Having lived in southern Florida for several years, I can tell you from personal experience that your father is going to need a compact size firearm. Otherwise, as was already pointed out, if the gun is to big, he's not going to be carrying it much and might end up not having it when he might need it.
While a .380ACP would not be my first choice as a defensive caliber, given the fact that your father has arthritis, I can understand the need for a low recoil caliber. I would recommend the Browning BDA380 (
pictured below on the right) ...
... It is larger than a pocket gun and fourteen rounds of .380ACP is nothing to sneeze at especially if he puts the rounds where they need to go. In 9mm, I would recommend the Smith & Wesson 3913TSW (
pictured above left). It's about the same size as the Browning but in a single stack 9mm.
May I remind you that most of the police forces across the USA have decided that the 9mm is not a capable firearm when it comes to stopping a man even if he is not wearing body armor? The most reliable auto loading pistol when it comes to stopping a man is the 45ACP. The 40 S&W is a good choice when a smaller gun is wanted and a larger magazine is preferred. Both are designed to stop a man - with or without body armor. The do that job admirably.
The 380 shoots a 100 grain bullet at 950 fps which is fast enough to penetrate but is so light that it does little damage to stop a determined attacker. The 9mm is equal to the 38+p in all loadings but only because of its higher working pressures. The 38 special was used for years after its introduction by police across the country although it was under-powered and anemic when compared to the guns that the police were defending themselves against. These two rounds - the 38 and 9mm are best suited to low powered games and target competitions in my opinion.
To parapharse Todd Green, the average 9mm load probably isn't as effective as a defensive round as the average .40S&W or .45ACP round. However, when you start to look at the best loads in each caliber, you begin to see that they're almost identical in terminal performance (
ability to penetrate, expand, and otherwise wound a violent threat).
As you can see, the 9mm versions of most "premium" loads are very close and sometimes superior to the .40S&W and .45ACP versions. It's all about bullet design, not bullet weight or velocity. If it penetrates the same, expands the same, and disrupts tissue the same, the end result will be the same.
