The problem many forget or do not understand is that the down to velocity threshold that bullet companies give as the minimum velocity for expansion is for ANY EXPANSION. Most of those bullets at that velocity will just barely open the nose a little bit. To get what most would call expansion you need 200 fps or so more velocity.
Then you also have to take into account the bullet getting unstable at a certain yardage. The bullet depending on what twist barrel it is fired from will start to yaw below a certain velocity, usually below 1125 fps (speed of sound). A bullet that is yawing or is on the verge of yawing will often tumble on impact with flesh. A tumbling bullet, even an FMJ, will do considerable damage to flesh. That is the principle of the design of most FMJ military rifle ammo. Even back a far as WWII the M2 ball round with it's short flat base spitzer bullet would start to yaw and tumble between 4 and 6 inches of penetration in flesh even at close range. As a teenager I had a 1903A3 Springfield rifle and a man out the road a couple miles from my home raised cattle. There was a pack of dogs that had gone wild and started causing him problems and kill one calf. He paid me and a friend to kill these dogs. The first one that I shot was about 100 yards away facing me and was about a 40 pound dog. I was using the 03 with M2 ball ammo. I shot it in it's chest. Upon going to it it's rear end had a pretty large exit hole with lots of damage. We cut it open and could clearly see evidence that the bullet had tumbled causing lots of damage. I talked to my cousin that was still in the National Guard and was supplying me with a 250 round can of ammo every month when he went to muster and was a silver star awardee that saw a lot of combat on Korea about my tumbling assumption. He said yes that the M2 ball ammo definitely would tumble because he saw evidence on the battle field.
Jump ahead a number of years and out comes the M-16 with a barrel twist that would just stabilize the little 55 gr FMJ. When it hit flesh it almost instantaneously started to tumble and combined with the high velocity at shorter ranges created some pretty nasty large wounds.
So bullets do crazy things depending on construction, spin rate, at different velocities.
As has been basically said, out to the yardage that a Hornady 123 SST can stay above 2000 fps at impact it will not matter what velocity it was started out at if it is put in the vitals of an animal most likely the animal will die from the result with no problem.
Then you also have to take into account the bullet getting unstable at a certain yardage. The bullet depending on what twist barrel it is fired from will start to yaw below a certain velocity, usually below 1125 fps (speed of sound). A bullet that is yawing or is on the verge of yawing will often tumble on impact with flesh. A tumbling bullet, even an FMJ, will do considerable damage to flesh. That is the principle of the design of most FMJ military rifle ammo. Even back a far as WWII the M2 ball round with it's short flat base spitzer bullet would start to yaw and tumble between 4 and 6 inches of penetration in flesh even at close range. As a teenager I had a 1903A3 Springfield rifle and a man out the road a couple miles from my home raised cattle. There was a pack of dogs that had gone wild and started causing him problems and kill one calf. He paid me and a friend to kill these dogs. The first one that I shot was about 100 yards away facing me and was about a 40 pound dog. I was using the 03 with M2 ball ammo. I shot it in it's chest. Upon going to it it's rear end had a pretty large exit hole with lots of damage. We cut it open and could clearly see evidence that the bullet had tumbled causing lots of damage. I talked to my cousin that was still in the National Guard and was supplying me with a 250 round can of ammo every month when he went to muster and was a silver star awardee that saw a lot of combat on Korea about my tumbling assumption. He said yes that the M2 ball ammo definitely would tumble because he saw evidence on the battle field.
Jump ahead a number of years and out comes the M-16 with a barrel twist that would just stabilize the little 55 gr FMJ. When it hit flesh it almost instantaneously started to tumble and combined with the high velocity at shorter ranges created some pretty nasty large wounds.
So bullets do crazy things depending on construction, spin rate, at different velocities.
As has been basically said, out to the yardage that a Hornady 123 SST can stay above 2000 fps at impact it will not matter what velocity it was started out at if it is put in the vitals of an animal most likely the animal will die from the result with no problem.