Big discrepancy between chrono MV and real world shooting. Why?

[Hoosker Doo]

I'm a long time hunter, but short time long range shooter (which will be plain to see, I'm sure). I worked up a load for my 7mm Rem Mag topped with a Trijicon Credo HX 2.5-15x42 using 162 gr ELD-X. With my Magnetospeed Sporter, I got an average muzzle velocity of 3033 fps, which seems right on according to my reloading manuals. Now for the fun part...

Full discretion, I am just using the Hornady Ballistics app and have been doing 3 and 5 shot groups. I sighted my rifle in at 100 yards. Center of the group was actually 1/2 inch high at 100 yds and that's how I left it.

I plugged all of my data into the app. Elevation - 7000 ft, 100 yd zero, scope height - 1.75", BC as stated on box.

I decided to test the ballistics app and my setup by shooting a 5 shot group past 400 yards. At an exact 422 yards my total come up was 5.45 MOA. (I also verified this after the fact with my brother's Ballistic AE app.) After shooting 5, I retrieved my target to find that 4 of the shots made up a 3.1" group, and but my first shot opened it up to 1.5 MOA (5.8"- pulled to the right) Still I am very pleased with this, being my first group shot at that distance. However, the center of the group was exactly 1 MOA above my point of aim (4.4")

After checking all my data entries and being at a loss as to what I might have done wrong, I decided to (informally) test it little more. I adjusted my muzzle velocity to match my results. Here's where I'm baffled. In order to get my total come-up to 4.4 MOA (Theoretical center of target) my muzzle velocity had to be adjusted to 3200 fps. That is smoking for a 162 gr projectile out of a 7 RM. I could see maybe 3100, seeing as how the Nosler book puts my load around 3050, and my rifle has a 26" barrel instead of the published 24" data.

With my new theoretical mv of 3200 fps, I shot at 422 again (all steel this time with a spotter) and hit, elevation was perfect. I jumped to 520 hards, adjusted my turret according to the app, and punched the steel dead center. For fun, I ranged the farthest steel plate at 812 yards, adjusted 13 MOA as prompted (13.08) and let her fly, and hit the bottom edge of the steel (within 1 MOA).

Please educate me on what you feel you need to. I'm very happy with my outcome so far. I feel like the data for one reason or another for my MOA adjustments is as close as I am equipped to to get it at the moment. However, I'm at loss as to the discrepancy between the chronographed MV and my "actual" MV. I still wouldn't bet money that I'm getting 3200 fps on a load under max powder charge. What say you?

 

[sndmn11]

Your zero isn't 100 yds since it is 1/2" high at 100yds.

You can test this by changing the zero to 150 yds, 200 yds, 167 yds, etc., In the app, until you get an adjustment value of 1/2" at 100 yds. While leaving your MV as 3033fps

 

[EdP]

Did you zero 1/2" high at 100 yds with the Magneto still on your barrel? The Magneto will change you POI so if that is what you did you really have no idea where your true zero is.

Assuming you zeroed with the Magneto removed, you still entered 100 yds as your zero when you were 1/2" high at 100 yds. That would account for for 2.1" of the 4.4" you were high at 422 yds.

Was there any mirage present when you were shooting? Mirage (heat waves moving air upwards) will make a target appear to be above it's actual position.

 

[Hoosker Doo]

Thanks for the responses. Here's a little clarification on my zero. Another disclaimer - 3 shot groups only repeated a couple times. My zero is at the top of the bull, but touching it. Playing with the app, I wanted to automatically assume that's a 150 yd zero, so I put up a target at 150 yards, and my groups were just below the bullseye not quite touching (3/4" low), with those 2 data points and the app, assuming a 100 yard zero seemed like my best bet.
And actually to get my 3200 fps to match my actual drop, I even had to back my zero up to 150 yards. So I actually had both those parameters in when I was hitting steel.

As for mirage, we've been between 50 and 60 degrees each time I've been shooting.

Edit - no magneto speed attached for zero or shooting at distance

 

[SDHNTR]

You are assuming too much. Zero dead nuts on at 100 with a 10 shot group. If your velocity is true then the other variable you need to true up is BC. Ammo boxes lie. Also make sure you have all parallax error cleaned out.

 

[PNWGATOR]

You are assuming too much. Zero dead nuts on at 100 with a 10 shot group. If your velocity is true then the other variable you need to true up is BC. Ammo boxes lie. Also make sure you have all parallax error cleaned out.

This!

 

[Hoosker Doo]

You are assuming too much. Zero dead nuts on at 100 with a 10 shot group. If your velocity is true then the other variable you need to true up is BC. Ammo boxes lie. Also make sure you have all parallax error cleaned out.

Thanks

 

[Stache Attack]

Hornady had a good podcast about zero angle but I think that must only be in their paid 4DOF application. If you have Spotify I would recommend listening to the podcast when they talk on the app I’m not a target shooter. Just a guy trying to get better and I learned a lot more than fooling with the app for 3 years.

 

[XLR]

https://xlrindustries.com/blogs/xlr-precision-rifle-blog/truing-rifle-data

Go over and read that article and it might help you out a little bit. It sounds like you are messing with a lot of inputs at once though so I would start at your zero and make sure you have a rock solid 100 yard zero. The only other thing that would make sense is checking the velocity with another chronograph to see how close the two numbers are.

 

[mxgsfmdpx]

You are assuming too much. Zero dead nuts on at 100 with a 10 shot group. If your velocity is true then the other variable you need to true up is BC. Ammo boxes lie. Also make sure you have all parallax error cleaned out.

This, other than I almost never adjust BC.

 

[SDHNTR]

There are way too many even numbers in your inputs. Red flag that you are off on at least some of your inputs. The amount that you have to true MV is a huge red flag. How are you determining your sight-in range? How are you determining your zero offsets? How are you getting your bore height? Have you checked that your scope tracks? Environmentals will play a very minor role at these distances, but it is an input that's easy to get right - how are you getting this data?

A 10-shot group won't get you any better zero offsets than a 5-shot group. If you are seeing a difference in zero offsets between 5 and 10-rounds, there's a shooter or gear issue. At the distances we are talking about, absolutely do not touch BC - that's terrible advice, and you'll want to be beyond 1,000 yards before you touch BC. Since you're using a magnettospeed, and assuming it's not broken, you should only need to adjust MV within about +/- 1-SD (I mean your actual SD, based on 15 - 20+ rounds), and that should be done 700 - 800 yards (minimum).

All of that assumes there isn't significant shooter error. Also, stop shooting steel. Focus on paper as much as you can and track all shots.

BC doesn’t matter at 812 yds? Right

 

[Hoosker Doo]

There are way too many even numbers in your inputs. Red flag that you are off on at least some of your inputs. The amount that you have to true MV is a huge red flag. How are you determining your sight-in range? How are you determining your zero offsets? How are you getting your bore height? Have you checked that your scope tracks? Environmentals will play a very minor role at these distances, but it is an input that's easy to get right - how are you getting this data?

A 10-shot group won't get you any better zero offsets than a 5-shot group. If you are seeing a difference in zero offsets between 5 and 10-rounds, there's a shooter or gear issue. At the distances we are talking about, absolutely do not touch BC - that's terrible advice, and you'll want to be beyond 1,000 yards before you touch BC. Since you're using a magnettospeed, and assuming it's not broken, you should only need to adjust MV within about +/- 1-SD (I mean your actual SD, based on 15 - 20+ rounds), and that should be done 700 - 800 yards (minimum).

All of that assumes there isn't significant shooter error. Also, stop shooting steel. Focus on paper as much as you can and track all shots.

Thanks! It is a red flag which is the main reason I decided to post here. I am not dissatisfied with my results, I just know something might not be quite right.
- I sight in by shooting at that range (I can probably dial that in a little better)
- I measured bore height with a caliper, center of bore to center of scope.
- Next I could probably do a tall target test to check scope tracking. To this point I have been trusting the Trijicon name, but I have had issues with other brands not tracking correctly in the past and that was actually my first concern, that my scope tracking could be off.

Thanks for the input and the check list. Much appreciated.

 

[ResearchinStuff]

zero angle is absolutely avaiable on the free version of 4DOF. the only thing you have to pay for is group analysis, which will automatically calculate your zero angle (maybe, group analysis doesn't work worth a crap on my galaxy S22).

 

[willfrye027]

Go back and to 100 yards. Get a perfect zero. Like, PERFECT. Double check your inputs in the app…then you should be dead on. Never had to tweak BC until things are way out there and bullet is really running out of steam. Before tweaking BC if you’re still off at 800-1000 and zero is perfect, all other inputs are correct in your solver…then adjust MV.

 

[WKR]

Sometimes I think my magnetospeed isn't as accurate as they claim

 

[Hoosker Doo]

Here is a very common workflow for me:

New scope is hard mounted and tracking, reticle subtensions, and reticle cant are checked in a tracking board. All components are dry torqued per mfg spec. New barrel is 'broken-in' with a couple hundred rounds to make sure it has sped up. Mounted new scope & bore sighted. Initial sight-in at range verified to be 100-yards with 5-shot group:
View attachment 567100

Adjusted 0.1mil Up and 0.1 mil Right. Confirmed zero offsets with 5-rounds.
View attachment 567101

I shot 20-rounds to get an idea of this ammo's SD (one round did not read). Side note, about 5 rounds is all you need for average muzzle velocity, but you need 15 - 20 to get an idea regarding SD IME.
View attachment 567102

I go a little crazier with height above bore, but calipers work fine. Twist rate is simply what's on the barrel, environmentals are from my Kestrel.

Inputs:
Sight-in range = 100 yards (range has been verified)
Elevation Zero Offset = -0.03" (see above target)
Windage Zero Offset = -0.03" (see above target)
Ammo MV = 2743 fps (need about 5 rounds over a quality chrono)
Scope Height: 2.02" (measure/verified with quality equipment)
Bullet BC/dopler file = provided by mfg and/or solver
Environmentals = from Kestrel

Quick gut-check/try-dope at 425-yards (primarily just concerned with elevation at this time, and I did not hold for wind). Range also went cold partway through shooting this group. No significant red flags popping up regarding inputs.
View attachment 567111

Traveled to a different range on a different day. Updated environmentals in kestrel, and dialed try-dope of 8.1 mils for target at 998-yards = impact.
View attachment 567112

Confirmed dope at various ranges. No drama. No changes to MV, and didn't even think to touch BC.

It is a red flag if I need to adjust MV more than about 15 - 20 fps for centerfire cartridges. It is a red flag if I need to adjust BC by more than a couple percentage points.

Great information! It's great to see someone else's process and how to improve my own. Appreciate it.

 

[Hoosker Doo]

Sometimes I think my magnetospeed isn't as accurate as they claim

I've wondered. I had a 6.5 CM that I shot through a friends prochrono with an average of 2710 fps. Then I bought my magnetospeed and it gave me 2635 with the same load. I automatically assumed the magnetospeed was more precise because I struggled to get readings from the pro. Now I'm wondering.

 

[Mule3006Elk]

I bought a Kestrel years ago.
No longer have this issue.
Solid investment which helped my shooting tremendously and saved tons of time/ammo/money/etc.
BC IME are overstated as others have noted. At 500-600 yards I always need to true my MV, which IME, is always MV drop never an increase. I have used Kestrel on Barnes, Hammer Hunters, Federal Edge TLRs. All needed a MV adjustment (decrease).

 

[Antares]

I don't want to derail, but I have a related question.

How many of you don't use a chrono at all?

I don't own one. I zero at 100 yards. Take the velocity of the box, minus 30 fps per inch if the barrel is less than 24 inches. Put that into the calculator (Shooter) to get "try" dope. Shoot at 300 and 500 yards. (my range only goes to 500). Tell the calculator what my actual drops are and let it figure out what it thinks my MV is. Anyone see an issue with this process?

 

[svivian]

I don't want to derail, but I have a related question.

How many of you don't use a chrono at all?

I don't own one. I zero at 100 yards. Take the velocity of the box, minus 30 fps per inch if the barrel is less than 24 inches. Put that into the calculator (Shooter) to get "try" dope. Shoot at 300 and 500 yards. (my range only goes to 500). Tell the calculator what my actual drops are and let it figure out what it thinks my MV is. Anyone see an issue with this process?

Never used one... just put in either the velocity on the box or what the load table says for reloading then shoot at various ranges true my solver. I shoot at transonic if I really want to dial in which at that point may effect the BC.

Then again I like to keep things simple so I don't second guess my equipment.