Re: RRA carbine to Dissy
If you use standard handguards or a non-freefloat type rail you will basically use a second gas block or front sight post to hold the handguard/rail. The standard carbine length gas block you currently have is still used, the second is not used as part of the gas system, but strictly to secure the handguard. You may need to replace uour existing gas block with a low profile unit so it will fit under the new extended handguards/rail. Basically, you have a standard carbine length gas system and a second non-functioning gas block in front of the handguard. If you go with a free float style rifle length tube, the second gas block is not required unless you want to run a standard front site base.
Kirk R