Sighting in new BCM 14.5 middy with LaRue BUIS...is this a problem?

JohnnyNumbers · #1 04-22-12, 17:21

Hey guys. I rarely post here as I'm basically a lurker taking in as much information as I can and learning as I go along, but I ran into an unusual situation today at the range and I'm curious to know if this is something to be concerned with or not.

Before I get into the issue...A while back I purchased a Magpul MBUS for my LE6920 as I had removed the carry handle in order to mount a RDS. Zero the MBUS at 50yds with the base of the front sight post flush with the top of the FSB. A couple windage adjustments and I was dead on without having to make any elevation changes. GTG.

A week ago...I got in a new BCM 14.5 mid-length upper. Now what I don't want to do, is move my red dot and MBUS from the Colt over to BCM since the 6920 is zero'd.

A year or so ago, I picked up a LaRue BUIS that I never really used...so I figured I would mount the LaRue to the BCM, zero it...and then whenever I can pickup another lower, I'll have another AR build to play with.

Here is the LaRue BUIS I'm referring too...

Ok...start out with base of the front post flush with the FSB, rear aperture (small) centered. Take 5 shots at 50 yds. In the picture you can see the first group at the top right...then you can see the adjustments as I walked in my rounds to the center of the target.

Now...this is what has me at pause...

My question is...is it normal for the front sight post to be this elevated after a 50 yd zero depending on what rear BUIS may be used? I was under the impression that rear sights be they folding or not...were supposed to be pretty close to the same height off the upper receiver...but I may be wrong. IDK.

Remember...the front post on my Colt needed no adjustment at all. It's still flush with the FSB and it's dead on at that same range.

What I may do, is mount the carry handle off the Colt onto the BCM and re-zero to see if there is a change.

Failure2Stop · #2 04-22-12, 17:50

No, that is not normal.
Something is off somewhere.
Easy answer is to get a taller front sight post.

JohnnyNumbers · #3 04-22-12, 18:07

Quote:

Originally Posted by Failure2Stop

No, that is not normal.
Something is off somewhere.
Easy answer is to get a taller front sight post.

I thought of that as well, but if it needs a taller front sight post, then I would think BCM would make that the standard.

I think I'll test out the carry handle and maybe even pull the MBUS off the Colt if I have too. If I still have the same problem, then I know it's the front site. If not...then the LaRue is screwed up.

sewvacman · #4 04-22-12, 18:11

Looks like the sight post is above the ears though.
I have the same LaRue buis on one of my rifles and several other brands as well (DD, Troy). None have the sight post that high, most are flush w/ the top of the FSB or lower. Try the carry handle, that should narrow it down to the sight or the upper.

JohnnyNumbers · #5 04-22-12, 18:20

Quote:

Originally Posted by sewvacman

Looks like the sight post is above the ears though.
I have the same LaRue buis on one of my rifles and several other brands as well (DD, Troy). None have the sight post that high, most are flush w/ the top of the FSB or lower. Try the carry handle, that should narrow it down to the sight or the upper.

Yeah that's where the sight post ended up after I got it sighted in. I started off with the front post flush with the FSB.

When my first group was like 6" high at 50 yards... I knew something wasn't right.

Failure2Stop · #6 04-22-12, 18:24

Quote:

Originally Posted by JohnnyNumbers

I thought of that as well, but if it needs a taller front sight post, then I would think BCM would make that the standard.

I think I'll test out the carry handle and maybe even pull the MBUS off the Colt if I have too. If I still have the same problem, then I know it's the front site. If not...then the LaRue is screwed up.

Is the FSB "F" marked?
Usually BCMs are good to go, but sometimes things fall through the cracks even with good companies.

I also misspoke, the "easiest" answer is to swap rear sights if you have one available.

JohnnyNumbers · #7 04-22-12, 18:34

Quote:

Originally Posted by Failure2Stop

Is the FSB "F" marked?
Usually BCMs are good to go, but sometimes things fall through the cracks even with good companies.

I also misspoke, the "easiest" answer is to swap rear sights if you have one available.

Yes...the FSB is most definitely "F" marked.

I'll get back to the range as soon as I can.

Cowtown556 · #8 04-22-12, 19:54

try another rear sight,my troy lets my front post set two clicks lower than my LMT or GGG rear sight.

JohnnyNumbers · #9 04-24-12, 16:54

Update...

Ok...back to the range today with carry handle off my Colt mounted on the BCM upper.

FSP reset to flush and the rear sight set to 6/3 and centered. Range is 50 yards...

Now from what I have read using M193 w/14.5" barrel and hitting about an inch high at 50 yards...should put me right on at 100yds. At least thats what I thought I read somewhere on this forum... I may be wrong.

Move to the 100 yard range...thinking I should be hitting around the general vicinity of the 10x with my grouping, granted my fundamentals are in check.

Fire a 5 shot group...

That first group is roughly 5" high from center.

Make a 1 click FSP adjustment and lower my POA to where I have it marked on the target (at least that's where it appeared looking thru the aperture at that distance and my eyes aren't as good as they used to be) which still has me printing 2" high from center of the 10x.

I could make further adjustments raising the FSP to bring my POI lower...but here we are getting back to raising my FSP up to a point to where it's as high or higher than the ears of the front sight base, which to me isn't right...and the whole purpose of this thread.

Here is where the FSP is set as of now...

So again I ask...do you guys think this a unusual? I could just remember that I'm hitting 3" high at 100 yards

or just go ahead and see about getting a taller FSP. It just seems to me that if a taller post is needed...BCM should make that a standard.

tsconver · #10 04-24-12, 17:07

Why not set it at 300 yards like that carry handle is designed for? Zero at 25 and you should be about 6 high at 100 and zero at 300. Once set at 25 verify at 300 and fine tune your zero.

Sent from my DROID RAZR using Tapatalk 2

Doc Glockster · #11 04-24-12, 17:53

Something is out of spec.

Another bad BCM upper?

I used one of the improved battlesight zero's on my BCM carbine. I zeroed at 50 yards and the front sight post's flange was just above the sight base hole, but not as bad as yours. A taller Bushmaster front sight post fixed my issue. It may fix yours but I suspect you will still have part of the front sight flange showing above the edge of the front sight base hole.

I'm not an expert on this but I'm wondering if your gun just shoots unnaturally high.

Tweak · #12 04-24-12, 17:56

Is your BUIS seated into a slot in the upper?

ETA: a 100 yd zero will be low all the way out.

If the BUIS is properly seated then contact BCM.

JohnnyNumbers · #13 04-24-12, 18:05

Quote:

Originally Posted by tsconver

Why not set it at 300 yards like that carry handle is designed for? Zero at 25 and you should be about 6 high at 100 and zero at 300. Once set at 25 verify at 300 and fine tune your zero.

Sent from my DROID RAZR using Tapatalk 2

That might be possible if I had access to a 300 yard range...but I don't. 100 yards is all I have to work with.

I'm grouping 4" high at 50 yards with FSP set flush and rear sight set at 6/3. I can only guess where I would be printing at 25....5" maybe 6" high? I can only imagine where my front post would be elevated after I zero'd at 25 yards.

Quote:

Originally Posted by Doc Glockster

Something is out of spec.

Another bad BCM upper?

I used one of the improved battlesight zero's on my BCM carbine. I zeroed at 50 yards and the front sight post's flange was just above the sight base hole, but not as bad as yours. A taller Bushmaster front sight post fixed my issue. It may fix yours but I suspect you will still have part of the front sight flange showing above the edge of the front sight base hole.

I'm not an expert on this but I'm wondering if your gun just shoots unnaturally high.

I'm thinking that I may go ahead and get a taller post because something isn't right. I'm also going to send BCM an email and ask their opinion as well.

Quote:

Originally Posted by Tweak

Is your BUIS seated into a slot in the upper?

I used the carry handle today. If you look at the picture posted, you can see it's mounted properly.

Canonshooter · #14 04-24-12, 18:13

Quote:

Originally Posted by JohnnyNumbers

I'm also going to send BCM an email and ask their opinion as well.

^^^THIS^^^

I also have a relatively new BCM 14.5 Midlength (LW) with a FSB and the same LaRue rear sight, and it sighted-in fine. The top of the post base about flush.

Something is amiss, IMO.....

Tweak · #15 04-24-12, 18:29

Quote:

Originally Posted by JohnnyNumbers

I used the carry handle today. If you look at the picture posted, you can see it's mounted properly.

I saw that, I've also seen people who don't know how to mount their sights and if they do it with one they can do it with another. The only thing to check is the rear sight, either construction or installation. If both of those are good then it's on the manufacturer and the innumerable variables that can lead to mis-alinement.

JohnnyNumbers · #16 04-24-12, 18:30

Quote:

Originally Posted by Canonshooter

^^^THIS^^^

I also have a relatively new BCM 14.5 Midlength (LW) with a FSB and the same LaRue rear sight, and it sighted-in fine. The top of the post base about flush.

Something is amiss, IMO.....

When I sighted in my 6920, all I had to do was make a couple windage adjustments and I was dead on at 50 yards. That's why this whole episode seems so strange.

I hate to go thru the hassle of having to send this thing back if need be...but for the money spent and my OCD (LOL) I don't see another option.

JohnnyNumbers · #17 04-24-12, 18:35

Quote:

Originally Posted by Tweak

I saw that, I've also seen people who don't know how to mount their sights and if they do it with one they can do it with another. The only thing to check is the rear sight, either construction or installation. If both of those are good then it's on the manufacturer and the innumerable variables that can lead to mis-alinement.

I agree.

I'm getting the same result using the LaRue BUIS as I am with the carry handle. At this point...I'm contacting BCM.

I shouldn't have to alter this upper on my dime to make it shoot right...i.e. change out the front sight post.

TacMedic556 · #18 04-25-12, 00:16

Quote:

Originally Posted by Tweak

Is your BUIS seated into a slot in the upper?

ETA: a 100 yd zero will be low all the way out.

If the BUIS is properly seated then contact BCM.

Definately check the mounting of the Larue as stated above. If it is not properly seated into the picatinny rail slot as designed it will be off.

ucrt · #19 04-25-12, 00:31

.

To me, a taller FS Post isn't a solution on a fighting rifle because the post will be above the protective ears.

Just curious, are you using the large or small peep sight? I can't shoot for jack with the large peep...but it's probably my old eyes.

.

JohnnyNumbers · #20 04-25-12, 00:40

Quote:

Originally Posted by ucrt

.

To me, a taller FS Post isn't a solution on a fighting rifle because the post will be above the protective ears.

Just curious, are you using the large or small peep sight? I can't shoot for jack with the large peep...but it's probably my old eyes.

.

I have ruled out getting a taller site post as of now. I sent an email to BCM and should get a response here pretty soon...I hope. I'll make a determination after they respond.

I was using the small aperture.