Why do sniper rifle extended barrel and concentration mods not exist in single player

This guide shows you where all the mods show up in the game and from which store you can buy them. If you're doing a newgame+ remember, that you cannot level them beyond V. Other than that - if they don't show up at all it looks like your gamefiles have become corrupt, so you either try a repair install or re-install.

What platform are you on? I've heard of these issues but not for PC.

I'm on xbox,

the thing is i can see some if them, i can click on them to make them disapear, but they just never show up in inventory or in shops, so for 2/3 of my games i have no sniper extended barrel of any kind

As far as I know this is a known bug, but I don't think it's been fixed. Having said that I'm a PC gamer so don't follow xbox issues all that much so could be wrong.

I can confirm this issue on PC as well. I've basically not been able to take/keep all weapon-mods I found.

Started right on priority:Mars. First I was able to take the SMG-mods in the beginning; then I died and used the restart-mission-save. From that point on, only the animation played while pressing space on the mods. The sniper-rifle-mods worked, though. Same problem on the Grissom-Academy; When I first played it I was able to take the mods lying around there, but when I had to reload it was animation-only again, with no window popping up. Meaning those mods aren't available on weapon-bench or shops, which made me stop playing 'till it's fixed.

I've tried scanning for corrupt files, but Origin says all my game-files are fine and up-to-date. Same for all DLC-contents. Also tested on multiple careers with normal and extreme difficulty (both with imported character from ME2 which also was carried over from ME1), same issue.