Help please, I'm interested in the acog scope for my 20" ar15 (UK spec before anyone asks).
Does this scope work with them as the ad says it was designed for the M4 carbine with 16" barrel and 62 grain bullet. It also states the recticle bullet drop compensator and range finding stadia lines are based on the 62 grain bullet from a 16" barrel.Now i'm using 62 grain bullets so no probs there but don't want to make the purchase if it's gonna be wrong. Sorry if this seems like a stupid question.    

Not a stupid question at all because there are several models that are designed for specific weapons.  Different models are meant for differnent rifle setups.TA31 and TA11, or at least some version of them, are setup for use on 20" flat-top ARs.  Others maybe available as well.

Any ACOG will work with your rifle...what you will need to do is figure out your own BDC for the lines given on the reticle.  The FAQ thread is a good place to start.  Check this out...

ar15.com/forums/topic.html?b=3&f=18&t=229420

Cheers for the advice!