeveloped advancements in weaponry. When Ben Franklin was a boy, there had been no Brown Bess. Did he, one of America’s brilliant founding statesmen, suddenly become ignorant of the fact that weapons advanced in his own life? To imply that is to also say that these men were ignorant of the rest of the Industrial Revolution, with all its daily advancements in all areas of industry and convenience, occurring all around them.