It's ridiculous to believe that a firearm is needed, or that it could kill in higher numbers, than any other method. With proper planning, preparation, education and practice, any weapon can be used to inflict the same, if not more, casualties as a firearm.
How much ammo is the person using? What caliber? How many mags? What capacity?
Lots of variables make one gun more lethal in a mass shooting than others. I'd guess a .22LR pistol would be less deadly than a .45ACP.
But I don't have to guess. History proved it. When the US Gov't banned so-called Saturday Night Specials, they did so with the knowledge that those small, rimfire, cheap and easy to hide pistols were involved in the vast majority of shootings primarily in urban areas. Logically, if you ban the most widely used gun, there will be fewer such shootings.
The consequences of that ban were anything but positive. Having fewer .22LR. .25 and .32ACP options, people began buying 9mm handguns instead.
The result was a huge increase in shooting fatalities whereas before most of the victims survived a small caliber rimfire injury.
So, all things are not equal, because not all guns are equal. A skilled knife user can easily kill someone quickly, whereas the biggest plus for a small caliber gun is you don't have to be as close to the victim. But the lethality isn't that much better than a knife that can be drawn across a throat versus a small bullet trying to penetrate clothing, flesh and bone before hitting anything vital.