HK roller delayed uses a numbering system for caliber for the last digit:
91, 51 the one stands for .308/7.62X51 (G3 is the German military designation like M16 for the US military)
93, 33, 53 the three stands for .223/5.56X45
MP5, MP5K, MP5-10, MP5-40 the 5 stands for pistol caliber.
HK used the first digit being a "9" for semi auto designation:
91 semi auto .308 rifle
93 semi auto .223 rifle
93 semi auto pistol caliber (9mm) rifle
They also used the first number to designate barrel length:
33 is select fire rifle in 5.56X45 with a barrel length of 390 mm (15.4 in)
33K select fire carbine in 5.56X45 with a barrel length of 332 mm (13.1 in)
53 is a CQB entry carbine in 5.56X45 with a barrel length of 211 mm (8.3 in)
The addition of the "K" designation stands for "Kurz" meaning "short".
Some manufacturers have made some models that HK never manufactured such as the 51. The five designates a barrel of approximately 8.85". That barrel length in .308 is not practical for much more than concussive noise and bright flash. I hope this description helps.
Scott