Note: only Rahim 7.62 mm and MAR-10 .338 have their mags visually present - see last screenshot link

When you take the weapon in hands -> it visually has the mag.
When you drop the weapon on ground -> it visually has the mag.

I myself respawn without backpack, vest etc. So I have 0 ammo for any of weapons spawned when I take them in hands.
But when I take them in hands, mag or rocket is instantly there, the one, which comes attached to the weapon by default.

https://i.imgur.com/V7MbeLr.jpg
AK-12 7.62 MM
AK-12 GL 7.62 mm
AKM 7.62 mm
AKS-74U 5.45 mm
AKU-12 .762 mm
CAR-95 5.8 mm
CAR-95 GL 5.8 mm
CAR-95-1 5.8 mm
Katiba 6.5 mm
Katiba Carabine 6.5 mm
Katiba GL 6.5 mm
Mk14 7.62 (classic)
Mk20 5.56 mm
Mk20 EGLM 5.56 mm
Mk20C 5.56 mm
MX 3GL 6.5 mm
MX 6.5 mm
MXC 6.5 mm
MXM 6.5 mm
Promet 6.5 mm
Promet GL 6.5 mm
Promet MR 6.5 mm
Promet SG 6.5 mm
RPK-12 7.62 mm
SDAR 5.56 mm
SPAR-16 5.56 mm
SPAR-16 GL 5.56 mm
SPAR-16S 5.56 mm
SPAR-17 7.62 mm
TRG-20 5.56 mm
TRG-21 EGLM 5.56 mm
Type 115 6.5 mm

https://i.imgur.com/oXJF6BF.jpg https://i.imgur.com/mD2t3Zm.jpg https://i.imgur.com/ANbnvIg.jpg https://i.imgur.com/hzXoM2Q.jpg https://i.imgur.com/2BwED8n.jpg
9M135 Vorona
MAAWS Mk4 Mod 0
MAAWS Mk4 Mod 1
PCML
RPG-42 Alamut
RPG-7
Titan MRPL
Titan MRPL Compact

https://i.imgur.com/D7FquY0.jpg
LIM-85 5.56 mm
Mk200 6.5 mm
MX SW 6.5 mm
Navid 9.3 mm
SPMG .338
Zafir 7.62 mm

https://i.imgur.com/7J3Es1M.jpg
4-five .45 ACP
ACP-C2 .45 ACP
P07 9 mm
PM 9 mm
Rook-40 9 mm

https://i.imgur.com/A1JjzGU.jpg
ADR-97 5.7 mm
ADR-97 TR 5.7 mm
ADR-97C 5.7 mm
ADR-97C TR 5.7 mm
PDW2000 9 mm
Protector 9 mm
Sting 9 mm
Vermin SMG .45 ACP

https://i.imgur.com/i2q3uli.jpg
ASP-1 Kir 12.7 mm
CMR-76 6.5 mm
Cyrus 9.3 mm
GM6 Lynx 12.7 mm
M320 LRR .408
Mk-1 EMR 7.62 mm
Mk14 7.62 mm
Mk18 ABR