I have started to reply to this thread a number of times. I seem to be having an bit of writers block in effectively conveying my thoughts. Here is a new tack to get my point across.
First, cut to the chase. Your question of the 10mm cartridge as a SD round. It is a great choice. It can do anything the 40 can do, and it can do things the 40 can't (see Cartridge Ballistics below). As far as over-penetration, last I checked, shooting in SD was not an excuse to ditch the 4 rules, one of which is be sure of your target and what is beyond.
Now, the small book. Those with attention spans similar to mine should stop here....
There are three things to consider when looking at a self defense package. Cartridge ballistics, cartridge cost, and available platforms.
Cartridge Ballistics
Ballistically, the 40S&W is inferior to the 10mm. The 40S&W is a shortened and lightened 10mm auto case. It operates at a lower pressure, and has less case capacity. Any external or terminal ballistics that can be generated in the 40S&W cartridge can be duplicated in the 10mm cartridge. The shoot the exact same bullets, and the the 10mm can push them the same speed, or if desired, faster. In cartridge ballistics comparison, the 10mm wins.
There will be those who say that there is a wider selection in 40S&W ammo on the market. This is based on a few additional bullet types being available commercially loaded in the 40 that arent' in the 10. This argument doesn't stand up to scrutiny. The 10mm is available commercially in bullet weights from 135 gr to 230 gr. The 40 loadings stop at 200 gr.
Cartridge Cost:
10mm new ammunition will cost a minimum of $3.00/box (50 rds) more than new 40S&W ammunition. If you reload, once brass is acquired, the costs can be the same, if you load light 10mm rounds. The only difference is the primer, and small and large pistol primers can be found at the same price. I just checked powder valley. I could make arguments about brass life either way. The short story is the 40 S&W will be 20% cheaper in new ammo, and may be slightly cheaper to reload.
Available Platforms:
No question, there are more guns out there for the 40S&W. However, that doesn't mean that your gun isn't out there in 10mm. Personally, I am a cocked and locked, double stack, no grip safety cheapskate. EAA has my number with the Witness. And it is available in 10mm from them.
The 10mm is longer than the 40. Therefore, guns with the same magazine capacity can be engineered with a smaller grip circumference in 40 than in 10. (Whether they are or not varies from model to model.)
Speaking broadly, there are four basic auto platforms as I see it. Striker fired, DA/SA w/decocker, DA/SA w/safety, and SA (with safety). There are sub-genre's of plastic and steel. I do not know of a currently produced 10mm that falls in the DA/SA w/decocker space. Striker = Glock(20,29), DA/SA w/safety = Witness (steel and poly frames), SA = SVI, Dan Wesson, Fusion, Wilson Combat, Kimber, Witness Match.
The only people really out in the cold are those married to a brand that doesn't make a 10, or those with small hands. My wife can handle a Witness just fine, and I still have the bulky rubber grips on it (Read, they have to be pretty small hands. Anyone who can wrap around a 1911 can find a 10mm they can wrap around.).
Psychologically, the human animal is a pack animal. As a group we don't like to be alone. If you feel funny when you are dressed a little different, or say something a little different, the 10mm isn't for you. You will be a loner, and will be challenged about your intelligence and pedigree if you choose one.
I deemed flexibility in the cartridge to be a high criteria. The 10mm was available in a platform I really liked, and I reload, so the cost difference in negligible.
