As someone has already pointed out on this thread, rifling hadn't been invented yet, and any competent gun writer oughta know that.
I suspect that you are correct. I will point out, however, that rifling had been experimented with as early as the mid 15th century. Odds are good though that John Alden’s weapon was not a rifle.
Excuse me but rifling had been invented at that time, actually, even earlier. It was not used often as rifles were difficult to load fast after being fired a couple times due to black powder residue build up in the barrel and were also expensive, therefore smooth bores, or muskets, were normally purchased.
This wheel-lock is in the NRA Museum. Its exhibit description says that "almost all traces of rifling have been worn away". Sounds like a very early rifled barrel to me. If anyone is near the museum, it's open today. Go ask!
No. Actaully most of the major developments: spring loaded matchlocks, the wheellock, striking locks, and rifling all date to the same period around or just after 1500.
But because military use was volley fire at the time, only the first and cheapest became normal infantry equipment.
Because of the additional advantages the wheellock gave to cavalry - and that fact that they were already more expensive and the relative cost increase was less, horse soldiers were carrying multiple wheellock pistols from 1520. But because their use was short range, rifling was not required.
That leaves the more expensive hunting arms, and there is no reason that a civilian wheelock rifle would not be available in 1620.
The book THE AGE OF FIREARMS by HELD disproves this. I am right now looking at a photo in the book of ...
“Fig 101.Wheellock RIFLE of the Tshinke type developed in the north German provinces between circa 1585and 1610.”
On page 64 is another wheelock RIFLE made between 1610 and 1632.
At that time Guns were considered to have a devil ride the bullet as you could hear the devil scream when the bullet went past you, and the woulds were exceedingly deadly.
Rifling was in use as far back as the 1520. Accuracy was so good that the Necromancer Moretius (Herman Moritz) said the spinning of the ball caused the devil to be thrown off.
Everyone who talks guns should have a copy of THE AGE OF FIREARMS by Robert Held. It is chock full of interesting tidbits of gun lore.