Rifle Scopes Possible tracking issue with Bushnell ERS 6-24x

[Hollywood 6mm]

The last two trips to the range have revealed some odd changes in my confirmed dope on my rifle. There were no significant changes that I can't account for, so I am starting to think that I may have a tracking issue.

This is my previously confirmed data on multiple KD targets at the range I belong to. It tracks perfectly with the AB app's data using Litz's info for the 175 SMK and my 2600FPS muzzle velocity:

200y - 0.6 mil
300y - 1.4 mil
350y - 1.9 mil
400y - 2.4 mil
500y - 3.5 mil
600y - 4.7 mil
700y - 6.1 mil
800y - 7.7 mil
840y - 8.3 mil
940y - 10.2 mil

Here is where everything ran two weeks ago:

200y - 0.5 mil
300y - 1.4 mil
350y - ??? mil
400y - 2.4 mil
500y - 3.3 mil
600y - 4.7 mil
700y - 6.1 mil
800y - 7.5 mil
840y - 8.3 mil
940y - 9.7 mil

Dafuq? I fired 3 rounds at 940 on a full size IPSC. The first shot was held center of the 'chest' and impacted dead center of the head - .5 mil high. I dialed back down .5 mil and put the next two in the center of the torso of the plate without an issue.

Now it gets really odd. I re-zeroed yesterday as I am now running an SDN-6 can mounted to the muzzle brake. It's worth noting that my zero had to come up .5, but the data out to 500 shows no major shift.

200y - 0.6 mil
300y - 1.4 mil
350y - 1.9 mil
400y - 2.4 mil
500y - 3.4 mil
600y - ~4.35 mil
700y - 5.6 mil
800y - 7.2 mil
840y - 7.8 mil

I didn't get to shoot 940 yesterday, so no data there. All of a sudden at 500 yards, my 175s seem to be shooting extra flat - or my scope is somehow jumping an extra .5 mil in the span of 1.0 mil dialed. At one point yesterday I did a quick tracking check by engaging a target (don't remember the distance), getting a confirmed hit at the right elevation, then dialing a full 5 mil (one revolution) up, then back down, and shooting the same target. The scope tracked through that without an issue. Even if I was just shooting at 200 yards, that full turn would have passed me through the 500 yard data to roughly the 600 yard correction. I think I did that check at a 250 or 350 yard target, though.

It's worth noting that I am using Varget, so I don't think that it's going to be a powder temp issue, and my dope inside 500 yards isn't being seriously effected like it would be if I was seeing a velocity spike.

If I have to, I might be able to make an extra trip to the range this weekend just to go run data and confirm dope at as many different distance as I possibly can. I'm hoping it's something obvious I have just missed, though.

So, what else could be going on here besides the scope suddenly developing a tracking problem? It has been hot as heck the past few weeks, but I can't see the ~15-20 degree temp difference causing that much of a shift.... right?

 

[Mendy300wm]

It's not the scope, did you chrono that load with and without the can? What was you environmental data those two days?

 

[fdkay]

Powder temp might not be an issue, but air temp/density certainly is.
Also, is this the same lot of powder, primers or same brass? Barrel clean or dirty?
Lots of variables here. The easiest thing to do is check your scope against a yardstick at 100 yards or at least a large sheet of paper with dots at 3.6" apart at a measured 100 yards. Again, I would go at least ten dots for a total of 36".

 

[Hollywood 6mm]

It's not the scope, did you chrono that load with and without the can? What was you environmental data those two days?

No idea on the actual environmentals on the last day I chronoed the data, but it was checked without the can on a Magnetospeed. According to my log, I last ran the chrono on 18 May. According to Weather Underground's historical data, the high that day was 88, with a max humidity of 97%. Yesterday was a high of 91, with a max humidity of 97.

I did try playing with the velocity in Applied Ballistics, and I couldn't find a velocity increase that replicated the 500+ yard data without also skewing the sub-500 data a noticeable amount.

Powder temp might not be an issue, but air temp/density certainly is.
Also, is this the same lot of powder, primers or same brass? Barrel clean or dirty?
Lots of variables here. The easiest thing to do is check your scope against a yardstick at 100 yards or at least a large sheet of paper with dots at 3.6" apart at a measured 100 yards. Again, I would go at least ten dots for a total of 36".

Same lot of powder, same primer but it might not be the same lot (I lost track of when I changed the lot on my primers). The brass is the same but was trimmed, and I cleaned after the 18 May trip.

All of the data checks on 1 June and 15 June were done on a fouled barrel.

What is still making me scratch my head is that there's no noticeable change in data until 500 yards.

 

[gigamortis]

Since your data inside of say, 400 yards is mostly the same, but your longer distances are changing, I would be suspecting bullet differences between lots. If you are running the same lot powder and primers and chronoing the same speeds as before, then your bullet BC would have to be higher in order to be getting less drop over longer distances like that.

I have seen lot to lot variances with Sierra bullets over the last 8-10 years. I have since changed over to 178 Amaxes and have a lot more consistency between lots vs the Sierras.

Usually when a scope goes goofy on tracking, the bias springs that hold the erector tube up against the adjustment turrets are weak. Along with spotty tracking comes bigger groups, too. How are your groups at distance?

 

[Killswitch Engage]

Not many mention or say this but I've seen it many many times. I have a target I shoot at 935yds and when then slightest mirage starts showing up on target my dope instantly .2-.3 low on target. It happens every time. Mirage very easily changes your perception of target location at distance especially beyond 800yd. I've seen so many people beat their heads against a wall chasing their dope when nothing is wrong. If I shoot same exact target in cool crisp conditions dope is perfectly normal. Something to consider

 

[Hollywood 6mm]

Since your data inside of say, 400 yards is mostly the same, but your longer distances are changing, I would be suspecting bullet differences between lots. If you are running the same lot powder and primers and chronoing the same speeds as before, then your bullet BC would have to be higher in order to be getting less drop over longer distances like that.

I have seen lot to lot variances with Sierra bullets over the last 8-10 years. I have since changed over to 178 Amaxes and have a lot more consistency between lots vs the Sierras.

Usually when a scope goes goofy on tracking, the bias springs that hold the erector tube up against the adjustment turrets are weak. Along with spotty tracking comes bigger groups, too. How are your groups at distance?

As long as I make sure the loose nut behind the trigger is properly secured, I've been shooting great groups. Hell, the last couple range trips have seen some of the best shooting I have done since I got into precision rifle. Edit: That said, I did have some issues Sunday morning when I first started shooting, but that could have been a combination of the can not being settled into the brake, loose nut, and not realizing fully that my dope wasn't agreeing with the actual performance.

I did change bullet lots as well - I had forgotten about it. I am also shooting factory second SMKs, so I'd say it is entirely possible that I am just seeing a lot to lot variation.

That said, I just went into AB and played with the BC in my load data, and I was over .275 G7 and still see more drop in the tables than I did on the range yesterday.

I still don't get it.

 

[Hollywood 6mm]

Not many mention or say this but I've seen it many many times. I have a target I shoot at 935yds and when then slightest mirage starts showing up on target my dope instantly .2-.3 low on target. It happens every time. Mirage very easily changes your perception of target location at distance especially beyond 800yd. I've seen so many people beat their heads against a wall chasing their dope when nothing is wrong. If I shoot same exact target in cool crisp conditions dope is perfectly normal. Something to consider

The mirage actually wasn't bad at all the last two times at the range. Two weeks ago, though, I finally started shooting with an anti-cant indicator on my rifle, but I don't see that causing a disagreement with the AB data and prior dope. If I hadn't matched the AB data before, and I did now, then I'd say adding the ACI did the trick.

 

[Killswitch Engage]

It doesn't have to be bad. Just visible. Hard to believe it's near 90f and you're not getting any. Would have to be complete overcast

 

[Hollywood 6mm]

It doesn't have to be bad. Just visible. Hard to believe it's near 90f and you're not getting any. Would have to be complete overcast

Even at 940, I tend to run my scope in the 14-18x range, I rarely run it all the way up, so I suppose I am just not seeing it as much due to the lower magnification.

 

[Killswitch Engage]

Shoot when it's cool and crisp or light rain and see what happens at 940. Then you'll know. That seems to be your discrepancy dope

 

[Mendy300wm]

G7 bc for that bullet is .243
Put that in your AB and use velocity to true it at distance

Also my gun shoots faster with a can on yours might too

Tracking tests should be done at 100yds, with a tape measure going from the turret to the paper

 

[Hollywood 6mm]

G7 bc for that bullet is .243
Put that in your AB and use velocity to true it at distance

Also my gun shoots faster with a can on yours might too

Tracking tests should be done at 100yds, with a tape measure going from the turret to the paper

Yep, it was set for .243. When I run the velocity up to match the longer range numbers, then the close range stuff starts getting way off.

 

[Hollywood 6mm]

I spent some time at lunch today going over my notes and checking my settings in AB. I also finally figured out that I have to hold down the task switch/menu button on my phone to access ballistic calibration. Loading the approximate weather conditions from Sunday and then doing velocity truing based off my 800 and 840 yard data resulted in AB tracking almost perfectly with my notes.
Lesson learned - garbage in garbage out will definitely screw up the numbers. Time to buy a kestrel and make sure this doesn't happen again.

Sent from the U.S.S. Sulaco

 

[delixe]

I did change bullet lots as well - I had forgotten about it. I am also shooting factory second SMKs, so I'd say it is entirely possible that I am just seeing a lot to lot variation.

I recently sorted some 175 BTHP seconds from Powder Valley.
There was significant variation in the ogive to base dimension (5 distinct lengths) with the longest .050" longer than first run bullets.

 

[Hollywood 6mm]

I recently sorted some 175 BTHP seconds from Powder Valley.
There was significant variation in the ogive to base dimension (5 distinct lengths) with the longest .050" longer than first run bullets.

Those are what I have been using for practice, and saving firsts for matches.

Sent from the U.S.S. Sulaco

 

[gigamortis]

Two weeks ago, though, I finally started shooting with an anti-cant indicator on my rifle, but I don't see that causing a disagreement with the AB data and prior dope. If I hadn't matched the AB data before, and I did now, then I'd say adding the ACI did the trick.

Scope/rifle cant will primarily affect just left to right hit POI. If you are canted way, way over then you will eventually see some elevation POI variation