Gunsmithing Zero moving with can off and back on.

[ColoWyo]

I am at my wits end with this. Every time I remove my thread on AAC Cyclone and put it back on, my zero moves an inch every time. It's USUALLY repeatable but not always. It's driving me crazy. I have replaced, the base (Badger), the rings, the scope (Nightforce), and even the stock (AX) in hopes that it had something to do with the equipment.

Today, I went out and shot at 100 to confirm my zero. (can still on from last outing) and below are the results.

Group A is my first group of the day. The can was not taken off from my last outing.

Group B, I took the can off and put it back on. Same point of aim as the first group.

Group C, I took the can off and put it back on again and was aiming for the dot below the holes.

Group D, I re-zero'd and then shot the rest of the dots on the target.

Now, I know I'm no benchrest shooter, but once everything get's zero'd, the bullet seems to go where the cross hares are pointing.

So, what the heck would you guys do? I've emailed AAC, but they are closed until after new years. So hopefully I'll hear from them next week.

Any suggestions? Thanks guys. I appreciate any help.

 

[Lumpy Grits]

Re: Zero moving with can off and back on.

Do you have an index mark on your "can" that lines up to a mark on the bbl?
Only way to be sure the "can" "clocks" to same position on install.
Check the torque on the action mounting screws/bolts
If it a std cal. Try some Fed. match ammo and see what happens.
Respectfully,
LG

 

[carlton]

Re: Zero moving with can off and back on.

Lots of factors, but here goes- and I'm simply looking at it from a mechanical perspective.
+1 on LG's suggestion of consistent indexing, though. That could help a great deal, and kinda goes along with my thoughts below.

I do thread lots of barrels -mostly for muzzle brakes, and 90% of those are "timed". I make sure to get that shoulder on the muzzle absolutely square to the bore/crown/threads so that the bullet exits each baffle of the brake with the same clearance on all sides. Having said that, I'm wondering if the can and barrel shoulders are mating up solid against each other. If they aren't, then that could explain the lack of consistency - ie, they don't mate up solid for the entire circumference of the barrel shoulder, so the harmonic changes with each shot, albeit slightly.

Maybe take some thin (I use .0015) shimstock/feeler gauge and screw the can on until it almost touches, then run the feeler gauge around it until it gets "sticky" in one spot as you screw on the can, and continue running the shimstock around the entire circumference to see if there are any spots in the can shoulder/barrel shoulder mate-up that are tighter than others. Next, you can take some inletting black or a sharpie/dykem to the barrel shoulder to double check contact/mate-up

It's just one of a multitude of things that could be happening, but that is where I'd start, and I wish you luck.

 

[ColoWyo]

Re: Zero moving with can off and back on.

Thanks for the ideas guys. I should have mentioned the fact that I did take a pencil and mark everything to make sure it was indexing to the same spot every time I put the can on. It lined up everytime. That's the frustrating part.

I'll get out the feeler guages tonight and see if I can find some variances in the area between the can and the barrel.

Thanks again. I appreciate the help. This is driving me f'n crazy.

 

[carlton]

Re: Zero moving with can off and back on.

OK.
For reference, I have a customer who takes his brake on and off for F-Class. I texted him after I responded to your post to ask him if his zero changed, etc -and if so did it repeat. His doesn't change...
I'd say that if yours mates up correctly and all is in line, I think you are on the right track with contacting AAC. They should have ample thoughts on harmonics and things going on inside the suppressor that should help fix the problem.

Be sure to let us know what they suggest, and I hope you get it sorted out.

 

[Rprecision]

Re: Zero moving with can off and back on.

Is it possible you are distrupting your bedding when you snug down the can each time?

Does this happen has this happened before the can ?

 

[ColoWyo]

Re: Zero moving with can off and back on.

Well, I followed 300jcc's advice and broke out the feeler guages. I noticed that there was a noticeable difference as I moved the feeler gauge around. I took the can off and gave it a good once over and this is what I found.

As you can see, the can is making far more contact on one side versus the other. I gave it some rough measurements and it would appear the can is not square.

Now hopefully the manufacturer will get back to me soon and I can get this taken care of.

Thanks 300jcc. Great idea.

 

[mscott]

Re: Zero moving with can off and back on.

Are you positive it's not the shoulder cut on your barrel that is off? Was it threaded by a known smith? If I was the manufacturer I would question the barrel over my own product. Do you have a muzzle brake or anything else to thread onto your barrel?

 

[ColoWyo]

Re: Zero moving with can off and back on.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: mscott</div><div class="ubbcode-body">Are you positive it's not the shoulder cut on your barrel that is off? Was it threaded by a known smith? If I was the manufacturer I would question the barrel over my own product. Do you have a muzzle brake or anything else to thread onto your barrel? </div></div>

GAP did the work. I sent the can in when I had the gun re-barreled.

I used the feeler gauge as 300jcc recommended with the muzzle break and everything seems square. The resistance is even all the way around between the barrel and the brake.

So unfortunately, I'm quite certain it's the can.

 

[Greg Langelius *]

Re: Zero moving with can off and back on.

Some folks may disagree, but it think harmonics make a difference; and that changing the polar moment of inertia (altering the amount of mass at the end of the barrel) will pretty definitely affect harmonics.

If I'm right, adding mass to the can in small increments will make a difference, and done right, might provide a solution. The idea is to retune the barrel's harmonics with the can so they work better together with the same load, with and without the can.

Meanwhile, getting even shoulder contact makes sense, too.

Greg

 

[Lumpy Grits]

Re: Zero moving with can off and back on.

You need to send that picture you posted to GAP and see what the builder sez........
That "can" should be making 100% contact of the shoulder. It is NOT doing so as pur your picture
LG

 

[Glen Seekins]

Re: Zero moving with can off and back on.

I have a few cans on rifles and they all have a different zero with or without the can. My best one is about 1/4" low with the can on. I don't think you can assume that your rifle will have the exact POI with and without. Taking the can on and off shouldn't move your zero though.

What is your zero without the can? Put the can on, shoot a couple and remove the can. Your POI without the can shouldn't change.

 

[ColoWyo]

Re: Zero moving with can off and back on.

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: heatseekins</div><div class="ubbcode-body">I have a few cans on rifles and they all have a different zero with or without the can. My best one is about 1/4" low with the can on. I don't think you can assume that your rifle will have the exact POI with and without. Taking the can on and off shouldn't move your zero though.

What is your zero without the can? Put the can on, shoot a couple and remove the can. Your POI without the can shouldn't change.
</div></div>

ALL of these groups are with the can ON. That's the problem. It is not repeatable by any stretch of the imagination. I know to expect a difference in POI with the can off versus on. But that's not the issue here. I wish it were that simple.

I'm just going to shoot it with the brake on till I can figure out WTF is going on.

 

[LRI]

Re: Zero moving with can off and back on.

Lets do an experiment:

First lets preface it by assuming the machine work is right and your not experiencing a crown issue, baffle strike, plugged up/dirty can, or any other little annoyance thats the result of poor PM or machine work.

Get yourself a 12" wood/plastic/steel ruler and clamp it to the kitchen table so that 10" are hanging off into space.

Now thump it with your finger and watch it wiggle.

Go get a penny and tape it to the end of the ruler and wiggle it again.

Changes doesn't it? So does your barrel when you go hanging chit off the end of it.

The repeatability of the change comes into play when you place the penny on the stick. You have to ensure it goes back to the same spot each time its installed.

This would be the torque/clock position of your muzzle can once installed.

The "thump" of the ruler also must be the same each time. You have to ensure you whack it with the same amount of energy and apply that energy with the same duration (dwell).

This would be the consistency of your ammunition.

Variations and things that don't (on the surface) seem to add up:

I'm sure someone has a rifle or knows of a rifle blessed with a zero or near zero point of impact shift whether a can is installed or not. No fair right? How come mine doesn't?

He must have slept with his smith in order to get that.

No, you or your buddy don't have to be gay. You just have to get lucky.

Amplitude and frequency. Two terms used when describing a wave. Any wave for that matter. Amplitude is how tall the wave gets at its peak. Frequency is how many of these waves get produced in predetermined amount of time.

If the naked barrel produces an amplitude of X and a frequency of Y its entirely possible to get lucky and screw a can on the end of it and not see much/any change in amplitude. The frequency however can very likely change.

Going back to our ruler analogy if we watch closely there's two points where the ruler is motionless for just a split second. When its at the peak of its amplitude at either direction. During this ever so small brief second nothing is moving.

If we catch this on the barrel as the bullet leaves the crown we can expect accuracy to improve as the repeatability increases. How many times per second/minute/hour it does this is really not of any consequence/interest. So long as we catch that "sweet spot" (what that really means) we don't care.

We add the penny and the frequency of how many times (cycles) this occurs changes within the confines of our given allotment of time. As I said earlier, the amplitude didn't change.

So long as you catch that same peak in the wave the bullet doesn't see a change and therefore goes on its merry way chasing down the hole the bullet before it just made.

If it were me:

I'd fiddle with this by going to an RC airplane hobby shop and buying some sticky back lead weights used to balance an airplane. Pay attention to your clock position when installing the can and start fiddling with how many/location you install the weights. The alternatives are to have a load with can on and one with can off.

That would be very gay imho.

Just might surprise yourself how fun this can be. (sicko!)

Hope this helped.

C.

 

[Glen Seekins]

Re: Zero moving with can off and back on.

I would be willing to bet your can is not going back on the same. I doubt its the barrel because of the way threads are made, the shoulder has no choice but to be square. Mine tighten to a dead stop and hand tight is dang hard to break loose.

 

[Cascade Precision]

Re: Zero moving with can off and back on.

Wait, so, hold on...if we sleep with our smith, we get better results??


Crap...time to find a female smith thats not married that I would consider sleeping with...


So, on Chad's note: That kind of thing going to work? (sticking lead weights on the can?) Yeah, it will work to make it ghey, but will it help to reproduce consistent hits with/without the can??

(edit: Chad, you have TOO much free time on your hands...)

 

[ColoWyo]

Re: Zero moving with can off and back on.

I got a hold of AAC today and they were extremely helpful and told me to send it in and they would re-face it.

I'm optimistic after talking to them that they will make it right.