I was able to purchase some Hornady critical home defense HP 9mm rounds for a very reasonable price. They only had 115 grain rounds left though. I noticed they also make 124 grain and 135 grain rounds and maybe even 147 grain. Will it really make that much difference? I've been told don't go below 115 or above 135. So many opinions.
You are correct about the plethora of opinions.... here is mine...
It very likely doesn't make much, if any difference to the average citizen.
The 115gr bullet is the lowest bullet weight in "normal" 9mm ammo. The 147gr is the highest weight in "normal" ammo.
There are differing schools of thought as to what makes the best defensive load. There is the "lighter, faster" school that says expansion is the holy grail, so a lighter bullet (for caliber) going as fast as possible will expand more/be better. This was somewhat "proven" by data collected in actual shootings back in the 80's or so, by Marshall and Sanow (IIRC) that found the 125gr hp load in .357 mag (light and fast) was THE best defense load, period. It had a record of one-shot-stops in the upper 90% range... 95? 97? can't remember exactly, but it was far and away the most effective. It even beat the .45 ACP and .44 Magnum.
Others say that penetration is paramount. Heavy/slower bullets. I think that is why the FBI adopted the 147gr HP in 9mm for a while, at least. Better penetration, without being OVER penetrative. Whole books have been written on this topic...
To me, bottom line, I will only be using lethal force as a LAST resort. It will very likely be close, because I have no business shooting at someone who is 25 yards away, unless they are running at me with a machete, or some other crazy, hypothetical situation.
And, for me, nearly ANY bullet weight defensive ammo will work, if it is applied correctly. (in the right spot, etc...) I try to not get too hung up on deciding on, and demanding what is "the best".... I try to buy the best I can afford at the time. If, at the time, all I can find/afford is FMJ, that's ok with me.
Like I stated earlier.... I carry the Federal 124gr HST's.... a friend and I found some at a good price, and split the cost of 3 or 4 hundred of them. I still have about 2 1/2 boxes of them. I shoot enough of them through my 9's to make sure they function flawlessly... then save the rest for "carry". Range shooting is done with 115-124 fmj's, mostly.