It could be the ACE issue, where maybe they die some time later and it takes the weapon you have in hand at that point.
I've seen data with kills for those weapons.
"_id": "e3a0b15121d2b39647908f7e92982439",
"_rev": "1-279d9420fcfb0468b16f327c6c5c6510",
"realTime": "25/07/2017 21:06:05",
"Server": "89.163.206.173",
"Group": "161stRR",
"Operation": "alive_liberty_test",
"Map": "lythium",
"gameTime": "0633",
"Event": "Kill",
"KilledSide": "EAST",
"Killedfaction": "Takistani Army",
"KilledType": "Asst. AT Rifleman",
"KilledClass": "Infantry",
"KilledPos": "029131",
"KilledGeoPos": [
2983.33,
13161.4,
0.00125122
],
"KillerSide": "WEST",
"Killerfaction": "British Armed Forces",
"KillerType": "Marksman (LRR)",
"KillerClass": "Infantry",
"KillerPos": "028129",
"KillerGeoPos": [
2849.31,
12905.9,
0.00140381
],
"Weapon": "HK417A2 16.5 Barrel (AFG) Dark Earth",
"WeaponType": "SPS_hk417_16_afg_fde_f",
"Distance": 290,
"Killed": "O Alpha 1-1:8",
"Killer": "B Bruiser:4 (Steve) REMOTE",
"KillerConfig": "CUP_B_BAF_Soldier_scout_WDL",
"KilledConfig": "CUP_O_TK_Soldier_AAT",
"Player": "76561198058551062",
"PlayerName": "Steve",
"playerGroup": "161stRR",
"missionTime": 1124