I am going to give you a terrible answer, can't be helped. You can suppress any length AR barrel. The issue that you mentioned (and others) is that it forces more gas back in a DI system and makes the gun unpleasant to shoot. You can spend money changing our your buffer, your bcg, adding an adjustable gas block but at the end of the day a DI AR is designed to push gas back so it is always there. I currently own one AR that is DI and it is a DD Mk 18. All of my other ARs are piston rifles, my current favorite is the Primary Weapon System long stroke piston rifle. The piston guns with adjustable gas blocks let you suppress anything and are easily tuned, no more gas in the face, no more over gassing (unless you want it). Effectively you end up with the modularity and adaptability of an AR with the reliability of an AK. I have shot and messed with the POF, the LWRC, the HK, a couple of Sigs and they all work great. I don't have personal experience with but I have heard that the Adams Arms and the Stag Arms piston systems work really well also.