Secret Service carry Sigs in .357Sig because it's a high velocity round, at about 1,500 fps, which penetrates.
Air Marshalls use either Sigs or HKs in .40S&W.
FBI Agents are issued Sig-Sauer also, but they have a roster of approved handguns they can choose from if they'd rather use something else.
Seals carry a variety of handguns.
The DoD encourages Special Ops like Seal units to not stray from NATO universal cartridges, which means 9mm for sidearms, but SEALs have been known to carry .45ACP and even 10mm into the battle.
Air Marshalls use either Sigs or HKs in .40S&W.
FBI Agents are issued Sig-Sauer also, but they have a roster of approved handguns they can choose from if they'd rather use something else.
Seals carry a variety of handguns.
The DoD encourages Special Ops like Seal units to not stray from NATO universal cartridges, which means 9mm for sidearms, but SEALs have been known to carry .45ACP and even 10mm into the battle.