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Posted: 1/13/2012 11:32:44 AM EDT
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I know Bushmasters do not have "F" FSBs and that their removable carry handles are lower than milspec. Does this dimensional issue also carry over to their A2 upper receivers?
I'm piecing together an A2 build using a DD 14.5" LW with M4 cuts. Bushmaster seems to be the easiest A2 receiver to find in black and with M4 cuts, but if it's dimensionally challenged it might be an issue with the DD barrel's "F" FSB. Thanks! |
| I have a middy with an "F" marked FSB that had issues with A1 and A2 receivers. The front sight had to be set way low to zero at 50 yards. I even filed off some of the front sight post to have some elevation room left. This is my first time dealing with fixed handle receivers on 16" barrels and may not be a real problem but is screws with my sight picture because the post is so short. May not be a deal breaker for others. |
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A1 and A2 carbines will work fine with F marked FSBs with very rare exceptions. All the pre-flat top Colt Carbines used a slightly taller (about 0.04") front sight post to account for the extra height often needed with the shortened sight radius. The dimensional changes in the flat top made this worse, so that's why they went to the F FSB that is 0.04" taller for flat tops and allows use of the standard height front sight post with flat top carbines (simplifying logistics). That LA_357SIG had a problem could be excacrebated by the fact he's using a middy barrel, or just the bad luck that everything (tolerenace stack) went against him on that build; sucks. But, plenty of folks have used F FSBs on A1 and A2 uppers without problem.
http://www.ar15.com/forums/t_3_18/467842_.html |
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Quoted:
If you want to use an A2 receiver then go with a NON F marked front sight. The F marked is higher to match the removable carry handles height. The bushmaster A2 is at the normal A2 height as far as I know. So you should use a non F marked front sight. Yeah, I'm aware of that - it's not ideal, but it usually works. |
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Quoted:
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It's .040 for Christ sake. If it's a problem get a .040 taller post. Did you even read the OP? Besides, the last thing you'd want with a taller than optimal FSB is a taller sight post. Lol. But anyway, the general rule of thumb is: flat top upper = F marked FSB fixed carry handle upper = non F marked FSB I have no idea how Bushmaster made this a clusterfuck, but in MY situation: Bushmaster 20" barrel with a non F marked FSB Bushmaster A1 and unknown 3 dot keyhole A2 upper - no problems Flat top upper: Bushmaster carry handle - no problems Troy Flip up rear sight - no problems LMT fixed rear A2 sight - no problems Rock River SS 16" Midlength barrel with F marked FSB: Flat top upper: Troy/YHM flip up sights and Bushmaster carry handle - no problems A1 and A2 fixed carry handle - problems Mechanical zero shot at least 1 1/2 ft low @ 25 yards. Delton 16" carbine barrel with non F marked FSB Flat top upper: YHM/Troy flip up rear sights, Bushmaster carry handle - no problems A1 upper - no problems No problems means upon initial sighting in produced groups on paper with sights set at mechanical zero @ 25 yards. Problems would indicate groups were not on paper from mechanical zero. On paper = 8"x11" M4 25m zero target. So, in closing: It seems like rifle length doesn't matter. Carbine and midlength may have issues. Flat top = F marked. A1/A2 = unmarked. |
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Quoted:
Lol. But anyway, the general rule of thumb is: flat top upper = F marked FSB fixed carry handle upper = non F marked FSB I have no idea how Bushmaster made this a clusterfuck, but in MY situation: Bushmaster 20" barrel with a non F marked FSB Bushmaster A1 and unknown 3 dot keyhole A2 upper - no problems Flat top upper: Bushmaster carry handle - no problems Troy Flip up rear sight - no problems LMT fixed rear A2 sight - no problems Rock River SS 16" Midlength barrel with F marked FSB: Flat top upper: Troy/YHM flip up sights and Bushmaster carry handle - no problems A1 and A2 fixed carry handle - problems Mechanical zero shot at least 1 1/2 ft low @ 25 yards. Delton 16" carbine barrel with non F marked FSB Flat top upper: YHM/Troy flip up rear sights, Bushmaster carry handle - no problems A1 upper - no problems No problems means upon initial sighting in produced groups on paper with sights set at mechanical zero @ 25 yards. Problems would indicate groups were not on paper from mechanical zero. On paper = 8"x11" M4 25m zero target. So, in closing: It seems like rifle length doesn't matter. Carbine and midlength may have issues. Flat top = F marked. A1/A2 = unmarked. Thanks for the info with your experience! I've always followed the F-with-flattop rule, but it seems sometimes at least you can get away with an F FSB with a fixed carry handle. I ended up finding a used CMMG A2 upper with M4 cuts so I won't be ordering a new BM after all. With any luck... (ETA - typos) |
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even with F marked FSP - a colt front site should be used with rear flip up sites like magpul and arms. Colt are .40
seeing the pictures of a3 with f fsb cranked all the way up should tell the shooter something....but it doesnt 
I have always changed all of my post to colt post. works like a charm. bushmaster sells the colt F site for about $6 |
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