Ok i have a bushmaster shorty and a BM with a 20 inch barrel. I want to replace the front sight bases on both to have bayonet  lugs. are they both the same sight base? also where can i find this part? thanks mike d
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FSB swap is not an easy task, as there is no standard as to where the pin holes are drilled.  Each barrel/FSB assembly is drilled as a unit.  Basically a FSB from one barrel will have holes different than 99.9% of all other barrels.  It is a task best left to people with the proper tools.

If you already knew this, Bravo Company USA sells the "F" marked FSBs and Yankee Hill Machine sells the standard FSBs, both of these are UNDRILLED.  Drilling a new FSB to an old barrel is not a task I would like to attempt.  Your measurements would have to be exact and your drilling perfectly square, or else you will end up with a canted FSB, which your rear sight windage adjust may or may not be able to compensate for.

you need a new barrel and front sight assembly.  150.00 should get you where you want to be.
or just not mess with the bayo lug

Or Kurt at KKF can put a lug on your existing site base. See his forum in the Industry section.

well i was going to get a front sight base that has set screws, so center it easily. someone tole me its not thats hard to do at all. i dont see this topic under that section- can u post a link? thakls mike