The round carbine handguards are the most common handguards on carbines, and the triangular or round handguards on rifles. The fat M4 handguards are not correct on carbines.
Often accessories are mounted on standard carbine handguards using small rail sections like the DPR16/4 or UPR16/4. Click Here for more info.
Polymer handguards with molded-in rail sections like the FGR-3 are also used. The FGR-3 has advantages over a rail attached to a standard handguard in that it is designed to be more secure have no movement. The FGR-3 fits very tight and may need minor fitting on some rifles.
Aluminum rail systems are used by specialized units, but are not used universally throughout the IDF. A standard rail system used by special forces units is the NFR. It is a simple, slim rail system that locks onto the rifle very solidly. It is also very light – not too much heavier than polymer handguards.
The VFR can be used when a free-floated barrel is required and can also be used with a carry-handle upper. Rails can be removed or placed in 8 positions around the handguard.
My recommendation for building a basic IDF carbine clone is to use the HG-S handguards with or without handguard bands. If there is a possibility that foregrips or lights may be mounted, then use either the FG handguards with UPR or DPR rails added (older configuration) or the FGR-3 handguards (newer configuration). You can use a handguard band with either configuration.
If you want and IDF Special Forces carbine clone, then go with the NFR rail system unless you need a free-float barrel for a marksman configuration, and then use the VFR.
For a rifle (20″ barrel), use triangle handguards or round handguards; either will be correct. If you need a rail system, use the NFR-RL (rifle length NFR).