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New 6.5 Grendel upper, drifting zero, poor groups.

2K views 9 replies 5 participants last post by  Rocket_Monkey 
#1 ·
Hey everybody, I just shot my new 6.5 Grendel upper for my AR15 today.

At 25 yards it shot 5 (I shot more than one because I was just interested in seeing it run) through a quarter right under the bullseye. I was supprised because I just slapped on the mount and the scope, adjusted for eye relief, leveled it and torque it down.

Jumped up to 100 yards. First group was about 1 inch 5 shot, 4 inches high or so, went down 16 clicks. Shot a 2 inch wide 3 or so inch tall group around the bullseye. Figured I was rusty/exicited and needed to relax. Shot my .22 for a bit.

Shot another group that looked like the prior one, switched ammo. Same thing. Decided that the scope was not holding zero, whacked it sharply but not too hard on the top of the tube with a rubber screwdriver bit set (couple ounces). Now the groups are huge and like 4 or 5 inches low. I've used this whack the scope and shoot method before to condem a wandering scope erector in the past. So im pretty much sold on the scope being bad.

I also tried single shot loading because I've seen people complain ar15 grendels mangled the long skinny tips. Made not difference, its all over like a loose ring. As I fired it more It did trend back up toward the prior zero but was still shotgunning.

Everything is tight, hand guard is free floated.

Ammo was horady American gunner 123 then the hornady black box 123.

I emailed Vortex about the scope. Its a viper, PN VPR-M-05BDC

Bought that scope in 2015 but never used it till this year.

I purchased a rifle length Grendel upper used this scope and it grouped so poorly, I returned it. Having a bore scope I gave it a peak and decided that...

1. There was a huge burr on the crown
2. There where flattened rifling edges and longitudinal scratches

I condemned that upper, got a refund and build this new one.

Does it look like the scope is the issue to you all?
 
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#2 ·
Rocket Monkey don't give up on the scope or upper just yet. I have one that shot 4-5in groups with Hornady ammo with 123SSTs. I went through 40 rounds very disappointing. I finally handloaded some 95gr Nosler and was able to get a 3 shot group you could cover with a nickel. And I am no bench shooter by any stretch of the imagination. This load was nothing special either just tried one out of the reload manual and my rifle loved it. Good luck
 
#3 ·
Well I don't want to sound like a jerk but IMO it should not take handloads to get a shilen barreled upper shooting factory ammo that groups under 3 or 4 MOA at a maximum and shoots to a repeatable POA. It really looks like something is wrong to me. I don't want to toot my horn (yes he does, the Arrogant jerk) but I have shot a couple thousand hand loads benchrest shooting in the past with a factory barreled rifle that was pretty darn good.

That gun would do around 1.5 moa with anything it didn't hate completely and under 1 MOA with heavier factory ammo it liked. Thou switching ammo did move POA it didn't drift during a string of slow fire. With good full length loads It could shoot 5 shots under .250 pretty regularly and had a best of .115 at 100 yards, magazine length loads ran around .300-.500 all day, dirty, extreme heat(you did have to really slow down) or cold(keep it hot). Perhaps I'm spoiled with a prior factory killer and I am rusty to be sure but I can get better groups out of my .22 than this Grendel setup. This performance is so bad it looks like a drifting POA, like something is loose.

Even if I had dies, components, time, etc, I think I still wouldn't start developing a load at this state. Sadly, if that's what it takes I might as well sell it as I no longer have the time for reloading. If the POA wasn't moving, I'd move on to testing for ammo it likes. But until its stable all ammo going through it is to text if the drift is fixed.

Got my old bushnell 6500 on top now, just got to go waste some more ammo i guess. Oh how I hope that Vortex scop is messed up.

I originally posted on here looking for diagnostic suggestions. I think its the scope, but what next if its not?
 
#4 ·
My experience with Hornady ammo is that it is accurate with most rifles. I would suspect the scope over the ammo. It will be easy to check by switching scopes. That would be my first move. Make sure to field strip clean and oil it up before shooting with the new scope!
 
#6 ·
The scope mount was solid, I guess I'll pull the hand guard off and check the barrel nut. However, I had to take it up to 80 foot pounds to get it timed for the tube so I'm quite sure its tight unless my torque wrench is way off. I had to pound the shank into the receiver so I think I'll wait till after I test the spare scope to go so far as removing the barrel. I imagine it will require obscenities and heat.
 
#7 ·
Curiosity got the better of me, I removed the barrel nut and inspected the barrel/receiver union, it sure looks fully seated and square to the eye. Even if I removed the barrel I don't think I have any good way to accurately measure how square it is.

Anyhow visually the shoulder on the barrel is seated and making contact all the way around. I torqued it back down in the same method as last time, hitting the 30 foot pound minimum, then setting the wrench to 80 foot pounds and watching the nut cutout time with the gas tube hole. This time it timed up without hitting the 80 pound limit. Ill assume because it stretched or my wrench isn't very accurate.
 
#8 ·
Do you have another scope to put on the rifle? Quick swap should tell you if it the original scope is the suspect.

Shifting zero tells me the scope might be the issue, though.

One of my AR's began suffering a similar problem a while back. Zero shifts, groups getting larger. It was indeed the scope (a Burris MTAC). Sent it in for warranty and they discovered the objective lens had come loose. It was repaired and back to me in about two weeks.
 
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