I have a 16" midlength AR that I am trying to buy a folding front sight for.  It has a Daniel Defense 9.0 rail system and a dovetailed gas block.

Which surface would be best to mount a front sight on?  The rail should still be farther our then the front sight on a carbine but I am not too sure about that.  Would I lose any accuracy if I mounted it on the rail or would I be better off finding one that mounts on the gas block?  I have noticed that there are alot more sight options for the rail then the gas block.

Also, what brands should I be looking for?  If I can mount it on the rail I was thinking either Troy or PRi.  I have a Matech BUIS in the rear if that makes any difference.  I also have an aimpoint that might occasionally be mounted on my AR, but currently it is not.

Thanks.

I am going through the same process right now.  I'm building a 9mm and am going to mount my front YHM flip up on the end of the free float.  From what I have been told people seem to think that mounting on the gas block is better because the free float has a far greater chance of moving.

If your rail moves you loose your zero.  This rifle will just be a fun sand pit gun so I am going to take my chances mounting on the free float.

Check out this thread I started:

sight mounting

It doesent get very detailed but it may answer a few of your questions.

I went with the flip sight / gas block because it is just about as low as the FF railed handguard
and putting one on the rail seemed like it was defeating the purpose.

Invisiblesoul

civprod...I have a 16" midlength AR that I am trying to buy a folding front sight for. It has a Daniel Defense 9.0 rail system and a dovetailed gas block. Which surface would be best to mount a front sight on?
you might consider a popular folding front sight such as PRI which includes an integral gas block...linky
http://www.pri-mounts.com/mm5/merchant.mvc?Screen=PROD&Store_Code=PRI&Product_Code=05_0025&Category_Code=AR15_M16_ACC
i'm not sure how any of these sights interact with a 'midlength' barrel which repositions the gas hole further forward, but this sight nestled into my DD 9.0 FSP rail on a regular 16" barrel is a great combo