Confusion on My Ballistics

[craigp40]

I need some help. I've been working up some loads for the rifle Chris Matthews built for me (7mm RM with 26" Broughton 7.75 and A5 stock). I found a pretty good load using 175 SMKs and RL-22 that very consistently prints at least in the 3's and better when I don't screw it up.

Here's my problem: I zeroed the rifle at 100yds. According to my ballistics program, 3.4 MOA should get me to 300. When I actually shoot it, it only takes me 2.5 MOA. This doesn't make sense to me. I know the charts aren't perfect, but I have never had one this far off. The physics of it should be fairly close. I admit, not the worst problem to have, but how in the hell can I make up drop charts without shooting at every 50 yd increment between 100 and 1500 yards? The scope I have is a new Nikon Tactical 4-16. I plan to do a box test with it to verify its calibration, but anything else I should be looking at or doing?

In case you wondered, here are the numbers I am using to calculate it:

Bullet weight: 175
BC: 0.608
Scope height: 1.80
Velocity: 2934
Alt: 800 ft
Zero range: 100yds

 

[chris matthews]

That's the thing- they are just computer charts- they get you close- they are not meant to be exact.....
Yes, you have to go out and shoot every 50 yards!!!!! I hate it when I gotta shoot!!!! Twist my arm some more!!!! LOL!!! /ubbthreads/images/graemlins/grin.gif
There is no substitute for actual field collected data.

But as to why the chart is that far off- I am guessing the BC is higher than the number you are using......

 

[craigp40]

Yeah, I know Chris. Shooting that much is hard work /ubbthreads/images/graemlins/grin.gif. You don't make it any easier building such a **** good rifle. Can't wait to get this thing out in the field on the next tactical shoot in Sep.

Anyway, I punched some higher BC numbers into my program and it didn't affect the numbers enough to explain the difference at 300 yds. Biggest driver is velocity. For me to get a 2.5 MOA drop at 300 yds, I would have to be pushing these 175s 3250 fps. No way I'm even close to that. Aside from the scope, I can't think of anything else to do besides shoot it. Only bad part of that is trying to find a good place to do it.

 

[chris matthews]

Have you shot it over a chrony for the actual FPS?

 

[JBM]

800 feet? That tells the program to use the standard atmosphere. I don't know what it's like where you live, but last month, here in New Mexico, it was 105 degrees at 4100 feet which is no where near the standard atmosphere. Try to use the actual temperature, pressure and humidity. I don't know that it will make all the difference, but it can help.

 

[308 nate]

A very important factor that sometimes gets forgoten is a very acurate measurement from center of bore to center of scope. I believe this could get you off almost that much in extreme cases. And everything else was pretty much covered above. Also those broughton barrels tend to give you faster velocities,so if you are using a projected muzzle velocity I would reccomend using a crony and getting an accurate muzzle velocity. The broughton barrel on my .260 is giving an easy 75-100fps higher than projected velocity
FWIW,
308nate

 

[Eaglet]

craigp40,
I agree with "308 nate", I believe it will be a combination of Muzzle Velocity and Scope Height. I agree in that the ballistic table should be closer.

 

[Michael Eichele]

This is one of the reasons I dont use a 100 yard zero for a referance.

For example: say you are zeroing at 100 yards on a 1" target and your .3" group is inside that circle but its .4" high inside that circle. It may be an acceptable zero for 100 yards, but now you are truly zeroed at 150 yards instead of 100 yards exactly, even though you are hitting small targets at 100 yards.

If you do the math at 90 degrees F. and 800' with slightly lower than standard pressure say .4 inches of mercury and are .4" high at 100 yards you will only need 2.8 MOA to hit at 300 yards. If you do the same math with a true 0.00" zero at 100 yards then it will be 3.27 MOA to hit 300 yards.

Bottom line: I might carry may rifle afield with a 100 yard zero, but when it comes time that I set up for a long shot, I use a verified 300 yard setting as a starting point.

Hope that helps

 

[craigp40]

Chris, I had it out yesterday taking measurements on that load. Five shot group avg was 2934, with an extreme spread of 9 fps and standard deviation around 4.

 

[chris matthews]

Not to start an arguement, but if you punch in different values for scope height it will change very little. Craig and I have already been over that.
That is a good point about having a true 100 yard zero. Zero means zero, not 1/4" high.....it would definitely change things further out.
I imagine his velocity is higher than what he thinks and the BC is too- Sierra tends to be conservative where most other bullet makers exaggerate their BS.

 

[craigp40]

JBM, the program I use only has inputs for altitude and temp. However, I did run the numbers on an online program that had those inputs. I got roughly the same numbers that I ran at home. At the very least, this ruled out any issue with the program I am using.

 

[chris matthews]

BC # or like I said- they are a program- meant to get you on paper- but nothing replaces actual dope....

 

[Ian M]

Craig,
Bottom line is to simply accept what your rifle and load is doing, get a good set of drops, shoot them so you are confident that they are doing what they are doing, and don't waste time wondering why the 'puter projections are different. The time you have spent wondering about this disparity could have been used to load more ammo /ubbthreads/images/graemlins/smirk.gif Just be glad that you are not getting more drop than the print-out predicts.

You might try some Hornady A-Max's sometime, I am getting amazing accuracy with them in some of the calibers currently being used. Shoot flatter than the HPBT's I am used to - but then that might make the difference from the 'puter projections even greater /ubbthreads/images/graemlins/smile.gif

 

[308 nate]

Chris,
No argument started I just did not know what has all been checked and when I run the numbers on my Sierra infinity program with my current conditions and enter 1.5 scope height I get 3.45 MOA at 300YDS.
But then when I enter a scope height of 2.1 which is the height of the scope on my .260 I come up with only needing 3.06 MOA at 300yds. So no, it would not cause all of the error. just throwin' out possabilities.I have had very good results with this program(if I enter all the variables as precise as possable) but yes, there is no substitute for real world drop chart. But the program has proven to be a real time saver for making real world drop charts.
FWIW,
308nate