Hi All ! I am newbie here , so would like to get some info from you all pro out there

I want to know is the regarding the gun barrel length , and which type of gun is using what type of length , sorry for my bad English .
1) what is the barrel length for the m16 ?
2) what is the barrel length for the sopmod ?
3) what is the barrel length for the CQB ?
4) What is the barrel length for the sniper gun ?
All the above mentioned gun I like to build using M4 Aeg metal body .
Maybe I am wrong , so need help and teach me how and what to do , cos I am planning to build myself a sniper gun , a CQB and a M16 .
Thank you for reading my msg and please help .