POI shifting down as the rifle heats up

[Phylodog]

I've been working on an upper to take coyotes off of the deer lease. I've done some ammo testing and have settled on a brazilian manufactured 77gr OTM round that shoots better than others I have tried (Black Hills 77gr, Federal Gold Medal 69gr & Hornady 55gr & 75gr TAP). I might have 150 rounds through the barrel at this point and it's all been in ammo testing. I just went out and shot 20 rounds of the 77gr to finalize the zero after setting it close the last time. My first round was 2.5MOA high and .5MOA left, not a big deal as long as it is repeatable. As I continued to shoot the remaining rounds the point of impact began to drop considerably and shift a bit left. The last 5 shot group landed approximately 1.5 MOA low and 1 MOA left.

I'm shooting a 14.5" Rainier Arms Ultramatch 8 twist barrel and Surefire SOCOM 556RC suppressor. Just wondering if anyone has encountered anything similar and if there is anything that can be done about it. It's not the end of the world as I can't imagine a scenario where I'll be shooting 20 rounds at coyotes in a short period of time but it would be nice to not have to deal with this kind of a shift.

 

[BCP]

I would not be okay with that at all. Have you tried a different optic + mount? If not that then maybe a loose barrel nut or hopefully not a bad barrel.

 

[Phylodog]

I don't think it's the optic or mount, the rifle is holding sub MOA groups, they are just shifting as the rifle warms up. I suspect it has to do with different metals heating at different rates, just interested if anyone else has seen similar results and if there is anything that can be done short of swapping out components. The suppressed SR25 rifles were reportedly experiencing something similar but that was several years ago.

 

[PiggiePluggin]

Have you given thought to Paralax adjustment and cheek to stock weld repeatability? When I notice a huge shift in groups, I usually go back to the fundamentals.

 

[Phylodog]

I'm not new to precision shooting with a gas gun, pretty confident it's not the shooter this time around. If the groups were opening up I might suspect I was contributing to the problem.

 

[culater]

get a bi pod and a scope level. How far are you plan on shooting ?

 

[j-dubya]

Try shooting another group with the suppressor off and note the consistency of the group. I tried a Rainier Ultra Match barrel once. It had all sorts of inconsistencies and drifts with groups shooting my 77gr load. The only thing hanging off the end was the Surefire flash hider and the Magneto Speed.

 

[Phylodog]

I've got an Atlas bipod on it and have been shooting it from that. I don't expect to have any shots past 200 realistically and 300 would be a very remote possibility. It'll hold minute of coyote which is why I'm not in crisis mode but with the money I spent on the setup I'd like to have a bit more consistency out of it. Perhaps it is the barrel, that would be disappointing as I read a lot of good reviews on the Rainier tube.

 

[culater]

You have only 150rd shot in the barrel. You need to break in your tube a little more and clean it. Look into Dave Tubbs Final finish

FinalFinish? & TMS Kits & Loaded Ammo - YouTube

or shoot the heck out of it to break it in about 500rd.

 

[EUBANKS]

Can you help clarify something? You are not adjusting the scope. Groups shift when it is warm and then return to zero when the rifle is cold? Sometimes scopes stick when an adjustment is made.

 

[Lowlight]

Do you have a mirage cover on the suppressor ?

 

[turtlesniper]

Phylo i know you have been around for a while , but i would try not to bridge that gap with the scope not saying it wont work but the heat can slowly travel from barrel nut to handguard giving you plenty of time to shoot a nice group but causing a slow zero shift

 

[Phylodog]

I know it's less than ideal but there isn't a scope mount that I've found which allows me to mount a scope low enough on a gasser. Most people can't shoot my bolt guns because the cheek rest is so high. I don't know if I've got the high cheek bones of a supermodel or what but I can't get consistent with a scope mounted higher.

I chopped up an ACOG mount to bridge the receiver and hand guard in an attempt to eliminate any movement.

 

[Phylodog]

No mirage cover. I've had two and shot them both off of the suppressor on my bolt gun so I never bothered buying one for the SOCOM RC.

 

[Phylodog]

Correct. Cold bore is good then the POI begins to shift low and left as things heat up. I'm not making any scope adjustments.

 

[Lowlight]

No mirage cover can certainly cause the shift you're seeing

 

[turtlesniper]

What hight are those rings nightforce makes a 1.125 and adm makes a 1.1 i use an adm sl on a mk12mod0 so i dont have the scope stuck way in the air

 

[Phylodog]

I'm not positive on the height but they're much less than 1"

 

[McCrazy]

Frank and Turtle have both pointed out a couple options that may help.

What contour is the barrel you are using? It is a new barrel to you and it sounds like it may not be stress-relieved which is causing POI to shift back and forth as the barrel heats and cools. You may not have noticed this behavior while load testing if you were being slow and deliberate, resulting in the barrel never getting hot enough. Might be something wrong with the way the barrel is mounted as well.

I would try changing variables one at a time. Try shooting with the suppressor off. Different optic. Factory match ammo (probably not the issue as you say groups are good other than drifting). Get another good shooter to give it a shot.

Somewhere you will find the answer.

 

[355sigfan]

You bridges the gap with your mount. A very bad idea and I assume it's part of you problem


Sent from my iPhone using Tapatalk

 

[Phylodog]

No mirage cover can certainly cause the shift you're seeing

I'll see if I can dig up one of them and try it.

 

[.Mark.]

You bridges the gap with your mount. A very bad idea and I assume it's part of you problem


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I had similar issues doing the same thing. Move your mount back so it is only on the receiver.


- Mark

 

[Phylodog]

Made some progress today. I pulled the scope and rings off and repositioned them to keep everything on the receiver. Zero shifted 11MOA when I put it back together so obviously there was significant alignment issue I hadn't seen between the receiver and handguard. Mounted it up and put 40 rounds through it and the downward shift is gone. It's holding POI well at this point but still loosens the groups a bit as it heats up. I still need to try to dig up a suppressor cover and see if that improves things further.

Thanks for the advice guys. I knew bridging the scope between the receiver and handguard was a bad idea but I wouldn't have thought it would produce the results I was seeing.

 

[hondacafe]

Looks like you're at least making progress on the culprit.

I want to vent with your frustration about mounting optics low on an AR too. I went with the LT SPS mount. The lowest option I could find that also moved the scope forward. I didn't think rings would work for my eye relief.

http://m.laruetactical.com/larue-tactical-spr-s-mount-lt-158

Even with the Larue mount it's not near low enough. I added a stock pack riser to help my alignment too.

I can't believe they don't have a lower more compact mount out there that moves the optic forward just a hair. Seems to me it would be a good product request.

Anyways good luck on the yote hunting! The one above is from last week.

 

[justme]

Another thing to check is to make sure your gas tube is NOT binding where it goes thru the upper. If it does bind there, when the gas tube starts to heat up from the hot gas it expands, and will put pressure on the barrel as it heats up. I make sure that my gas tube has plenty of room to pass thru the upper so this problem does not exist.

 

[LRRPF52]

This thread seems to be textbook example of why not to bridge the gap between upper and handguard.

 

[tickled pink]

355sigfan is correct. You bridges the gap with your mount. A very bad idea and I assume it's part of you problem


I haven't read all the posts, but the first BIG PROBLEM is your mount for your scope. I can not imagine how you can even see through it. Your cheek weld must be lined up with your eye socket. For me I need 1.5 inch from the top of the Picatinny rail to the middle of the scope rings. Your rings are made to be used on a bolt action rifle where the stock drops down to fit your shoulder. The AR 15s stock comes straight out from just below the charging handle so to get a good cheek weld you have to move the scope UP. This means new rings.

The second problem with your poi shifting as you shoot is usually caused because the barrel is too thin. A heavy barrel that is Fluted does not heat up as fast and the Fluting helps to dissipate heat and saves some weight.

Take a close look at mine which is set up pretty good. It has a Colt lower with a Geissele SSA-E trigger. The upper is a WOA 18" Varmint upper with a 1 in 7" twist and is Fluted. I have a Bushnell AR/223 scope with 4.5-18x44mm zoom with tactical turrets which are very repeatable. The scope mounts are Warne QD ULTRA HIGH ALUMINUM rings. They are very strong and light.

PS: The scope is not bent the photo is distorted.