I think he's saying that with the larger calibres, there is a greater margin of error.
We all have had the odd shot that didn't go where we want it to, even the experienced ones... So if your shot is off by an inch with a .308 there's a much better chance of the deer dropping instantly as opposed to the same shot with for example a .243 http://www.thehuntinglife.com/forums/pub...
What was of interest to me was his comment " a .243 with a well placed 100 grn bullet will drop Fallow no problem ", there seems to be an Internet myth that the .243" is a good all round option for a deer rifle, it's not.
A well placed shot with any calibre will kill a deer and if it didn't kill then deer by definition it would be a poorly placed shot.
I was also interested to find out how he would compare the .243" to the .308 which in the main is a superior deer calibre, yet we all seem to love the .243" for certain job (myself included).
There isn't a greater margin for error with either calibre, either you hit the kill zone or you don't which will result in a wounding.
The main advantage of the .308 is the extra kinetic energy it can transfer to bone and the greater wound channel it will produce for like to like deformity with bullet varieties. For example a 100 grain .243 with a muzzle velocity of 3000 fps will produce 1727 ft/lb energy at 100 yards, this is a massive amount of energy. In comparison the .308 will produce 2369.9 ft/lb (2900 fps MV). If we extend our ranges to say 200 yards, the .243" will produce 1481.3 ft/lb and the .308 will produce 1986.2 ft/lb and it's here that you can start to see the benefits of the .308, which produces more energy at 200 yards than the .243" at 100 yards. Either will kill, experts seem to agree that just 4 ft/lb energy is sufficient to destroy a vital organ.
The most important factor is of course hydraulic shock, the .308 in general will produce a great wound channel and thus hydraulic shock. This shock can completely disrupt vital organs in the body cavity. Another benefit of the .308 is jacket thickness, in my experience .308 jackets are slightly thicker than .243, which helps when hitting bone.
Bottom line is that in my opinion both calibres will kill deer, the .308 slightly better and when you are stalking fallow the extra effect of the .308 produces more consistent results in dropping the beasts.