Range Report Variations from calculated POI (JBM)

[XTR]

I took a new load out to the range today to start collecting dope.

Shooting 175 SMKs over 44.1gr of Varget, BH x1 fired with CCI #200 primers.

My chrono says I'm getting 2660 FPS.
Ran the JBM simplified calc and printed up a dope sheet.



Out on the range today the it looks like either my Chrono is slow or something missed buy a little.

Drop at 200 is pretty spot on
Drop at 300 is about .425 off
Drop at 600 is about a full MOA off.

Here's the good part, substituting numbers till I get the points to line up it looks like my initial 1000 yd calc was off by as much as 2.7 MOA.

Weather permitting I guess I'll find out on Monday when I can shoot 1000.

 

[jbailey]

Re: Variations from calculated POI (JBM)

environmentals?
also, you sure your scope tracks w/o cant and accurately?
I would be very surprised if its a ballistics issue. I would rule out everything on the gun/chrono/weather first.

 

[gstaylorg]

Re: Variations from calculated POI (JBM)

Velocity required to calulated values to match up? How different than 2660 fps? It the difference realistic?

 

[Rob01]

Re: Variations from calculated POI (JBM)

Make sure you put all the right info into JBM. The old saying "garbage in, garbage out" really applies to ballistic computers. I have never had JBM off more than .2 mils at 1000 yards which is about 7".

 

[XTR]

Re: Variations from calculated POI (JBM)

I'm not to surprised by the difference, it was pretty close.

Plugging in 2700 vs 2650 put it pretty spot on.

I plugged in the environmentals from my Kestrel. I'm at about 900 feet in Oak Ridge, TN so it's not like I'm in thin air. The conditions weren't weird or anything. 988.2 mb 74F and 40%

At 600 yds I had a light wind from about 4 to 5 o'clock (4 to 5 mph) but even when it died my shots never dropped low. Just oddly right. I didn't start out doping anything for wind and my first shot was a 10 and the next 3 were Xs then I lost 2 to the 9 ring right when the wind died. Then I had 3 loopy where I was chasing the dope 10/8/X but the movement on the 10 and the 8 wasn't what i thought it would be. Anyway I ended up right back at zero. and shooting 10s an Xs again.

Out of 20 shots I only lost about 5 or 6 out of the 10 ring, but I was shooting at HP targets. I was working in with some service rifle guys that were at the range when I got there.

The point of the post wasn't to complain about the mismatch, just to give folks a data point.

 

[damoncali]

Re: Variations from calculated POI (JBM)

Yeah, that's the limitation of ballistics software. It's very difficult to get the inputs exactly right. A few things off here or there can have you scratching your head. A few feet per second in chronograph error, a couple of yards of range, a couple of percent off on the BC, and you'll start to see some changes. And that's ignoring things like cant and scope tracking.

 

[XTR]

Re: Variations from calculated POI (JBM)

Looks like I found one correction to the data that gets me closer. I was using 1.5" for the height of the scope above the bore. Measuring from the ocular to the center of the bolt it is more like 2 inches. That changed the data quite a bit. Puts the 600 yard dope at 14.9MOA vs the 14.625 that I actually dialed, and put the 300 yd number spot on at 4.4 vs the 4.375 that I dialed. Those are much better numbers. (G7 BC)

Garbage in = garbage out. I didn't realize how big a difference the change in scope height would make. It matters, a lot.

 

[gstaylorg]

Re: Variations from calculated POI (JBM)

2700 vs 2650 is about the difference I'm seeing between my new chrony and the same model it recently replaced (same weapon/loads). It seems as if my new one is reading a tad low compared to the old as I gather may possibly be your situation from your post.

 

[XTR]

Re: Variations from calculated POI (JBM)

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: gstaylorg</div><div class="ubbcode-body">2700 vs 2650 is about the difference I'm seeing between my new chrony and the same model it recently replaced (same weapon/loads). It seems as if my new one is reading a tad low compared to the old as I gather may possibly be your situation from your post. </div></div>

I was ready to believe the Chrony was off till I changed the scope height in the JMB data and the numbers got really close. I was happy with the data being as close as it was before the scope eight correction, now the 600 yd number was only off by 2 clicks because I have 1/8 clicks. Off by .275 is not very far off. It still put me in the 10 ring.

 

[XTR]

Re: Variations from calculated POI (JBM)

Actually got out on the 1000 yd range this afternoon. This was my first time ever at 1000 yds with anything, so I was going completely off of the JBM.

No spotters, 20 rounds shooting at a HP target, not an F-class target, shot 180 7x.

Oddly 1000 yds took about 38 MOA, closer to the numbers I calculated before I changed the scope height in the program. I didn't get a chance to look at today's atmo numbers though. I know the humidity was way up.