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
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!!! [img]/ubbthreads/images/graemlins/grin.gif[/img]
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......
Yeah, I know Chris. Shooting that much is hard work [img]/ubbthreads/images/graemlins/grin.gif[/img]. You don't make it any easier building such a damn 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.
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.
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
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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.
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