It absolutely fits the c308 (read below) - AIM Sports Inc M-Lok Handgaurd for H&K 91 Rifle Length

Does it or doesn’t it? The answer is that is absolutely DOES fit the C308. The problem is that some people may not feel comfortable wrenching the nut down to fit - but this is really no different from most of the AR free-floating handguards that clamp-on to the barrel nut. Once tightened to the point where the front no longer wiggles - you may have some wiggle in the back end by the receiver, but this is also normal for the initial fitting. If you have wiggle in the rear, you just need to loosen the nut and use a bit of loctite around the lip of the rear portion of the handguard. Apply a bit of force when fitting it back in, and then tighten the heck out of the nut until there’s no more wiggle. I used blue loctite to secure mine around the receiver about a month ago, and I have had absolutely no issues with it coming loose. Furthermore, the m-lok slots are well machined - no issues with attachments either!