Vertical zero shifting with new AR15 build

[Packfan]

New guy here. Been lurking for a while, but never posted.

I recently built an AR15 and am having some unusual zero/point of aim shifting between shooting sessions. I am getting about a 2 MOA shift-vertically only. Once adjusted, the zero stays put for the rest of the shooting session and is consistently under 1 MOA accuracy wise.

For example, last week my initial shots were 2 MOA high. Today my initial shots were 2 MOA low. It did this previously and I re-torqued everything: barrel nut, hand guard, scope mount, scope rings, and flash suppressor.

Build is an Aero Precision lower, Aero upper, Aero Atlas handguard, Toolcraft bolt and carrier, Geiselle SSA-E trigger, with a White Oak Armament 18 inch 1 in 8 twist barrel. Scope is a Vortex Viper PST Gen II 3-15x44 SFP and the mount is a DNZ Freedom Reaper Forward rail.

Any ideas on what could be causing this and what else to try?

Thanks!

 

[msgriff]

Is that one of those extremely cantilevered mounts? Was there any temperature differences between range trips?

 

[Packfan]

It is a cantilevered mount, but it is really solid. I bought it from White Oak Armament and used it on an NRA service rifle for several years. Picture below.

The temp was different, colder last week that this week by about 10 degrees. I didn't record temps when this happened previously, but loads included factory Federal GMM 69 grain SMK. This week and last were my predominately my reloads: 25.0 grains RL15 with a 69 SMK and 24.0 grains with a 77 SMK.

 

[Captrjc]

Are you resetting your zero with a hot barrel? Try to rezero with plenty of time between each shot so not to heat things up. Then let’er rip and see if your poi raises during your session.

 

[Powder_Burns]

This may be a stupid question but does your poi shift correspond with you switching loads?

 

[ballisticdaddy]

Just to be clear, if you zero using the 69 grain and then move to the 77 grain it SHOULD shoot low.

 

[Longshot231]

Are you shooting from a bench or a bipod? If a bipod, are you shooting with the rifle on the bench or prone? Are you shooting with the bipod on dirt or on something like concrete?

The reason that I'm asking is that I had similar problems. I discovered that the problem stemmed from the amount of load that I put on the bipod. If I zeroed it with a certain amount of load on the rifle then shot it later wit more or less then I got the vertical shift.

Also keep in mind that the type of surface material the bipod is rested on makes a difference with the dynamics the rifle is exposed to during recoil.

I'm not saying that would be definite cause but something to consider.

 

[Packfan]

I am accounting for the change in loads. I always start with the 24.0 R15 77 SMK which was my "go to" for service rifle. It shoots good in this rifle as well. I developed the 69 grain load as another option due to current supply of components. The point of impact between these loads is pretty close.

I will monitor heat more closely and have not been resetting. I haven't noticed a big point of impact shift though. If it starts high, it stays high until I dial down with the scope. Same when it has been shooting low.

So far I have been shooting off a concrete bench with a Midway front rest and rear sand bag. I am going back out today and will see what happens. I plan on shooting some prone with a bipod. The outside temp should also be more consistent with yesterday.

 

[kl3309]

It is a cantilevered mount, but it is really solid. I bought it from White Oak Armament and used it on an NRA service rifle for several years. Picture below.

The temp was different, colder last week that this week by about 10 degrees. I didn't record temps when this happened previously, but loads included factory Federal GMM 69 grain SMK. This week and last were my predominately my reloads: 25.0 grains RL15 with a 69 SMK and 24.0 grains with a 77 SMK.

View attachment 7595795

I'd argue that your cantilever mount isn't solid enough for the added weight of the current optic. You were using a fixed 4X for service rifle? There's a lot more glass in the Viper PST and it's all hanging off of the rear ring.
It's not the sole cause of your POI shift, but it's not helping the situation.

 

[Packfan]

You may be on to something here. I was running the Viper PST 1-4x24 which is a lot lighter and smaller.

 

[TimK]

I've been struggling with a rifle with a very similar problem. I'd put a zero on it and then halfway through a match the zero would shift by like a lot. Not heat related. It's a Valk, Bartlein chambered by Craddock, Wilson upper and lower, blah, blah, blah.

After screwing with it extensively I determined the scope was the culprit so I sent it back to Germany. During the six month wait to get it back I put another known reliable scope on it. Same damn problem. After several long conversations with Paul Craddock, I did two things. First, I pulled the barrel and glued it in with Loctite 609. Second, I switched to a much lower profile (and non-adjustable) gas block. The original gas block had clearance under the handguard, but it wasn't very darn much. Paul commented that he'd seen a couple of guns that inexplicably wouldn't shoot. Changing gas blocks solved it. Neither he nor I could come up with a mechanism to explain it, but it seemed like it was worth a shot. He specifically recommended a block from Troy.

The scope came back so I put it all together and shot it in a local match last weekend. I had a few errant shots, but I might have had a hangover. To my surprise, the two changes seem like they might have fixed the problem. Elevation was good all day. I have no idea which one actually fixed it (if it's indeed fixed), but it definitely wasn't sending shots a half a mil low.

 

[Packfan]

Thanks Tim. Something is definitely not right. The gas block is a standard low profile block from White Oak Armament, so nothing fancy. It has good clearance with the hand guard so nothing is touching. I think it is good to go, but I guess you never know.

It started out low again today. I came up about 2 minutes and shot a .680 5 shot group. So something is shifting and doing so in a consistent manner. I think kl3309 may have it with the cantilevered scope mount. It is a lot of scope that sits way out there. I am going to try a regular set of rings and see if that helps. If not, on to the scope?

Regards.

 

[Neurotic]

I don't know if it's the issue, but how much clearance do you have between the gas block and handguard? The Aero Atlas handguards are some of the tightest fitting handguards out there.

 

[Packfan]

Well, a regular set of rings will not work as I would have to place the scope too far to the rear to have a good sight picture and cheek weld, especially when shooting prone.

I could go with a mount that has less cantilever, but first, I am going to try to adjust the mount I have. I moved the scope further to the rear in the mount and moved the mount up on the receiver. This will center the scope in the mount and will reduce the weight of the scope that is forward.

We will see what happens next time at the range.

 

[Neurotic]

That mount will put pretty large forces on the rear attachment point. I'd recommend looking for a mount with a longer 'base' that interfaces with the rail and has less cantileveredness. Where the forward most rail attachment is not under/behind the last 'ring'.

 

[Packfan]

Thanks Neurotic.

The Atlas Handguard is tight, but I have good clearance on the gas block with no signs of any contact.

I will start looking for a new mount with the characteristics you mention. If it is the mount, shifting the scope as I have done may not make a sizeable difference given the forces and leverage involved.

I know there are some mounts that have much less forward cantilever than others: 2 inches versus 3 inches for example. Do you have any recommendations?

 

[Packfan]

Maybe something like this Vortex Mount? The leverage on the base from the front ring would have to be far less than what I am currently using.

 

[msgriff]

Geissele is considered by many as the best, but very expensive. Warne is also a good choice, but is also expensive. Aero has nice mounts at a good price that many use, but I'm not fond of the mounting design. I do like Vortex very much due to their "no questions asked" warranty, and wouldn't hesitate buying their mounts. Which offset to get depends on cheek weld style, eye relief, scope length and keeping the mount off the handguard. Use the least amount of offset to meet those criteria. Don't forget to rack the mount forward in the rail slots before tightening. I'm also a fan of Vibra-Tite VC-3 for all my optic screws.

 

[Longshot231]

Geissele is considered by many as the best, but very expensive. Warne is also a good choice, but is also expensive. Aero has nice mounts at a good price that many use, but I'm not fond of the mounting design. I do like Vortex very much due to their "no questions asked" warranty, and wouldn't hesitate buying their mounts. Which offset to get depends on cheek weld style, eye relief, scope length and keeping the mount off the handguard. Use the least amount of offset to meet those criteria. Don't forget to rack the mount forward in the rail slots before tightening. I'm also a fan of Vibra-Tite VC-3 for all my optic screws.

+1 on Vortex and Vibr-Tite VC-3.

 

[Packfan]

Thanks guys. My local gun store has several types of Vortex mounts and the Warne QD mount. I plan on making a visit there today.

 

[Neurotic]

For larger optics I use the Warne XSKEL. Which, as far as I can tell, Warne makes versions of this mount for many others in slightly heavier ring patterns... Like the Vortex listed above, primary arms and several others.

Sig makes some interesting mounts in steel at the same weight as the Warne.

 

[diggler1833]

I run ADM mounts on most of my ARs, and am pleased for the money. The one that holds my Pulsar Thermion XP50 has been on four rifles now, and about 500 hours in the field and has only given me one issue where I managed to inadvertently unlock the front latch. I still hit the hog I was shooting at, but it made it a ways before expiring.

I recently made a post about not being able to get a WOA gas manifold under an Aero ATLAS handguard...there was about .177" needed still to clear.

 

[Packfan]

I pulled the hand guard and checked gas block clearance again. Everything is still centered with no signs of contact. There's at least 3/32 clearance on each side where the gas tube enters the block and 1/8 or more everywhere else. I think that should be sufficient given the fairly heavy contour of the barrel. If the scope mount doesn't pan out, I will try a different hand guard to rule this out.

I ended up going with the Vortex Pro Series mount. My gun shop guy said they have had good luck with this mount and heavier scopes. It probably has half the cantilever length and more surface contact with the rail than the DNZ.

I am going to shoot again tomorrow, but since I will be re-zeroing with the new mount I wont know if the shifting is gone until next time out.

Thanks again for all the experience and advice.

 

[Neurotic]

What are you checking clearance between? I'm not quite following your comments about the gas tube and barrel profile.

 

[AMGtuned]

ADM scope mount, and knowing the Atlas handguard, I'd guess it could be touching when hot under recoil.

Does the shift occur if you do not shoot from a rest?

 

[MontanaMan]

I pulled the hand guard and checked gas block clearance again. Everything is still centered with no signs of contact. There's at least 3/32 clearance on each side where the gas tube enters the block and 1/8 or more everywhere else. I think that should be sufficient given the fairly heavy contour of the barrel. If the scope mount doesn't pan out, I will try a different hand guard to rule this out.

I ended up going with the Vortex Pro Series mount. My gun shop guy said they have had good luck with this mount and heavier scopes. It probably has half the cantilever length and more surface contact with the rail than the DNZ.

I am going to shoot again tomorrow, but since I will be re-zeroing with the new mount I wont know if the shifting is gone until next time out.

Thanks again for all the experience and advice.

I'd be betting more on a shitty Vortex scope than the mount if it does it again. Try a good NF & I imagine that your problem will be gone..................wouldn't be the 1st time a Vortex has been bad. Just sayin.

MM

 

[Packfan]

I was referring to the clearance between the gas block and the inside of the hand guard. The barrel reference was to the weight of the barrel and the ability for it to flex enough to contact the hand guard.

The more I look at it, it is pretty tight in there, so I am going to try a different hand guard with more clearance and weight to it in case it has some flex. That will take both the hand guard and scope mount out of play. A different hand guard will also mean re-torquing the barrel nut which could be another variable.

After that, I guess it could be the scope. I used this scope on a bolt gun previosuly and it zeroed and tracked well, but I guess it could have gone bad.

I built numerous uppers for service rifle use and never had this much of a challenge.