We are not any more bullet proof, but we are harder to kill- we weigh considerably more now, and have much better medical care.
a .36 roundball weighs what? 95grains? At 7-800 f/sec? 1000, even? and you have five shots?
You empty that into a big determined individual and you might kill him .... after he beats you to death with your "gun".
Is true, we weigh more but a hundred years ago more people were walking to work splitting wood and spending more house with a hoe or shovel in their callused hands instead of keyboards and forklift controls.
Big determined guy might beat you to death with 7 rounds of 230 gr hardball from a 45 or he might die with a single 40 gr LRN 22LR stuck thorough the middle of his forehead. Only problem with a firm and fast rule are all the exceptions to the rule. .36-caliber with a full charge pushes an 80-grain ball at 1,000 feet per second, that is gonna hurt.