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Consistently low?  

Bushmaster carbines, in general, flattop or not, don't come with proper height FSBs or FSPs.  

That being said, your rifle should fail to zero high, not low, if this was the issue.  

What range / zeroing procedure are you using?  

~Augee

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All of mine came with the right FSP/FSBs.

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I donno, I remember seeing a post a long time ago about bushmasters sights being marked differently to reflect the taller sight. What says it HAS to be marked with an F?  In your first post you said they dont come with the right sights. Not the right markings on the sights. Just sayin...

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Some Bushmasters have been spec'ed by purchasers to have the proper components.  

When Colt developed the carbine back in the sixties, they found that a small percentage (less than 20%) of carbines could not be zeroed properly due to the reduced sight radius.  Their solution at the time was to issue carbines with taller front sight posts.

When the flattop M16A4 receiver was developed, the upper receiver had to be thickened as well, because Dick Swan was reportedly able to stick a sharpened pencil through the thin aluminum of the top rail.  Coupled with the large scale adoption of the M16A4 and the M4 flattop rifles, Colt switched from taller front sight posts to new FSBs with the front sight pocket .040" higher and marked them as "F" to designate "flattop."  This was primarily to simply the military supply system where "bench stock" of spare parts are used, meaning spare parts are pulled from a bin.  It is easier to simply have one front sight post in the inventory, than two different ones to install since the taper pinned FSB is semi-permanent to the barrel and not usually removed.  

Bushmaster's carbines and flattops do not follow this convention of using the .040 higher front sight as it would mean they would have to complicate *their* manufacturing process to install the proper FSBs and/or FSPs.  Rather they installed rifle FSBs and front sight posts on all of their offerings and instead chose to drop the rear sight base on the detachable carry handle.  No modifications were made to their fixed carry handle carbines, however.  

What this means is that their fixed carry handle carbines have a built in failure rate.  Approximately 15% of those carbines cannot be properly zeroed from the factory.  They do, however sell a .040 taller front sight post.  Bushmaster calculates, probably accurately, that the failure rate is low enough, and most users don't shoot enough and don't care enough that they can slime on by without addressing this issue.  The few customers who do contact them about it are invited to purchase a .040 taller front sight post, or if they complain more publically, Bushmaster will be "gracious" enough to send them a free taller FSP.  It's just good business in their eyes, if less than 15% of the rifles fail, and only a small percentage of those users will actually bother to complain, they can send out quite a few free FSPs before it approaches the cost of sourcing, purchasing, and assembling with proper components from the first place.  

Their flattops then, might have a backwards solution with their proprietary detachable carry handle, however, a great many people who purchase flattops purchase them with the expectation of installing an aftermarket rear sight.  The problem with this is that almost all aftermarket front sights are calibrated for military height front sights - i.e. using the "F" marked FSB.  Meaning that the innate failure rate has re-appeared.  Again, the risk is taken that the majority of people who install rear flip ups probably do not even bother to properly zero them - and only a small percentage of rifles fail - again, they save money by not correcting their rifles, and simply addressing individual issues as they pop up.  

In other words - Bushmaster rifles typically *do not* come with the right front sight height, but since they consider the failure rate "acceptable" they do nothing about it even though they know it's an issue.  Again, as a business, can I say this is a bad decision on their part?  Perhaps not.  But it makes you wonder if a 15% failure to zero rate is considered acceptable, what other parts do they apply the same logic and cost cutting procedures to?  

As a counter-example, LMT rifles do not come with "F" marked FSBs, but they are "F" height.  LMT does not produce non-flattop rifles, therefore there's no need for them to distinguish between "F" and non-"F" FSBs.  They are not marked "correctly" but are functionally and dimensionally correct.  

~Augee

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Well I went to the range today with the new MBUS2 and I finally got it in paper, sort of...

I mis-remembered when I said that I was shooting low. My point of aim was way low to hit the paper, and at 50 yds, I was about a foot or so above the target. I called BM and spoke with one of their techs who reiterated what you all have been saying; I need a .040 taller front sight because BM "tried to save money on not engineering a proper flat top front gas block and reusing the A2 one, thus causing them to have to redesign the carry handle lower to work on the carbines."

After basically telling me that BM was being cheap by not just supplying the M4's with the taller FSB, he wouldn't just send me the taller FSP. $5.50 to get it to me in about a week...  

I hate to say it, but this has really soured my on my BM. I liked it when I was using the iron sights, not a single problem. Now I would be happy just getting rid of if it and starting from scratch...