If they are no longer in the military then you don't have to worry about what they would be issued. But even if your a retired Vietnam vet does that mean you have to use the old M16? No.
Currently the Navy SEALs have chosen the HK45CT (compact tactical) over the Sig P226 and the MK 23 which was never really used much in the first place because it was a huge sidearm. The switch over to the HK45CT was recent.
And the SEALs use a lot of HK firearms period. HK416 (their version of the M4) HKG36 and now the HK45CT
HK are very expensive but IMHO they are the best. I have a USP .45 and I trust me life to it no problem. Its a tank. Never failed and feels a lot more refined than my glock 30.
Reason I have the glock 30? Easier to carry than a USP and didnt have the $$ for a new HK45c but one should be in my possession soon