I have a Knights RAS on an M4 and have been happy with its performance. Assuming however that the information regarding failures experienced by the 75th Ranger Regiment using the RIS/RAS are true,  www.biggerhammer.net/stoner/wwwboard/index.cgi?read=2047 there is obviously a problem with this system due to the heat retention combined with pressure on the barrel.

Therefore, the four most popular models including the SIR, RASII, FF RAS & FIRSH offer a possible solution to this problem. The heat issue aside, these models will all free-float the barrel at least partially eliminating the possibility of the failure described above.

However, I am undecided about the barrel mounted (FF RAS & FIRSH) vs. receiver mounted (SIR & RASII) arguments mentioned in several earlier threads since both mounting methods seem very durable and secure. As I see it, there are three major issues to be addressed. Heat transferred to hands or mounted accessories, a change in POI due to thermal expansion and ease of maintenance. The two receiver mounted models are arguably better with respect to ease of maintenance.

However, regarding the first two issues, I would be interested in hearing if any FF RAS & FIRSH owners have 'experienced' (not hypothesized) a shift in POI after long sustained fire or have observed that the heat transferred to mounted accessories was excessive and potentially damaging.

FIRSH is not the only free float tube with rails.  There are similar tubes available from Les Baer, CMore and others.  I think KKF was actually first to market, though not by much.