Many saloons functioned as bars/pubs in the modern sense of the word while also selling products for off-site use (as a modern liquor store does). A patron could buy whiskey by the glass or opt to purchase a bottle. I have not seen any analysis of alcohol content, but I see no reason to assume that it was anything but within the spectrum of possibilities available today (not all modern whiskeys have the same alcoholic content).

If someone bought an entire bottle, he could take it with him and wouldn't necessarily finish it at one setting - which would, indeed be impressive. I've seen primary source diaries, where it is recorded that "we demolished a bottle of whiskey" while traveling on a stage or during some other outing. Here we have two or more men sharing a bottle that has been taken off premise.