The GenII forends lock in place both with a jam collar and with set screws that push against the barrel nut.  They are a little tricky to install but once in place they are very secure. (This is the version seen on the original SPR's where the collar is the same diameter as the forend tube.) Long ARMS rails are made to slide onto the receiver and forend rails from the rear.

The GenIII is the current model with the larger collar. It requires relief cuts on the ARMS rails to clear the collar and also to allow the rail to "tip" onto the receiver and forend top rail. The are also solid and, as already mentions, align to the barrel nut with aligning pins mounted to the inside of the forend tube.

While the gas tube does go through the entire assembly on both models and would prevent the forends from turning too far in either direction, this is not an appropriate function for the gas tube to be performing. It needs to stay centered but should not have side pressure forward of the receiver or functioning of the upper could be affected.

Paul