I've seen two different types of IARs. one with the big unusual heatsink front end that is like the one in Kileys post, and the other one is basically any 9 or 10 inch free float ris.
For example, take a look at this link:
http://www.airsoftpacific.com/viewtopic ... sc&start=0Here is another IAR:
http://i26.photobucket.com/albums/c137/ ... 6/LWRC.jpgTo achieve that IAR format, all you would need is a 9-10 inch free float RIS, a real steel IAR sight, and a heavy outer barrel that I've seen on sale. I've even seen some guns that have this RIS, but I have not seen just the ris.
For example, here is a gun with that Ris already on it:
http://www.airsoftpost.com/product_info ... s_id=32911For an IAR, you must have that free float Ris with that half inch space or so where the body meets the rails.
All you would need is a heavy that does not have the indentations or groves like a m4 outer barrel would have.
I've seen IARs with different length barrels. Barrel length is varies. the Madbul dissipator barrels I belive would work.
And the barrels from DMR style guns would also fall under heavy barrel
One more thing. Most IARs usualy dont run with a drum mag. they usualy run with extended magazines.