I have a bushmaster 14.5 inch carbine with A1 fixed carry handle,The FSB has a crown embossed on it. So now I want to build a new upper for it.I have ordered a new 16 inch heavy barrel from Windham weaponry which also has crown embossed on the FSB . Also ordered a BCM flattop receiver which I'm going to install the new barrel on. And now here comes real question I have just bought a stand alone Troy battle site ( which is made to use with a standard FSB )to put on my BMC flattop receiver . I believe I'm going to have to install a taller front site post in the FSB. CAN anyone advise me what the differences between the standard front FSB and the F marked and the Crown embossed one ?