Project Appleseed

I didn't have time to save up for a decent scope for a new AR15 so I put on medium priced YHM BUIS (9584 front and 9680 rear) on it.  It's a 20" Wylde chamber, low profile gas block and shoots nice.  I had a problem at our KD day at 300 and 400 yd.  The front sight bottomed out and I couldn't adjust to raise impact at 400. 

I did some research when I got home and found out that the front sight is only for rail height not gas block height so I'm exchanging it for a Midwest MCTAR-FFG.  But as I'm waiting for these to cross paths in shipping it strikes me that the rail height bottomed out then the gas block height will bottom out sooner.  The rail on the gas block is 3/16" below the receiver rail.

What am I doing wrong here?

If your intent was to go full distance with a short sight radius carbine (or any rifle), why did you not get a rear sight with elevation adjustment?  That is what I believe is the first thing you are doing wrong.

It sounds like you need a taller/higher profile gas block. I beleive the receiver rail and gas block rail should be of equal height.
As accuracy of sight alignment is greater with an increased sight radius, or to put it another way, any inaccuracy is multiplied less at the target, you might want to consider getting that front sight as close to the muzzle as practical.  This might be a good time to consider doing so.

I'm not exactly familiar with the individual products you've mention. 
But there are a few other variables worth considering before you fret too much.


The gas block sight assembly will be taller.  Therefor, conceivably could have a greater amount of overall adjustment.
The actual height of the front sight post, could indeed vary.  Which could be changed, assuming they've gone with the standard A1/A2 sight - They come in all sorts of shapes and sizes.
BUIS sights aren't generally designed for any more adjustment than BSZ to begin with - At that, I would also suspect they're designed with the 16' carbine profile in mind as well.  The longer sight radius of your 20in(assuming you don't have a 20in barrel with the carbine sight radius - I've seen that before!  But anyway...) gives any adjustment a smaller moa value.  So, if they did their R&D with a 16in in mind - They probably wouldn't have even noticed their sight lacked suitable adjustment range.  If this suspicion is correct.

Never the less, with so many possible platform/accessory configurations for the AR, you're bound to run into this from time to time. 

Since you've already exchanged sights - Give the new one a try.  If you don't have enough adjustment one way or the other, you could try a taller/shorter front sight.  If you need more elevation, you could simply shave a few thousandths off the front post to save a few bucks.  Otherwise you could try an adjustable rear sight.

This is all under the assumption that your rifle itself and/or ammo are not to blame.

A real cheap fix for this is to throw a 1/4" or 1/2" riser under one of the sights. Should give you enough travel on the other sight.

If you are hitting low, you need a shorter front or a taller rear.  The one you have now  (9584) is the tall style for gas blocks as you can see in the picture below so you need a rail style (shorter) or an adjustable rear.  Like slim said, you could use a riser with your rear sight and then you may have enough adjustment to raise your current front sight back up.


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Quote from: GEmanuel on September 13, 2012, 08:03:59 PM
It sounds like you need a taller/higher profile gas block. I beleive the receiver rail and gas block rail should be of equal height.

The higher you raise the front, the lower you hit so he would be going in the wrong direction.

Until your parts come in it sounds like you need a needle file, a steady hand, and some instant blueing material to reblacken the sights as you slowly file off a little at a time until you get it zeroed where you want it.

File a little off, reblue, shoot a group, and repeat as necessary until the desired height is achieved - make sure you keep the front sight square during the process.

This is similar to what I do when initially zeroing an M14 after I get the rear sight calibrated with how many clicks I want off bottom for a certain range (I set rear sight 6-7 clicks off bottom, reset the 200 yard witness mark to correspond with this height, and shoot a group.  If the group is lower than desired, I begin slowly filing down the front sight until my 200 yard elevation zero is achieved.)  After this I have plenty of adjustment up or down on my rear sights depending on the load and/or distance.

If you have no rear sight adjustments and rely on the front sight for elevation shifts on your BUIS, you will need to know your front sight click adjustment values in MOA so you can be sure to leave enough adjustment available (down) before bottoming out so you can shoot to the maximum desired range you wish your sights to allow.

Remember - shoot a group, file a little if needed, reblue the sight, shoot another group, and repeat until the results are achieved.

Don't worry about filing down the front sight - it's obviously not working for you now.  Improve it!

Git-R-Done! 

Quote from: fepowered on September 13, 2012, 07:34:00 PM
If your intent was to go full distance with a short sight radius carbine (or any rifle), why did you not get a rear sight with elevation adjustment?  That is what I believe is the first thing you are doing wrong.

I wouldn't consider this a short sight radius carbine; it's a 20" barrel with a rifle length gas action.  The overall sight radius is 19.5" and the gas block, front sight is 6.5" from the muzzle.  My intent was to scope this but have BUIS as a backup.  I have a scope on it now, but I want/expect the BUIS to be able to work.  Am I wrong to expect the flip up BUIS to reach out to 400 yd?  I didn't have this problem with am AR10, but of course, it's a 308.

Thanks for all your help!  I really do appreciate it.

BUIS should offer same range as A2 sites - no question. They often lack rear site elevation adjust, but they should be dimensionally same as standard sites, as it relates to elevation required to make 400 yards. I have a set of YHM on a flat top with gas block rail. No idea of model #, but have shot it to 500 using them, and aside from my poor marksmanship, the sites worked fine. If you need for me to look at this more closely - send a PM.
PH

Why is the front sight mounted on your receiver in that picture??

Quote from: fepowered on September 14, 2012, 01:48:08 PM
Why is the front sight mounted on your receiver in that picture??

That isn't the front sight on the receiver, it's been sent back for exchange.  That a ARMS 40L rear that I took off my AR10 to check the sight plane level of the YHM 9680.

Quote from: TN_Dadx5 on September 14, 2012, 02:41:56 PM
That isn't the front sight on the receiver, it's been sent back for exchange. 

Are you exchanging what you had for a shorter front sight?

Quote from: fepowered on September 14, 2012, 03:07:58 PM

Quote from: TN_Dadx5 on September 14, 2012, 02:41:56 PM
That isn't the front sight on the receiver, it's been sent back for exchange. 

Are you exchanging what you had for a shorter front sight?

No, I sent the rail height (short) for a gas block height (taller).  But that's my question, won't a taller front bottom out sooner?