Rifle, shotgun, or handgun? The 10 gauge shotgun at close range would be very high on the list, as would some of the buffalo rifle cartridges, such as the 45-110. The .45 Colt revolver was possibly the most powerful handgun of the day.

Not even close. There are several more powerful handgun cartridges, not counting the single shot pistols that use rifle cartridges. While Dirty Harry may have proclaimed it to be "The most powerful handgun in the world"- it is not.

There was no "the". Many cartridges were used. Some of the more popular were 44/40 and 45 LC.

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