I'v only been able to find 10, has anyone else found the remaining 3? Or am i just a blundering idiot whose missing something obvious?

Any help/discussion would be welcome.

What difficulty have you completed it on? I only ask because i'v only done it on normal so far, i'm guessing that when you finish super soldier difficulty, you unlock some new ones, maybe?

I have 12/13 and I don't think difficulty matters. I swear I think the last one is just glitched and doesn't exist.

what are your other attachments? i'v got reflex, assault scope, laser sight, silencer, extended mag, grenade launcher, gauss and light shotgun for the rifles. Pistol silencer and the laser sight, and thats it.

There are 12/13 attachments in single player. The 13th is presumably the hologram decoy which was counted as an attachment but not added to the single player campaign (so, probably a bug).

Difficulty doesn't matter either.

Hairymary, are you perhaps missing the shotgun attachments?