If I were choosing a pistol for an agency, I'd go for the Beretta 92G or 96G (one or the other, but not both). The barrel is longer than most other DA autos that are suitable for duty use, which will give better accuracy and higher velocity. In my experience, the Beretta in both 9mm and .40 are extremely reliable and durable. The size of the pistol isn't even a factor since it will be a duty sidearm, but the weight helps tame the recoil. I would not authorize anything other than the issued weapon due to two main reasons. First, if my officers were in a firefight, I want them to have interchangeable ammo and magazines. Second, for some reason, if one officer needed to use another's service pistol, he or she would be familiar with it's operation.
Issued ammunition would be Speer Gold Dots in either caliber:
9mm- 124 gr. Speer Gold Dot
.40 S&W- 180 gr. Speer Gold Dot
Both rounds are reliable and accurate.
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I'm going to get carried away and continue on with the rest of the issue firearms.
Off duty weapons and backups would be just about anything, as long as they are cleared with the department armorer and are suitable (no cheap-o Jennings or other such low-quality guns)
Patrol shotguns would be either the Remington 870 or Mossberg 500 (again, one or the other). Each would have 00 buck and rifled slugs for duty ammo.
The patrol rifle would be the Bushmater 16" Shorty Carbine (LE version with telestock, bayo lug, etc.) and would be supplied with 55 gr. ball ammo and a few 30-round magazines.
SRT would be equipped with select-fire Bushmaster 14.5" carbines. Being that they are a tactical team, they would get an HK USP in .40/9mm or a GLOCK 17/22 (depending on whether the duty pistol was 9mm or .40). Both are famous for their extreme durability, accuracy, and reliability.