They can be added in compat tab:
https://steamuserimages-a.akamaihd.net/ugc/2366166697362820608/69F1502393F8C7FE98127E473F6491937C9B41DF/

All in one collection:
https://pastebin.com/RePYsDTq

Note: johnb432's Weapons Balance mods have collect a lot of weapon classnames for various mods, suggest checking his github first.
https://github.com/johnb432/Weapons-Balance-IFA/blob/main/addons/main/weapons/CfgWeapMG.hpp
https://github.com/johnb432

This is what @Country Doggo shared for RHS: (edited in 24-9-12)
//All the RHS machinegun and heavy sniper rifle classnames for those of you who need it, dump this at the end of the already present code in the heavy weapon array (don't replace what's already there)

,'rhs_weap_m249','rhs_weap_m240B','rhs_weap_m240G','rhs_weap_m249_pip','rhs_weap_m249_light_L','rhs_weap_m249_pip_L_para','rhs_weap_m249_pip_L','rhs_weap_m249_pip_ris','rhs_weap_m249_light_S','rhs_weap_m249_pip_S_para','rhs_weap_m249_pip_S','rhs_weap_m82a1','rhs_weap_fnmag','rhs_weap_minimi_para_railed','rhs_weap_mg42','rhs_weap_pkm','rhs_weap_pkp','rhs_weap_M107'

//UPDATE!!
//IFA3 (include M1918 BAR, BREN, PIAT, DP, DT)(edited in 24-11-4)
,''LIB_DP28','LIB_DT_OPTIC','LIB_DT',LIB_MG42','LIB_DP28','LIB_M1919A4','LIB_M1919A6','LIB_MG34','LIB_PTRD','LIB_PIAT_Rifle','LIB_M1918A2_BAR','LIB_Bren_Mk2'

//GEIST BAR series (M26, WZ1928)(edited in 24-9-15)
,'GLIB_FIN_M26','GLIB_POL_WZ28'

//Faces of war (include M1918 BAR, BREN, Japanese Type99 LMG, PIAT)(edited in 24-9-15)
,'fow_w_m1919','fow_w_m1919a4','fow_w_m1919a6','fow_w_mg34','fow_w_mg42','fow_w_piat','fow_w_bren','fow_w_m1918a2','fow_w_m1918a2_bak','fow_w_type99_lmg'

//UNSUNG (include 'support' version of MGs, and M60grip, M60shorty, RPD, M1918 BAR, Stoner M63a, DP)(edited in 24-9-15)
,'uns_PK_base','uns_PK','uns_ukvz59','uns_DP28_base','uns_DP28','uns_MG42_base','uns_MG42','uns_mg42_bakelite','uns_m60_base','uns_m60','uns_m1919a6','uns_M63a_drum_base','uns_M63a_LMG','uns_rpd_base','uns_rpd','uns_rpdsog','uns_RPDsupport','uns_PKsupport','uns_MG42support','uns_m60support','uns_m60grip','uns_m60shorty','uns_bar_base','uns_bar'

//3CB factions (include Bren, L4)(edited in 24-9-16)
,'UK3CB_Bren','UK3CB_Bren_L4_LMG','UK3CB_Bren_303',
'UK3CB_M1919A4_Browning','UK3CB_M1919A6_Browning','UK3CB_M60_base','UK3CB_M60','UK3CB_M60_mounted','UK3CB_MG42_base','UK3CB_MG3','UK3CB_MG3_KWS_T','UK3CB_MG3_KWS_B','UK3CB_MG3_KWS_G','UK3CB_MG3_Railed'

//CSA38 (include Bren, ZB26)(edited in 24-11-4)
, 'CSA38_zb26', 'CSA38_zb30', 'CSA38_zb30j', 'CSA38_MG30', 'CSA38_MG13', 'CSA38_MG34', 'CSA38_boys', 'CSA38_BRENMKII', 'CSA38_BRENMKII2', 'CSA38_BRENMKI', 'CSA38_BRENMKI2', 'CSA38_BRENMKIII', 'CSA38_BRENMKIII2', 'CSA38_31m'

//Northern Fronts (include DP, DT)(edited in 24-11-4)
, 'NORTH_Lahti_L39', 'NORTH_Lahti_L39AA', 'NORTH_dp27', 'NORTH_ls26', 'NORTH_Madsen1914', 'NORTH_Madsen1922', 'NORTH_DT_hand'

//WW1 mod (edited in 24-11-4)
,'wwi_chauchat', 'wwi_lewis_mg', 'wwi_LMG0815'

// East Asian War 1937 (include ZB26, Japanese type99/96 LMG)(edited in 24-11-4)
,EAW_ZB26_Base', 'EAW_Type99', 'EAW_Type96', 'EAW_Type11_Base'

// Vanilla addition (include Zafir, Mk200, LIM-85, GM6 Lynx, M320 LRR)(edited in 24-11-4)
, 'B_Patrol_Soldier_HeavyGunner_weapon_F','B_Patrol_Soldier_MachineGunner_weapon_F','LMG_03_F','LMG_Mk200_BI_F','LMG_Mk200_black_BI_F','LMG_Mk200_black_F','LMG_Mk200_F','LMG_Zafir_F','srifle_GM6_camo_F','srifle_GM6_F','srifle_GM6_ghex_F','srifle_LRR_camo_F','srifle_LRR_F','srifle_LRR_tna_F'



--------template-------------------------
// xxx (include xxx)(edited in 24-11-4)
,