How to True Trajectory Drop Charts for 7mm Rem Mag

[Herd_Bull]

I have the following actual field POI numbers for a 7mm Rem Mag. I'm using Strelok Pro software and can't get the drop charts to match up real well. Compared to what the software is telling me, I either have a higher POI at 100yds (about 0.75") or a lower POI at 391 yards (about 3-4").

.284 caliber
160gr Nosler AB bullet G1 BC 0.531

Avg fps at 55-57 degrees 2,741 (StDev 23.21)
Avg fps at 57-59 degrees 2,767 (StDev 20.29)

100 yard (+1.69 inch vertical POI)
200 yard zero (+0.39 inch vertical POI)
287 yard (-6.18 inch vertical POI)
391 yard (-19.26 inch vertical POI)

I've played with the Trajectory Validation options (both velocity and BC) and still can't get it real close, I'm not even sure on how close would be reasonable and sufficient. Wondering if any of you much more knowledge folks would have any insight for me. I just want some accurate drop charts and POI distances for the hold overs in my reticle. Thanks.

 

[Kyle87]

Are these numbers consistent every time you go out?

 

[Ds1971]

Hello I have been using Strelok + for quite a while now and it has been dead on for me out to 700 yards. I also shoot a 7mm rem mag and have several loads programmed into the app as well as a buddys 300 win mag. All I can say is go back and double check and make sure you have all your input data correct including scope height and slope and temperature on the day you shoot. I only say that because when I first started using it I was forgetting to make temp and baro changes for that particular day and it was leaving me wondering what was wrong too. Sorry I cant be of more help and hopefully you can get it figured out soon. I used this app to make my longest kill shot ever back in April on a 200lb boar at 590 yds. Dont give up on it keep messing around with it and you will get it figured out.

 

[rudysts]

Might be that your load is not consistent. I use Strelok pro with no problems all the way out to 1400 yards dialing up the scope. you should recheck your load with your chrony and possibly another one to verify your numbers.

 

[Herd_Bull]

Thanks for the replies guys. Yes, my POI numbers are pretty accurate, all taken at the same shooting range and temps have been similar here in the last month or so. Wish I had another chrony to check my velocities against for accuracy. I'll try rechecking all the inputs in the software and see if that gets me anywhere. Thanks.

 

[Auto-X Fil]

Thanks for the replies guys. Yes, my POI numbers are pretty accurate, all taken at the same shooting range and temps have been similar here in the last month or so. Wish I had another chrony to check my velocities against for accuracy. I'll try rechecking all the inputs in the software and see if that gets me anywhere. Thanks.

I bet your scope height over bore is incorrect. Very carefully measure the scope height over bore axis and plug that in. Having it off will cause what you see.

 

[boltfluter]

You could also check to make sure your click value is correct. Many are not in my experience. Good luck, and keep us posted.

Regards, Paul

www.boltfluting.com

 

[Brians356]

I bet your scope height over bore is incorrect. Very carefully measure the scope height over bore axis and plug that in. Having it off will cause what you see.

+1

Few people bother to measure and enter actual scope height. The height with modern scopes and mounts is almost always greater than the default of, say, 1.5 inches. Also ambient factors (temp, RH, elevation or pressure) need to be accurate.
-

 

[rduckwor]

Your standard deviations are very high for hand loads(?). Try to get them down into at least low double digits if not single digits. Statistically, there is no consistency in your velocity and thus your loads.

RMD

 

[hogpatrol]

+1 on the scope height. I've found the factory B.C.s don't work well all of the time. Have you done a test to see if your turret corresponds to actual vertical movement @ 100 yards? The big question is, what brand/type of scope are you using, high or low end?

 

[Herd_Bull]

Lots of good suggestions. Thanks guys. I'm using a Leupold VX-3 4.5-14x 40mm with the Boone & Crocket reticle. I have not done a test with the turret clicks to actual movement because I do not plan on adjusting the turrets for target distance. This is a hunting rig and I plan on using the hold over hatch marks in the B&C reticle for additional target distances. Strelok Pro has a reticle function where it'll calculate POI for each hatch mark in the reticle at each given power setting. But in order for it to work correctly with accurate POI's I need to map an accurate trajectory referencing the main crosshairs. I will double check my scope height and using my new calipers. I don't know why my standard deviation numbers are what they are and others have told me my 2700+ fps is slow for a 7mm rm...??? Maybe my chrony should be checked. I've been shooting early in the mornings with really hard sun angle if the sun has been out...??? I like early morning because I have the range to myself and the cooler temps are nice for the barrel and no wind. I use diffusers and never miss a reading, but maybe I should try middle of the day. Anyway, when you guys say that Strelok Pro is right on for you... how close is that to actual POI at distances beyond 300yds? Exactly on, within an inch, within 2 inches, etc...???

 

[boltfluter]

Just noticed your SD is much to high. Nothing is going to match with numbers like that. Shoot a group at your max distance and you will see what I mean. Es needs to be 20 fps or less in my experience. I would rework your load before going any farther. Good luck.

Paul

www.boltfluting.com

 

[Zero333]

I hope you meant to say your ES is 20 fps.
If your SD is 20 fps, then that's a bummer.

 

[Herd_Bull]

My Standard Deviation (SD) has been in the low 20's so far and it has given me 0.60 to 0.80 MOA groups out to 391 yards so I was never too concerned with it. I'd have to take another look at my numbers to see what the Extreme Spread (ES) was. Like I said, maybe my chrony is the source of error I should be looking at...???

I measured scope height very carefully with calipers this weekend and got 1.649" which very slightly helped the POI numbers but there's still room for improvement. I haven't had a chance to shoot again yet.

 

[284winner]

Definitely measure all points and make sure they are true to what you install in your formula screen. Then the very best way to tell exactly is do the field work at the range. I have a 500 yard range and i punch in all the accurate data and run velocities and get my drop data. Then I shoot all 5 ranges from 100-500. My moa numbers are always off. Mostly because the BCs are not exact. I can actually fine tune the BCs to very close to what Brian Litz prints as actual BC numbers. Once I get 500 perfect, it only takes very minor adjustments to 1000. It's beyong 1000 that the struggle begins. 7 mm mag or 223, it's all the same process.

 

[rudysts]

My Standard Deviation (SD) has been in the low 20's so far and it has given me 0.60 to 0.80 MOA groups out to 391 yards so I was never too concerned with it. I'd have to take another look at my numbers to see what the Extreme Spread (ES) was. Like I said, maybe my chrony is the source of error I should be looking at...???

I measured scope height very carefully with calipers this weekend and got 1.649" which very slightly helped the POI numbers but there's still room for improvement. I haven't had a chance to shoot again yet.

This is a good way to measure scope height
http://www.accurateshooter.com/optics/determining-scope-height-above-bore/

 

[Lapua40X]

Your standard deviations are very high for hand loads(?). Try to get them down into at least low double digits if not single digits. Statistically, there is no consistency in your velocity and thus your loads.
RMD

Those were my first thoughts too. It's hard to count your marbles when they're rolling all over the table.

 

[Herd_Bull]

Here's my last 16 and 12 shot strings

2,732 fps
2,732
2,749
2,749
2,754
2,754
2,760
2,766
2,766
2,772
2,772
2,783
2,783
2,789
2,789
2,795
2,795


2,688 fps
2,715
2,732
2,737
2,737
2,737
2,749
2,749
2,754
2,760
2,760
2,777

 

[Ds1971]

Thats a pretty large ES between those numbers. How long did you wait between shots? Reason I ask is because the way they keep gradually gaining more velocity leads me to think it probably has to do with barrel and chamber heating up. I went to the range to test some loads for my 7 mag a few days ago and I took almost 2 hrs to fire 14 rounds. I live in east tx though and the heat and humidity is terrible right now. I would shoot a round and then remove my bolt and take rifle over to a shop fan the range owners have set up and sit it in front of it for a few mins. I only chronoed the last 5 shots and they were 2750 2748 2736 2742 and 2751. The way yours are gradually increasing a few fps at a time only thing I would think it could be is barrel heating up

 

[Auto-X Fil]

Those numbers have been sorted - there's no way he actually shot those velocities in that order.

I agree the ES is pretty huge. Something is very wrong with that load. Weak primers?