Having played extensively through Wildlands, and in the process amassing a sizable arsenal of weapons and attachments, I've come to one conclusion:
For a tactical shooter, this game's attachment system is full of oversights and illogical omissions, which is weird given the series pedigree.
Here's my examples/gripes:
*The SPAS-12 gets an 8-round tube in Remanzo, which has rails attached to the tube's fore-end. Despite this, it cannot mount laser sights or grips, despite both attachments being beneficial for a shotgun (and both are conspicuously absent).
*The HTI, MSR, DSR-1, SRSA1, heck all of the bolt-action rifles except the M1891 have railed fore-ends, the HTI especially has a giant railed fore-end. None of them can mount foregrips or lasers, despite the G28 and MK14 in the same class having this option. The M1891 I can understand given it's chassis, but if I can nitpick, the M1891 in that configuration has to have a curved bolt, because the rail is in the way and cycling the bolt would be impossible.
*The Range Finder on assault rifles is useless and does nothing.
*The M4, P416, PSG, and any rifles that have similar stocks have no options to change or move their stocks up and down. There's no "Compensated Buttstock/Extended or Retracted Buttstock" for these weapons, but there is for the G36C and SCAR-H.
*The M4A1's stock is fully retracted, and no option to extend it is available, making the weapon look like its always set up for close-quarters.
*Some weapons have different selectable trigger groups, others don't. The bolt-action rifles have a category for them, but you can't change anything, so why have the category at all? The Remington R5 has no select fire, despite being an AR-platform assault rifle.
*I do not understand why there are multiple magazine models for 5.56. There's STANAG, PMAG and G36. Most rifles that are STANAG compatible can take PMAGs. Have one extended mag for 5.56, but different selectable styles (PMAG, STANAG, etc)
*Assault rifles have no option for drum magazines, only 60 round quad casket magazines for STANAG (which only holds 50 rounds), despite C-Mags existing and being an option for many rifles.
*The MG121 despite being equipped with rails, can barely accept any attachments and can't be suppressed.
*Underbarrel grenade launchers are in the game, but underbarrel shotguns are not, which would be useful for breaching and close quarters without having to lug a SPAS-12 or Saiga around.
*The TA31 ACOG that goes on sniper rifles and the ACOG that goes on assault rifles are two completely different scopes, despite looking identical. You can't mount the ACOG on an assault rifle once you remove it from the sniper rifle and replace it with a better scope. Its not just scopes; most attachments are weapon class specific; a laser sight meant for sniper rifles will not fit assault rifles, despite them having a rail mounting system.
*Its painfully obvious that Ubisoft did little to no QA testing before they shipped out Wildlands. Attachments are in different provinces than what their text says (the POSP is in Koani, not Ocoro, the shotgun suppressor is in Inca Camina, not Mojocoyo, etc), certain gun attachments don't work properly (G36 folding stock is locked, despite unlocking the folding stock in Flor De Oro, SCAR-H shows extended mag when regular mag is equipped in third person, Micro T-1 and G33 is stated to be compatible with SMGs but this is not the case, AUG's 30 round magazine is laughably oversized). My favorite is the Type 95 in Remanzo having a "5.56 round belt box" when it clearly has a magazine and that is completely the wrong caliber for it (if you're curious, its 5.8x42mm)
There's more, but these are the ones I have found so far (currently in Remanzo dismantling El Gato and the Smuggling operation, RIP Security)