The bullet is supposed to penetrate 2" to 5" before the expansion or fragmentation begins. Depending on the impact velocity and size of the animal this may or may not produce an exit hole. Also, depending on these factors the exit hole could be small or very large. The vast majority of the energy is spent inside the animal so using the exit hole to determine bullet performance is not as clear an indicator.
To have a large hole at the entrance is not how they should perform. We have not seen this in our testing so it is difficult to say why or how this occurs. There are a few things that may cause this result.
Bullets traveling at high speed and at various angles can do strange things (or things contrary to expectations) when impacting with material that is not consistent. This is pure speculation on my part but it may be possible that forces and factors unknown produce greater pressures to the entrance wound resulting in tearing and/or blow back. A famous man was said to have his head go "back and to the left" when shot from behind. How such things happen is not known to me but they've been reported to happen.
Having thrown out pure speculation, let me relay a potential cause that has more genuine evidence. It is a fact that we made our Target bullets with thicker jackets to protect them from blowing up on the way to the target. What happens is that the heat from the friction between the bearing surface of the bullet and the rifling gets hot enough to melt the lead. Once the lead becomes liquid, the fact that it is so dense causes the jacket to tear open and the bullet fails.
This is a result that happens infrequently and under conditions that are uncommon in hunting rifles and situations. It happened to target competition shooters who use long, heavy barrels, hot loads and who are shooting many shots in a short period of time while racing against changing wind conditions using barrels that aren't cleaned all day after seeing multiple dozens of shots. Occasionally, under these conditions these shooters would experience bullet failure (melted core).
These are conditions that do not exist for hunters. Barrels are shorter, loads aren't typically as hot and shots are much fewer. The one aspect of this scenario that might apply to hunters is the condition of the bore. If the barrel has a rough bore or if the barrel isn't cleaned often it is possible that this will produce high levels of friction between the bullet and the rifling.
If this condition gets the bullet hot enough that the lead gets "soft" then it is possible that the bullet doesn't penetrate like it should. There is one way to test this theory and that is to try shooting an animal with our Target bullets. I'll admit up front that our Target bullets do not expand and fragment to the same extent as our Hunting bullets but they do penetrate and expand after penetration.
In your specific situation (large entrance wound) if you were to try shooting our Target bullets to see if this result changes I will be very interested to know what happens. We have not been able to produce either a large surface wound or a pencil through so we are having trouble sorting out this question. If anyone has experienced large surface wounds using Bergers and is willing to try this test, please let me know what happens at firstname.lastname@example.org
Very well put.