The following, is a list of small arms which could reasonably appear/Not appear in both the Classic Fallout's, Van Buren and Fallout 3:

Possible:
Beretta M92FS(More likely would have been the replacement for the Browning Hi-Power)
M1911 Pistol(Would have been a common side arm in the US Military)
Mauser C96(Fallout 1/Fallout 2)
Desert Eagle(Fallout 2)
Smith & Wesson M29 .44 Magnum Revolver(Due to Smith and Wesson's existence since 1852)
Flamer Pistol(Possible, due to the abundance of flame based weaponry)
Casull Revolver(The round in first appeared in '59)
Browning Hi-Power(Introduced in 1935)
Zip Gun(Fallout 2 (cut content))
The Grenade Pistol(Homegrown weapon)
MP 38(1938, and became popular after WWII in different variants)
Thompson M1928(Fallout 2)
Uzi(Brought into usage in 1950)
Scorpio(Based on Czechoslovak ?korpion vz. 61 from '59)
FN P90c(Fallout 2)
Sten Gun(Extremely reliable & had a long period of service)
FN FAL(Fallout 2)
Colt Rangemaster Hunting Rifle(Fallout 1/2)
M16A1(AR-15 first appeared in '58)
AK-47(1949, a tough and reliable weapon)
XL70E3(Fallout 2)
DKS-501 Sniper Rifle(Fallout 1/2)
M1 Garand(Long service life)
M1918 Browning Automatic Rifle(1917)
M-14(Sucessor of M1 Garand)
Pump-action Shotgun(Van Buren aka Fallout 3)
Combat Shotgun(Fallout 1/2)
H&K CAWS(Fallout 2)
Pancor Jackhammer(Fallout 2)
Bren Gun(Long Service)
M2 Browning machine gun(Long Service)
M60(Fallout 2)
Lewis MKII(Multiple users, and decent period of service)
Chauchat(Probably only used by raiders)


Generally Not Possible/Limited Issue Only:
Calico M950(Did not appear until the 80's OTL)
Heckler & Koch MP5(Probably existed only in limited usage(1966))
Sig Sauer Pistols(SIG Arms AG did not begin producing pistols until the 70's)
Walther MPL(Not introduced until '63)
Steyr AUG(Not introduced until '78)
Ruger AC556F(Not introduced until '73)
Calico Liberty 100(80's)
Neostead Combat SG(90's)
M79 grenade launcher(May have seen limited usage)
M249 SAW(1984)