The rifle is a 16" DPMS flattop with a front YHM BUIS YHM-9835A
Front Flip Sight Tower With Bottom Rail and the rear is a ARMS 40L.
...Well it’s a new one and upon shooting it today I noticed it was grouping nice but low.
At 25 meters all the rounds came in around 4-5" low. So I figured I would turn down the front post, did that tested (still low) did it again -(still low) till it was all the way down and the rounds are still about 3-4" low at the 25meter mark. Tried to adjust my sight picture and put the front post to the top of the aperture but she still hit low (about 1“ @25m).
What gives? Are the two different BUIS not compatible?  
The Arms rear sight has no elevation adjustment only windage so what should I do to solve this dilemma? Suggestions please.

You could always try the 50m zero………….more folks prefer it to the 25m zero anyways. It gives you a flatter trajectory out to about 250m or so.  

link to 50 yard zero

groups.msn.com/TheMarylandAR15ShootersSite/improvedbattlesightzero.msnw

Thanks for the response but how does this help?
My rear sight has NO elevation adjustment and I have taken all the adjustment out of the front by turning it down and it till shoots low.
This is not a marksmanship problem- all the groups shot held tight, just low.
Any ideas if the sights work together?

Quoted: Quoted: Thanks for the response but how does this help? My rear sight has NO elevation adjustment and I have taken all the adjustment out of the front by turning it down and it till shoots low. This is not a marksmanship problem- all the groups shot held tight, just low. Any ideas if the sights work together? I think people are trying to show you the 50 yard zero so you don't have to modify your front sight, or buy something different.  They are giving you the cheapest help possible.   Did you build this, or did someone else?  You may be looking at the F/non-F FSB issues.

Could you please elaborate on the " F/non-F FSB issue"
I don’t know what this is. ^
I guess I didn’t understand the 50yd zero application here because it involves adjusting the rear sight and my rear sight has no elevation adjustment, can someone explain in more detail how this would work given the BUIS im running. The FSB appears to be installed correct.
The rifle was built by a local gunsmith who has a good reputation from what I know- supposedly built a bunch of AR's for our Metro PD.

completely remove your front sight post,take a very small amount of the post off with a file (few thousandths should be enough) the put back in. the shorter front sight post will solve the problem. and if you screw up, a new frontsight post is only a few bucks.

Quoted: completely remove your front sight post,take a very small amount of the post off with a file (few thousandths should be enough) the put back in. the shorter front sight post will solve the problem. and if you screw up, a new frontsight post is only a few bucks.

I already lowered  the front post like 6-8 clicks. So do you think a few thousands filed off will correct this problem?
I would have to think someone has used this BUIS combo and/or could offer some insight.

yes a few thousandths removed will fix the problem. DD

Quoted: I guess I didn’t understand the 50yd zero application here because it involves adjusting the rear sight and my rear sight has no elevation adjustment, can someone explain in more detail how this would work given the BUIS im running.

Gotta head off to work so I’ll try and explain as simple and to the point as possible.

At 50m the rounds trajectory is higher at the point of impact than at 25m. If you were to zero at 50m the round could possibly strike the target closer to the point of aim than at 25m, so less elevation adjustment would be required.

If I had time I’d dig up a bullet path trajectory chart to illustrate what I mean. Maybe another member in the know can help me out.

Quoted: Quoted: I guess I didn’t understand the 50yd zero application here because it involves adjusting the rear sight and my rear sight has no elevation adjustment, can someone explain in more detail how this would work given the BUIS im running. Gotta head off to work so I’ll try and explain as simple and to the point as possible. At 50m the rounds trajectory is higher at the point of impact than at 25m. If you were to zero at 50m the round could possibly strike the target closer to the point of aim than at 25m, so less elevation adjustment would be required. If I had time I’d dig up a bullet path trajectory chart to illustrate what I mean. Maybe another member in the know can help me out.

Now i am starting to understand  previously posted responses from others on this thread.
Thanks this is starting to make sense now. -
Anyone have links to Battle Zero / trajectory / poa/poi info?

I have this problem with one of my ARM 40 flip up rear sight with the standard front sight.  EXCEPT that it shoots 7-8 inches to the left.  And all the windage I put into it doesn't help.  If I put the Carry handle with A2 sight back on it shoots fine.  ARMS has no clue what I am telling them.  I've had this problem for 2 years now.  

Sorry not trying to hijack your thread.

Quoted: Anyone have links to Battle Zero / trajectory / poa/poi info?

This might help.

groups.msn.com/TheMarylandAR15ShootersSite/zeroingmethods.msnw

Quoted: Quoted: Anyone have links to Battle Zero / trajectory / poa/poi info? This might help. groups.msn.com/TheMarylandAR15ShootersSite/zeroingmethods.msnw

Thanks for the link - its chock full of info.
So if i understand this correctly -
every rifle/carbine using the various military battle zero procedures - Strikes low at close range ,within 20m for example?

If this is the case and you are zero at 50m/200m then what is your poi at 25m? -1",-2",-3"?

I am still doubting my BUIS if i adjusted my front sight all the way down and still couldnt hit center at 25m.hinking.gif

If I was you, this is what I would do:

Set my sights to mechanical zero. Attempt to zero at 50m and see if the 50m zero cures the problem. If not then I would consider removing a few .001’s off the front sight post to achieve the correct 50m zero. Sorry, can’t offer any more help than that.

Anyone else?


ETA: Here is an example of the trajectory of the M855 round taken from the Army Marksmanship manual. Your ammo used for zeroing will probably differ from the M885, but this chart gives you an idea of how the bullet path rises from 25m out towards 50m, peaking at about 175m. If you notice the POI compared between 50m and approximately 275m you will see about the same POI.

The difference between a 25m zero and a 50m zero on this chart appears to be about a 4.5-5.0” difference. This is why a 50m zero might benefit you.

Went out and tried to zero at 50 yds. as suggested.
Same as before, cant adjust down the front anymore and the bullets are still hitting low.
So I went to a buddys house and used his Aimshot bore sight. Checked the sights at 25 yds. and it shows the laser right where my hits had been -3" low approx.

At this point I am debating replacing the front sight -
Guess any suggestions would be great.

I hear Troy is liked by many - only concern would be if it will do the same thing?
So questions are...
1)Anyone know if a front Troy BUIS is compatible with rear Arms 40L?
2)Should I be checking height of sights before I change out the front BUIS,
3)If so does anyone have a link or explanation as to proper procedure to check sight height above the rail?