My apologies if this is a really obvious question, but I'm pretty new to the world of ARs and I can't seem to get a concrete answer to this one:

If one were to zero an A3-style rifle at a middle distance (say 50 or 100 yards), then later tighten the mounting knobs on the carry handle a little more, would the zero change?  The knobs were snug to start with, but you gave them an extra tweak.

Would the same hold true for an optic like an EOTech?

Finally, each time you change sight systems, do you hav to re-zero the rifle?  How critical is getting the torque the same when re-installing the sight?

Thanks for any info.

Within the context of "torque", IMO unless the thing was falling off loose, NO, any POI change will be negligible.

Mike

ps - although I sold my EO for another Aimpoint, mine was always quite close to it's original zeroing providing I got it back exactly where it was on the flat-top and again torque was not an issue

I'm going to disagree with Mr Wilson.  Yes it will change POI, I've seen this a number of times (mostly on cheap mounts).

The one High-End thumbscrew mount I have (the QRP) it has a built in torque limiter so the screws are always tighted the same.  They did this to keep the tension (and POI) consistant.

It's funny - for every "no it won't " answer, I seem to get a "yes it will."  The mounts in question are the A3 carry handle on a Bushmaster and an EOTech 512, so I'd say the quality is pretty good (thinking about Forest's comment).

Have any forum members routinely removed their sighting system (be it iron sights, electronic, or glass) and re-installed it?  I figure it doesn't happen too often, but I bet that once a sight is zeroed it's common to tweak it to make sure it's still tight.  Or let's say it was originally tight but  was shot loose.  In re-tightening it will the zero change?  That may be the more common scenario.

Quoted: Have any forum members routinely removed their sighting system (be it iron sights, electronic, or glass) and re-installed it?  I figure it doesn't happen too often, but I bet that once a sight is zeroed it's common to tweak it to make sure it's still tight.  Or let's say it was originally tight but  was shot loose.  In re-tightening it will the zero change?  That may be the more common scenario.

Yes I do.  However I've got LaRue mounts so it's not an issue.

However when using my EOTech and it's built in mount tightening was an issue.  If I only 'finger tightened' it (vs finger tight then add a 1/4 turn) it would shoot to a different point of aim.  Not enough to matter at near ranges (sub 100y) but it would at longer rangees.

Of course doing this with my 'cheap optic' tests, I've noticed that it makes a significant difference.

I don't know how much of a difference it makes with the detachable carry handles, for that I'll defer to the experiences of others.

FWIW, my comments above were regarding just the EOTech mounted on the flat-top.

No mounts (which I consider completely un-necessary) and when tighten down, cranking harder on it at a later time never moved it's POI, (but then I generally tightened it properly to begin with and only checked it later to sure it was still tightened/ occassionally getting it abit tighter).

When removed and replaced (fully forward in the correct/original mounted slot) the POI always hit within 1/4-1/2 inch of where it had hit before removing.

I have no experience with the mounts that came along later to fix a problem which I thought never existed.

Mike

Thanks, guys, this is really interesting to me.  Per Mr. Wilson's comment, I have the 512 mounted directly on the upper, no rail or accessory.

The reason this even peaked my curiosity is that I had my AR zeroed pretty well (for me, anyway) with the 512.  I gave the mounting knob a slight tweak to ensure that after shooting it hadn't loosened, and when I resumed shooting the POI seemed to have moved a little to the left.  This wasn't after a long session, so I don't think barrel temp was an issue, and the sight was secure before and after I started.  Of course, it could have been just me.  Nevertheless, I thought I'd ask those who know more than I whether mounting screw tension factored into this.

I appreicate your input, and that of any others who want to chime in.

T.

Quoted: ....when I resumed shooting the POI seemed to have moved a little to the left. ...

Define "a little'.  The EOTech mount - if you follow the published procedure - will return to zero within 1 moa.

If your 'little' is within the MOA than I wouldn't worry about it.

If it is really bothering you, get an inch-pound torque wrench and a ransom rest and head to the range.  The only real data is accurately obtained data, not heresay.  

1.  Fire some test groups at correct torque.
2.  Loosen and retorque to a minimum, fire groups
3.  Torque tighter and fire groups.
4.  Apply correct torque and fire groups.
5.  Control all other variables.

Optional:  completely remove sight assembly and kick it and the rifle around the room a bit.  Remount, start at step 1.

Now sleep soundly.

My Matech Buis had come loose without my notice and was slipping fore and aft.  At no point did groups shift in any manner at 100 yards with the irons. (loctite is my friend...)

If you have a loose scope, you will start to see some movement in point of impact as ranges increase.  All of this degree of looseness is subjective.  If it is loose as a $2 whore, it will shoot all over the place.

If you had a bad day, were dehydrated, had some trigger pull issues, had vision issues, or breathing issues-- your group was probably not due to a few inch pounds variance in your sight screws.  All of us have one day where we just can't hit a barn from the inside, as in lucky to hit a pie plate at 100yd.  The last time it happened to me, it was dehydration.  It affects your concentration and metabolism to an unnoticeable degree and you just can't work your way out of it.

YMMV!

Again, thanks for all the info, guys.  By a little I mean about one to two inches at 50 yards.  And I wouldn't be surprised if I (the shooter) were to blame.  But I'm curious by nature, and you won't know until you ask.

I'll loosen it, re-mount it, shoot it, and fiddle some more.  And it gives me another opportunity to get in more range time.  Besides, as you have mentioned, an MOA difference for plinking is not really a difference at all.

Thom

Remember MOA at 50 yards is 1/2 inch.