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M4 shooting 1' to right @ 25 yds

Discussion in 'Gunsmithing and Repairs' started by backbencher, Jan 18, 2013.

  1. backbencher

    backbencher Well-Known Member

    Received the weapon mechanically zeroed, ie, windage of the rear iron sight was dead center. 6 rnds later, was not even on paper, so we threw up targets to the left, right, top & center. I was a complete target over to the right, and to zero, had to crank the windage knob so far so that the rear sight is almost completely to the left - literally about 4 clicks from the mechanical stop. The Aimpoint M2 required similar adjustment to zero - I've never had to crank that much on a scope to get on paper.

    The only thing I did to it before zeroing was to throw in an AccuWedge, as there was considerable slop between the upper & the Colt lower. Cleaned it last night, & qualified on the pop-ups today. I'm afraid to remove the forward rail system to clean, as I'm concerned there is some pressure on the bbl forward that is causing the deflection.

    The front sight/gas block looks normal, the front sight post is not bent (had one of those before, thought I'd never zero), the weapon functions fine on M855 - only limited by my 40+ yr old eyes.

    Any ideas? Would an AccuWedge (in the rear of the lower, pressing on the rear of the upper) cause such deflection? Is it just an extreme random variation, & hey, I zeroed, don't worry about it? Should I take it to the armorer b/c the crown is shot/random other thing is wrong?

    3 shot groups when zeroing @ 25 yds/m(?) were about 1" - probably down to my shooting ability & eyeballs. Groups were consistent throughout zeroing, just took an amazing amount of adjustment to zero. Wind was negligible yesterday & today.
  2. rcmodel

    rcmodel Member in memoriam

    I don't know what is causing it, but it isn't an Accu-Wedge.

    The lower receiver is just something to hold on too.

    The sights are mounted to the upper and just go along for the ride if an Accu-Wedge moves it on the lower.

    It could be a bur under the barrel extension cocking the barrel off to the side of the upper a little.

  3. carbine85

    carbine85 Well-Known Member

    How much slop is there between the upper and lower. Contrary to what most say too much play between the upper and lower will effect accuracy. If enough play is there that the upper can actual be turned in one direction it will throw off the straight line of sight through the sights.
    What does the crown look like?
    The only proper way to check the upper for cant or barrel alignment is to remove it and chuck it up in a vise and use a level. It sounds to me like the barrel or gas block is canted.
  4. rcmodel

    rcmodel Member in memoriam

    Since the sights are attached to the upper?

    Could you go into more detail as to how exactly how that could even possibly happen??

    You might see a 1" change or so with a tight sling, or off a benchrest.

    But the sights are attached to the upper.
    And thats where the barrel is pointing, regardless of where the lower is pointing.

  5. backbencher

    backbencher Well-Known Member

    On further examination, confirmed by sighting @ a Conex w/ corrugated walls, the front sight tower is visibly canted by more than 5 degrees, perhaps 10 or more. And now I recollect a helpful MSG who lollipoped the Aimpoint to the mechanically zeroed sight before I went to the zero range, thus the massive adjustment I had to make to the Aimpoint as well. Mystery solved - now I have to convince my supply SGT to send it in for maintenance - and when I get it back zero it again.

    Is this really a Colt upper? The lower is black - the upper almost purple. I'd hate to think Colt would send such a product out - 4' off from mechanical zero @ 100 yds seems a bit far from standard. It will zero - but any wind from the right direction would flummox this carbine.
  6. carbine85

    carbine85 Well-Known Member

    It's pretty simple. A small amount of play isn't noticed. A large amount is given the fact that the gun is 2 pieces and a lot of play is twisting, turning, rocking, or moving the upper while one hand holds the upper and the other holds the lower. Tight fitting AR will normally outshoot a loose fitting upper.
  7. carbine85

    carbine85 Well-Known Member

    I'm glad you found it. I knew it had to be in the upper and something canted. I have seen this so many times in the AR it isn't funny. People get in a rush and things go wrong.
    Was the front site loose? Perhaps the pin was overdrilled or just drilled wrong.
    I saw one where the guy torqued his barrel incorrectly and actual twisted the key at the lower
  8. backbencher

    backbencher Well-Known Member

    No, nothing's loose - it either came from the factory this way (apparently Colt), sometime down the line the bbl was replaced & the FSB installed canted by the depot, or - someone wanted a real Colt upper & substituted an aftermarket unit. I've got some pics on my phone, will see if I can post 'em later.
  9. SilentScream

    SilentScream Well-Known Member

    Yeah I would get that front sight assembly confirmed centered, get the rear sight on the center index and go from there.
  10. backbencher

    backbencher Well-Known Member

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