For those who are looking into upgrading or lengthening their AEG barrels, I've comprised a comprensive of the majority of the most common AEGs. List is from shortest to longest.
Barrel Length - AEG
110mm - MP5K
141mm - MP5-PDW
182mm - MP7A1
195mm - UMP/UMG
208mm - G3 SAS
229mm - MP5A4, MP5A5, MP5SD5, MP5SD6, AK-47 Beta Spetsnaz
247mm - G36C, P90, CAR-15, SIG 552
280mm - CA SAR M41 FS Offizier, CA M15A4 CQB
285mm - MC-51
300mm - M1A1 Thompson, TM M733, G&P M4A1 Commando
304mm - G&G FS51 Fixed Stock
355mm - G36C
363mm - M4A1 (CA M15A4 Carbines), SR-16, S-System, TM XM177-E2, SCAR, G36K, M249 Para
420mm - SOC-16, CA M14 Scout
422mm - M249 (non-para)
429mm - CA 33E
455mm - AK-47, AK-47S, AK-74M
463mm - SLR-105, RK-103
469mm - G3 SG1, CA SAR Taktik
472mm - FAMAS F1, FAMAS SV
475mm - VFC MK43 Mod 0
495mm - G36E
500mm - M14
509mm - M16A1VN, M16A2, AUG
510mm - L85
534mm - SIG 550
550mm - M16A2 (Long), M16VN (Long), AUG (Long), King Arms FN FAL
590mm - TM PSG-1
650mm - TM PSG-1 (Long)