This was fixed in CBA very recently it seems:
https://github.com/alganthe/ACE3ganthe/pull/20
magazineWells introduced with 1.80 update
Weapons progress list:
[ ] AA-12
[x] AK/RPK
[ ] AS50
[ ] PP-19
[ ] CZ-750
[ ] CZ-805
[ ] CZ SCORPION EVO 3
[ ] FN FAL
[x] FN Mag
[x] FN Minimi
[ ] G22
[x] G36/MG36
[ ] KVSK - No CBA magwell
[x] L110A2
[ ] L115A3
[ ] L129A1
[x] L7A2
[x] L85
[ ] M107
[x] M110
[x] M14/M14 DMR
[x] M4/M16
[ ] M24
[x] M240
[x] M249
[ ] M40A3
[x] M60E4
[x] Mk 48 Mod 0
[x] Mk 12
[x] Mk 16/Mk 17
[x] MP5
[ ] PKM/PKP
[ ] SA-58
[x] Saiga
[x] SVD
[ ] UK-59
[x] VSS
[x] XM8
[x] MAAWS
[x] RPG-7
[x] SMAW
[x] CZ 75 COMPACT
[x] CZ 75 DUTY
[x] CZ 75 PHANTOM
[x] G17
[x] M1911
[x] M9
[x] PM
[ ] PDW - No CBA magwell
[x] PB
[x] SA-61