"they carried M-14s and CAR-15s and Swedish Ks and grease guns."

Swedish K
Public DomainSwedish K - Credit: SPEC. 4 DAVID SHAD
The Swedish K is a submachine gun that was developed in Sweden during 1944-5. It fired a 9mm bullet and weighed 9.25 pounds fully loaded with a 36 round magazine.

"Grease Gun" refers to the M3A1, a submachine gun introduced in World War II to replace the Thompson M1A1 submachine gun.  It has a high rate of fire and can be converted to use the ammunition and magazines of several other weapons, such as the British Sten gun. These characteristics made it a perfect weapon in Vietnam.  SMGs were used widely by all groups in Vietnam; many were weapons originally used in WWII.  Although the M3A1 became outdated, it was employed by several other nations and even by U.S. helicopter pilots.