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Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by jeff 300, Aug 15, 2008.
What has every one found them to be? Are they close to the .768 as stated by sierre?
I use .8 in my 338AM and it's very close.
The drop charts from Exbal are excellent using a BC of 0.8
I guess it depends on what gun your shooting them in, speed wise. I found at 3300fps Like AJ said....800 to .810 comes real close.
there coming out of a 338edge at 2850
I think a lot of it also depends on your elevation and humidity. Your not gonna get the same BC at sea level as you are at 5000ft. Usually the higher altitude will yield higher BC. For this I cannot fully explain, just what I've read. Kinda sucks when your only 600-700 ASL like I happen to be...
I am shooting the 338 EDGE w/300g SMK at 2840fps and my charts weren't matching up real well from the start...From 300-1000 they were anywhewre from a couple clicks low at 300, to about 12 clicks or 3 MOA low at 1000. So either I'm not getting the .768 BC value, or my speed really isn't 2840fps,which I highly doubt either as they've been time tested by others on this board. Everyone seems to be getting .770-.800 for a BC on the 300g SMK, and I"m using the standard 92g H-1000 load that everyone else is using in the EDGE to get 2850fps, give or take a few FPS. So I just figure that at my elevation, which aint chit, the BC must not be .770. Thats about the only conclusion I can come up with. If I enter 2700fps instead of 2840fps, my clicks match up perfectly out to 1000 yards, based on my actual shooting, thats all I know...Never did work the BC out backwards, but I have my drop charts refined to my actual shooting and thats really all that matters in the end.
I have found that the Sierra listed BC of .768 to work out very well in my 338 Lapua.
The first time that I Chronographed the 300 SMK i got 2820 FPS and I found my drops to be low at distance. I played with the BC all the way to .830 and that didn't make a large enough difference tyo match my drops. Next I decided to recheck my zero and at the last minute I decided to set up the Ohler 35 and re-chrono at the same time. This time I got an average velocity of 2791 and this matches my acctual drops perfectly with the Sierra BC of .768.
A 29 FPS error in velocity made the difference. The first day that I chrono'ed was a bright sunny day and the chrono was in the sun. The light diffusers were not enough to prevent the false readings. The next time was overcast and I have learned that direct bright sun light can cause a slighltly higher reading.
At 3200 fps clocked with an infrared chrono, I use .800.
From 2500-2800 fps 0.770 is about perfect at my elevation.
At 2900-3100 fps I have found that 0.790 is perfect.
At 3300-3500 fps, I have used 0.800 to 0.820 depending on rifle.
These are all out of 1-10 barrels of different chamberings for the various velocity ranges.
For an all around average I always use 0.800 and that will get you extremely close with any velocity out to 1K so you can tweak your drops from there.
Yes---Kirby is right about the BC---your software will tell you what the drop is at a specific range. For guys like me who shoot 1k BR this is very important esp. when I am shooting longer ranges at the ranch. When you look at the BC and the TOF along with the wind speed and how long the velocity stays within a specific speed it will make your head hurt to try and figure this out---use software!! The drop is only one part of the equation windage is the other!!
Does anyone know what the new 300grn scenar is? I havent found any published BC not even on lapuas sight?
I'll second Kirby's observations on the low and high end. Those #'s work real well out of an edge and out of a 338 AM.
Sierra's published B.C is right on with my rifle. My velocity is around 2825 fps. The barrel has 9.3 twist.
I'm confused. No more!
Sorry guys I need to rest.