Where is the gas port situated on the barrel? What type of guards are you considering? Not all guards can be installed over a gas block, tube will need to be same length since distance from port to BCA is not changing.
Another option is to switch out to a free float tube which can be installed over a low profile gas block, you will lose front sight tower so a rail will be requird for BUIS, if desired.
In the picture below you can clearly see the different positions of the port relative to the front of the upper receiver. I am assuming your Carbine is in the traditional M4 setup as shown in the lower of the 2 rifles pictured and you desire a guard like the one in ( in length anyway) the upper rifle..
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Last edited by Jpyle; 11-11-2010 at 04:12 AM.