Why should irons be re-zeroed after adding/zeroing scope????

[Ironbarr]

In the Maryland Shooters "zeroing" article, author makes a strong point to re-zero iron sights after adding and zeroing a scope.

...If you have previously zeroed your iron sights, then attach an optic, BE SURE TO REZERO YOUR IRON SIGHTS. Adding a optic (epscially mounted to the carry handle) can change your zero...

http://groups.msn.com/TheMarylandAR15ShootersSite/zeroingmethods.msnw

While I understand zeroed sights "work best", I don't understand the relationship between scope and iron zeroes in that they are, IMO, independent of each other.

Input requested.
Thanks,

-Andy

 

[Art Eatman]

Plumb new news to me!

Never hurts to check the irons, against the possibility you might have moved something when messing around with the mounts, but I wouldn't seriously expect any problem...

Dangfino.

Art

 

[gun-fucious]

Well, you should re zero after adding a flashlight barrel mount...

adding the mass of a scope & mount to the upper might change your weld
and you muscle memory would also be corrupt

 

[boing]

I've read that, too.

I suppose the added mass might cause the gun to recoil a little less, lowering the POI, or change the 'vibration harmonics' of the rifle/barrel...?

 

[Dave R]

I think Boing has it. I think the new mass on the barrel will change its harmonics slightly. Might change POI a bit.

Might also change "pet loads" as the harmonics change.

OTOH, if all the change is back at the receiver and, it may not make MUCH of a distance.

 

[Kharn]

You probably cant get your nose to the charging handle with an AR15 with a normal scope on it; putting the nose to the charging handle is how the military teaches recruits in order to keep their heads in the same spot for every shot.

Kharn

 

[G21NE]

They might mean the combination of an optic like the Aimpoint or Tasco red-dot sights, and a "scout" style mount that puts it in the same plane as the iron sights.

I am thinking there might be some distortion through the lenses?

I also think, like boing, that attaching more mass to the firearm changes the harmonics, Even though its way back on the receiver.
In my case, an AR-15 with a carry handle mount and a Tasco PDP5. That mount only attaches thru the hole in the handle, leaving the dot and rest of the mount kinda hanging in space.

Like those vibration damping things on compound bows. Is that a good analogy?

 

[Art Eatman]

With a limit of reasonably "standard" attachment systems, I'd think that anything affixed to the receiver wouldn't affect the barrel...

Oh, well...One test beats a thousand expert opinions...

, Art

 

[Bainx]

Sounds like he's had a few

 

[Mike Irwin]

Adding a scope can be like adding a Browning BOSS. It can change the harmonics of the barrel and action dramatically, leading to different point of impact.

I've seen this happen a couple of times, with changes in impact anywhere from a little bit to a LOT.

 

[Duke of Lawnchair]

Re-zero

Have any of you folks ever mounted a scope to an AR15 platform? Particularly one with an A2 receiver?

I have, and I had to rezero my rifle. The mount I used was the ARMS #2 with ARMS #22 rings and Tasco SS10x42mm scope. Once the ARMS#2 was tightened, my point of impact with open sights was out of whack. This was WITHOUT the rings and scope, just the mount.

I've heard that some A2 receivers will actually flex at the carrying handle when a scope mount is tightened in place.

As far as other rifles go, who knows...

Just my experience.

BTW, I removed the entire optics setup because I'm not a "scope guy." Irons, I dig the irons.

Jim

 

[Art Eatman]

Hokay. I missed the phrase "carry handle" in the original post. On a "real" rifle--a bolt action, that is--it's not something for concern. Certainly not, if one uses two-piece mounts. But, the idea that little 6x48 screws will bend a receiver is a bit much.

Art

 

[Mike Irwin]

Actually it is something for concern in a bolt action, Art.

Not as much, but still a concern.

Attaching ANYTHING to the beam (barrel and action) at any point along its length will chance the rate at which the barrel vibrates, which in turn can change the point of impact of the bullet. It can also lead to larger, or smaller, group sizes with a given load.

 

[Art Eatman]

Mike, have you, yourownself, observed this phenomenon with see-through scope mounts on a bolt-action rifle? Sight in with irons, mount the scope, sight in the scope and then have to re-zero the irons?

If so, I'll have learned sumpn--which is always a Good Thing.

, Art

 

[Mike Irwin]

Check my first post to this thread, Art.

Ah, I'll take pity on an old guy's eyes and copy & paste it...

"I've seen this happen a couple of times, with changes in impact anywhere from a little bit to a LOT."

I've also seen it happen with lever action rifles.

If you think about it, lever guns may actually be even more prone to this. In a bolt gun, the stock carries the weight of the action and barrel.

In most lever guns, the action and barrel carry the weight of the buttstock and the foregrip, which are usually separate pieces.