Thermal expansion and wondering zero

[flyfisherman246]

I've never been able to keep a consistent zero with my ar15's. I've tried different scopes as well. It has a bartlein barrel and shoots very well, but my 100 zero is usually off by .1 or .2 as much as .3 mils at 100 yards. I don't really get that on bolt guns. They are usually right on or within .1 mils. Never been able to keep a good zero with an ar15. I haven't started tracking temperature vs zero shift, but I think I'm going to start.

Ar15's consist of different types of aluminum, stainless steel, carbon steel, polymer.... all kinds of different materials interfacing with each other with different coefficients of thermal expansion. This makes since to me how during different temperatures the muzzle could end up pointed in different directions. Had anyone experience this or looked into it?

 

[bigjake83]

This is where cryogenically treated barrel blanks come into play and JP has it down.

 

[flyfisherman246]

Other than just the barrel though, all other parts of the system will expand at a different rate and amount. Would that cause issues?

 

[bigjake83]

No you're overthinking it, unless you do a couple mag dumps and then try to shoot for groups the other components won't come into play. The only variables that are going to affect your cold bore shot is your atmospherics, barrel temperature and the ambient temp of your ammo.

 

[RTH1800]

Unless you are shooting .25 MOA or less it could be hard to tell.
My .6 to one MOA 5.56 rifles won’t show .1-.2 MOA shift clearly.

My very accurate .204 would show it but they don’t seem to shift.

 

[acudaowner]

I have been lucky not to have my scopes loose zero yet except once they were slammed around in the car what was set was off had to re zeroed other than that are you sure they did not become loose from shooting blue or purple loctite could help , maybe bounced around in the car ride or kids changed it without telling you ? it don't take a whole lot to looses and then push or pull a scope a tiny bit enough to throw off your zero . good luck figuring it out and fixing what ever the problem is . I also use a cheap padded rubber mat in the car to keep the guns separated and from moving around .

 

[TimK]

I went down this particular rabbit holes a few years ago. I'd see my AR go from hammer to shotgun in the middle of a match. At least, that's how it looked to me. After a long string of fire I thought I'd see accuracy go bonkers. The best answer I could come up with was that the aluminum receiver was heating up faster than the barrel and affecting the fit of the barrel extension. Step one was to glue it in with locktite. Didn't appear to help. The next step was to buy a JP. They use oversize barrel extensions that have to be thermo-fitted to the receiver. I think that helped.

I've only ever seen this behavior when the gun was good and hot. If you're seeing zero shifts on a cold gun, I think it's something else.

 

[Molon]

Here's some data on the subject matter.


Observations on the Points of Impact of
Statistically Significant Shot-Group Sizes



The Set-up

For this ballistic exercise I used a semi-automatic AR-15 with a 20” stainless steel Lothar-Walther barrel. This barrel has a 223 Wylde chamber with a 1:8” twist rate. This Lothar-Walther barreled AR-15 has produced 10-shot groups with extreme spreads measuring in the “sixes” (0.6xx inches) when fired from the bench at a distance of 100 yards using match-grade hand-loads. Prior to the beginning of this exercise, this barrel had approximately 2,040 rounds fired through it.

The ammunition used for this exercise was factory loaded Black Hills red-box 223 Remington ammunition seated with the 69 grain Sierra MatchKing with a cannelure. Since I had been testing other ammunition with the Lothar-Walther barreled AR-15 prior to the beginning of this exercise, as well as throughout this exercise, I fired three “seasoning rounds” of the Black Hills 69 grain Sierra MatchKing ammunition to condition the bore of the Lothar-Walther barrel with the powder used in this factory load. This process was repeated immediately prior to shooting each of the groups evaluated for this exercise.

All of the shooting for this ballistic exercise was conducted from a concrete bench-rest at a distance of 100 yards (confirmed with a laser rangefinder.) The Lothar-Walther barrel used in this exercise was free-floated in a LaRue Tactical railed free-float handguard. The free-float handguard of the AR-15 rested in a Sinclair Windage Benchrest (with the aid of a Sinclair forend bench-rest adaptor) while the stock of the rifle rested in a Protektor bunny-ear rear bag. Sighting was accomplished via a Leupold VARI-X III set at 25X magnification and adjusted to be parallax-free at 100 yards. A mirage shade was used. Wind conditions on the shooting range were continuously monitored using a Wind Probe. The set-up was very similar to that pictured below.

The Wind Probe.

The Groups

No changes were made to the elevation or windage settings on the scope throughout the entire course of this exercise and the exact same point of aim was used when shooting each group. After firing the three “seasoning rounds” of the Black Hills 69 grain Sierra MatchKing ammunition, I settled-in and fired a 20-round group of the aforementioned ammunition. The group is pictured below. It has an extreme spread of 0.94". The center of the 20-round shot-group is located in the lower-right quadrant of the two inch circle on the target. After shooting this group, I continued testing other ammunition from the same Lothar-Walther barreled AR-15.

A little later that day, I fired a second 20-shot group of the Black Hills 69 grain Sierra MatchKing ammunition from the Lothar-Walther barreled AR-15. The second group is pictured below and this group has an extreme spread of 0.85".

As you can see, the center of the second 20-shot group showed no significant shift whatsoever in its location on the target as compared to the first 20-shot group. The next image shows the first and second 20-shot groups over-layed on each other using Adobe Photoshop with the blending opacity set at 50%; further illustrating that the centers of the two 20-shot groups showed no significant shift in their locations on the targets compared to each other.

On an additional trip to the shooting range, I repeated this entire ballistic exercise just as described above. The findings were the same; the centers of two 20-shot groups fired from the Lothar-Walther barreled AR-15 using the same lot of Black Hills 69 grain Sierra MatchKing ammunition showed no significant shift in their locations on the targets compared to each other. Nor did the centers of these third and fourth 20-shot groups show any significant shift in their locations on the targets as compared to the first and second 20-shot groups fired in the previous ballistic exercise.


The graphic below shows all four of the 20-shot groups over-layed on each other (forming an 80-shot composite group) illustrating that the centers of all four of the 20-shot groups showed no significant shift in their locations on the targets compared to each other. Of those 80 shots in the composite group, 95% of them are contained in a covering-circle that has a diameter of 0.97 MOA. All of the 80 shots in the composite group are contained in an area-of-dispersion (bounding rectangle) that measures 1.07 MOA wide by 1.08 MOA high.

….

 

[flyfisherman246]

Damn, that's pretty good!!!! Hahahah. Why can't I get this kind of consistency? I'll get it out if my lmt mws with a bartlein, but nothing even close to that out of an ar15?