Three stupid things.
Every year after deer season I get a few contacts from disgruntled hunters complaining of the inadequate performance of whatever combination of cartridge-bullet they happened to be using. Most fall into three categories. The first is the match-grade bullet under-loaded "because it was the most accurate load." Match bullets are not good hunting bullets. They can certainly kill, but their performance is iffy at best. Reducing theft velocity further handicaps their lethality, and increased "drop" with range eats up that increased accuracy in a hurry.
The second category is the hunter who simply shot the animal in the wrong place. Unskilled trees-tand shooters tend to place the shot too low in the body and at the wrong angle. Ground shooters also often fail to take note of the direction the deer is facing. "I shot him right behind the shoulder" is common when the deer is facing the hunter at a 45-degree angle. That shot usually misses the majority of the vitals and should have been placed at the front of the shoulder at the joint of neck and shoulder.
In general, the most lethal shot for most hunters is to place the shot so it will penetrate the center of the chest of the animal from whatever angle the shot presents. If on the same level as the animal, shot placement is between 1/3 to 1/2 of the way up from the brisket, with the proper side angle. If in a tree stand, the shot should be placed higher on the animal, and if uphill, it should be placed lower on the animal.
If you don't believe me call Sierra's help line at (800) 2238799) and ask them. Note that Sierra match bullets are called Matchkings and hunting bullets are called Gamekings--imagine that. Match bullets are designed for extreme accuracy with no part of their construction devoted to lethal performance.
Game bullets, on the other hand, are designed to give adequate accuracy and expansion, combined with penetration on game. Velocity is the single most important facet in bullet performance. Reducing the velocity a couple hundred feet per second to get the 1.25" group down to .75" is simply a foolish mistake, resulting in a reduction of lethality at impact and a less effective trajectory. Choosing the right bullet is also important. A 7mm TCU may give adequate performance within its limitations with bullets labeled "single shot" pistol bullets, yet simply become a nail-type penetrator with a bullet designed for a 7mm Magnum.
Likewise, shooting a bullet designed for a .45 Colt in a maximized-load .454 Casull can give serious problems. Cast bullets are great penetrators, but usually don't do much damage going through when compared to a good expanding bullet.