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A Novice's AR-15 sighting problem

Discussion in 'Rifle Country' started by 230RN, Oct 10, 2008.

  1. 230RN

    230RN Marines on Mt. Curibacci

    UPDATED: A Novice's AR-15 sighting problem

    I recenty got an AR-15 on the principle that I know nothing about them and figured it's about time I learned. After all, it's been around for like 60 years and I've been around for almost seventy, and it was about time we met.

    DPMS Flattop Panther in .223/5.56.

    Sixteen inch barrel.

    Stock front sight.

    CAA rear sight flip-over peep. (I can't find this sight on their site.)

    Sight radius 14 7/8 inches, call it 15 inches.

    XM 193 ammunition.

    Long story short, I took it to the indoor 25-yard range and found that even with the Battle Sight cranked down to its lowest, and the front sight set so far up that it's loose, I could not bring the point of impact (POI) to the desired 1 inch or so below point of aim (POA).

    Present groups, even with the loose front sight, are about 3" above where I want them.

    (I had figured about 1" below point of impact would get me about 1 1/2 inches high at 100 yards, which is how I sight all my centerfires, and yes, I read about the "8/3" and "6/3" settings for the two flipover peeps, but for now, I just wanted it about 1 1/2 inches high for 100 yards.)

    So, I figure I need either at least another 0.050" more on the front sight post, or, to somehow lower the rear sight that amount below its lowest setting.

    QUESTION 1: Are the CAA rear sight and the DPMS rifle itself simply incompatible?

    QUESTION 2. Are higher front sight posts available? Or, alternatively, is there a trick to lowering the rear sight the required extra 0.050"?

    Pic shows the general configuration --nothing out of the ordinary.

    Attached Files:

    Last edited: Oct 12, 2008
  2. a little help

    A few pointers cheek weld-making sure your face is in the same place every time. A good way to do this is to put the same amount of your nose on the charge handle everytime.Screw fromt site back in until it seats then turn back out 5 turns(1 turn is about 1moa).Use small site on rear site and start at about 5 clicks from bottom with half minute rear site. Most important part of this game is consistent site picture. Good Luck and have fun:evil:
  3. Armueller2001

    Armueller2001 Well-Known Member

    I think you can order higher front sight posts. I had to crank mine up pretty high to get it zeroed
  4. Emfuser

    Emfuser Well-Known Member

  5. 230RN

    230RN Marines on Mt. Curibacci


    I don't think cheeck weld or hold or anything like that is the problem. I've shot light rifles before (which would be more sensitive to "hold") with no problem getting the sights to register. And I've shot very well with peep rear sights before.

    The problem seems to be that the CAA sight mounts too high on the flat-top for the front sight on the DPMS.

    From what I think you're saying, I should crank both sights down, and then back up your recommended distance ("clicks"), but the problem is that even with front sight all the way up (to the point of looseness) and the rear sight all the way down, it still prints way too high.


    What kind of rear sight were you using? Where might one obtain a higher front sight post? Was that front sight loose when you cranked it up "pretty high?"


    Yes, I read that, and I understand the "proper" military way to set the sights --quite simple, really, once you understand what the "8/3" and "6/3" are for. For this flattop, I'd use the "6/3" setting stamped on the vertical knob and do it right. But when I found I had to use the front sight like an extension ladder :) that method went out the window, which is why I started to go to my "hunting rifle" method... i.e., POI one inch below POA at 26 yards so the rifle would print about 1 1/2" high, sorta, kinda, at 100 yards.

    My feeling about all this is that the CAA sight is just built too high for the DPMS Panther factory sight.

    I sure like that XM193 ammunition. I think I'll get another 500 rd.
  6. taliv

    taliv Moderator

    230rn, if you've been around 70 years, i doubt you're doing anything wrong here. the system is pretty simple and there are in theory, only two points to adjust (moving the front and rear sights up and down)

    based on your post, i'd say there's about a 90% chance they machined something wrong. I'd go borrow someone else's rear sight and attempt to zero again.
  7. DrPerry

    DrPerry Active Member

    Here is a link to an ar15.com thread


    Real good info here

    I think either Bushmaster or KN makes a taller front sight post.

    I would call Brownells or Model 1 sales

    Brownells 1-800-741-0015

    M1S 1-903-546-2087

  8. rob_s

    rob_s Well-Known Member

    You need a taller front sight post. Common problem with makers that don't use an "F" front sight base. Your rear sight is for milspec rifles, and your front sight isn't milpec for a carbine.
  9. entropy

    entropy Well-Known Member

    Don't make me older than I am! :mad: I am @ the same age as the AR-15, and I am not yet 50. They were first used by USAF aircops about the time I was born.

    We zeroed our M16A1's with M193 ball at 25m in BRM, and fired COM on sillhouettes out to 300m with that sighting. (actually I found no problem in making headshots on the sillhouettes at 100m and under, and shot COM past 100m.)

    Start from mechanical zero. (to get to mechanical zero, run the front post all the way up or down , whichever is closer, then count the settings until it runs out. divide by 2, and set. Do the same for the rear with the windage drum. That is mechanical zero.) Now fire 3 rounds at 25m, note group and adjust accordingly.

    If you are using an A2 type barrel and sight setup, go to M855 ball instead of M193 ball, M193 ball is for the A1 barrel and sights. I am not a fan of clamp-on BUIS's, too many variables in sighting, as you have found. HTH.
  10. rob_s

    rob_s Well-Known Member

    this is not a mechanical issue, it's a manufacturer and end-user issue.

    The solution is as I posted above.
  11. HJ857

    HJ857 Well-Known Member

  12. Emfuser

    Emfuser Well-Known Member

    I agree with Rob S. Sounds like you've got the wrong front sight for your rear sight.
  13. 230RN

    230RN Marines on Mt. Curibacci

    This one:


    looks like a kit for a 'smith. I don't need all that, just one like I have, but I'd say .0625 taller. (The calculation shows about .050" as I mentioned, but it's a lot easier to cut metal off than to put it on. "Cut once, try twice," as the saying goes.)

    This one:


    is a sight adjusting tool. I "Murphy'd" something up out of a round piece of bamboo fondue stick shaved down on one side to fit in the semicircular slot to depress the detent button, and heated a piece of plastic ball point pen ink tube and shoved it down on the post to form the internal flats. I figured I'd only have to use it for an hour or so and then discard the "apparatus."

    Will the front sight post come out if I just keep cranking upward on it? What happens when it comes out? Do the detent and detent spring fly out, or are they "kept" in there somehow? I know a precision welder who could tack a blob on the top and I could finish it off square. (I don't see this as a heat-treated part.)

    Apologies in advance to folks named Murphy.

    I think rob s is correct:

    ...which was my original diagnosis --either that or the rear sight is just too high. There's definitely a mismatch there. What is an "F" front sight base, and what front sight base do I have, if you would hazard a guess?

    I don't understand "and your front sight isn't milspec for a carbine."

    Obviously, it's easier to raise the front sight than mess with the rear sight. Even machining won't work because the height of the rear sight is regulated by where the vees on the sight match the "male vees" on the flattop of the rifle, and not the flats on either the rifle or the bottom of the sight.

    Thanks, all, so far! Very helpful! Especially since you indicated it's a "common" problem.

    entropy, sorry, I was only guessing at the age. Your recommendations are what I tried to follow, more or less, when I discovered there simply wasn't enough adjustment available.
  14. rob_s

    rob_s Well-Known Member

    Bushmaster part number 9349056-M

    "F" marked front sight bases are slightly taller than the ones used for a 20" rifle. The "F" is what the military uses on the issue M4s. Almost all rear BUIS are made to work with the taller FSB.

    If you think about it, it makes perfect sense. The boreline is the bare of a triangle, and the sight line is the hypotenuse. Since a carbine front sight is further back along this triangle, it needs to be taller.

    Manufacturers that don't care much about doing things the right way tend to use one FSB for both their rifles and carbines, and make up for it by using a shorter carry handle and/or taller front post.
  15. HJ857

    HJ857 Well-Known Member

    Yeah, the post will come out, but the spring will not, you can just screw the post back in without any trouble at all.

    The cool thing about the KNS post kit is that it has a few different configurations and you may be surprised by which one works best for you. It's also nice to have an extra one or two on hand, especially if you have any tendency to apply a dremel tool for "customization".
  16. 230RN

    230RN Marines on Mt. Curibacci

    Thank you all, again! This information is enough to get me to confidently try a temporary fix, that is, silver solder either a blob or a tiny piece of metal to the top of the sight post, trim it square, and try again. That is, if I can't find a taller post locally. We're blessed with a lot of good gun shops/smiths around here.

    You guys have been great!

    OK, now maybe I'm only a 98.45% novice at the AR-15.

    Sure is a lot to learn about that rifle and its variants.

    10e6 TNX !

    Terry, 230RN
    Last edited: Oct 10, 2008
  17. 230RN

    230RN Marines on Mt. Curibacci

    Semi-Final regarding this thread

    E-mail to my son on this AR-15 sight matter. The "precision welder" and machinist to whom I jokingly refer is he, who lives near Boulder CO. :)
    The "precision welder" is a guy I know in Boulder.

    The best answer was from Rob S. and, having been given a hint in this direction from his posts, I have more or less confirmed that the business of using an A2 front sight (20" bbl) on an M4 (16" carbine bbl) by the manufacturer is the basis of the problem. I guess they figured if you wanted a flattop, you would be sticking all kinds of optics on it anyhow.

    If you read rob s's post # 14 on the triangle thing, then look at the attached picture, you will see what he meant. (He typoed and used the word "bare" instead of "base" of the triangle.):

    Going from the top down in the attached pic, we have

    M16 A1 20" bbl
    M16 A2 20" bbl
    M4 carbine 16" bbl (mine)
    M16 A4 (or A45?) 20" bbl.

    Since every rifle barrel has to point UPWARD from the sight line (this is not true of handguns) you can see that if you took a 20" bbl with an A1 or A2 sight and slid it backwards (yeah, the forend is in the way, but just assume you could slide it back), you can see that the barrel up-angle would have to increase as the sight is moved backward along the barrel.

    Bring it back to where it would be on the 16" barrel (note the shorter forend for the M4, the carbine) and it would raise the barrel angle so the gun would shoot too high.

    You have to therefore RAISE the front sight (or the sighting post inside it) to bring the 16" carbine bbl back down to the right angle.

    I calculated from looking at the 25 yard target with the front sight post raised all the way up --to the point of being loose in the front sight base -- that the sight would have to be taller by an additional 0.050", so I'm calling it 0.065 (or more) inches.

    (Just raising it 0.050" would bring me to the correct point of impact, but it would still be loose in the base, so I would need more than that to have it set down firmly in the threads of the sight base and still give me the extra height I need.)

    When I pull it out, I'm going to check the thread sizes on it. I might know a machinist somewhere who can duplicate this thread on his lathe and maybe make a new front sight with the proper dimensions. I think there's one in Boulder who might be able to do this kind of work, but I don't remember his name offhand. He's also that precision welder I mentioned. ;)

    So. I'm going to call around today to see if I can get a higher sight post, otherwise I'm going to "attach" a blob or chunka metal to it to drop the bbl angle. Or maybe a chunka J-B weld.

    By the way, the sight base on my DPMS has " <B>2 " molded into it, which I would assume is the DPMS factory code for "A2." Just an assumption, you understand.

    Oh, incidentally, I looked up BUIS and it means "Back Up Iron Sights."

    Damn acronyms!


    Terry, 230RN

    (I don't recall where I got the pic from or whether it's copyrighted or not. Apologies in advance to any copyright holders.)

    Attached Files:

  18. 230RN

    230RN Marines on Mt. Curibacci

    Ah, the heck with it.

    I pulled the guts out of the front sight and figured out what was wrong. The threads are too loose on the sight post, but the detent plunger holds it fairly good until the sight post is raised too far and the plunger itself is loose in its own hole.

    Hence the whole assembly is loose when raised too high.

    So you know what I did? I went down to the hardware store and bought a good stiff spring of the right size, put it under the flange of the sight post, and screwed it down to the proper height. (I had previously taken measurements of the height of the sight post to the bayonet lug. FWIW it was 3.605".)

    Without the detent plunger.

    It's in there pretty solid with that stout little spring under it, no flexing, no ups and downs, no wiggleation*.

    The only problem is with the flats on the sight post for a sight post wrench, you've got to screw the sight in so that the flats face your eyeball, since there's no longer any detent plunger to index it at 90 degrees per "crank" on it.

    I doubt that the recoil of that pipsqeak little M193 varmint cartridge is going to shake it loose with that stiff spring under it, and of course it's protected by the sight protector wings.

    Only problem is that the little "sight post wrench" I made out of a plastic ball point pen tube probably won't give the torque I need for screwing down the sight post with that spring under it. So I'll go down to the hardware store again tomorrow and buy a brass tube with the inside diameter to slip over the post (ought to be about 0.099" in inside diameter), slip it over the post and clamp it flat with a pair of parallel-jaw pliers.

    Voila! A brass tubing sight post wrench!

    That orta do 'er!

    "Wiggleation is a technical term.

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