Looks like a Daniel Defense MK18 12" RIS, so yes 14.5" barrel, standard M4 length will work. You'll need a low profile gas block, or remove the front sight base.
Madbull makes a replica of the DD MK18 rail, here's a link to the 12" one, or rather 12.25".
http://www.airsoftgi.com/product_inf...oducts_id=6380
Daniel Defense also has a rail that is more drop-in ready, without the need to use a low profile gas block.
Not sure if Madbull carries that though, but there are other similar 12" rails that have the cut out for the FSB, if you don't want to use a low profile gas block.