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How much does a 308 drop?

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Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: April/30/2010 at 04:11
paulsfin View Drop Down
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There's alot of postings about running out of elevation adjustment with a scope that has a 1 inch tube. I bought a zeiss conquest and is advertised in the manual that it has 45" of elevation adj at 100 yrds- (the non hunting turret model has 68"@100 yrds) assuming that there was 22" @ 100yrds or 88 clicks of adj available as the reticle was centered, just how far could I shoot without resorting to "hold over or Kentucky windage methods"? I'll be shooting a 308 on a 20" barrel.
 
I guess what I'm really asking is how much does a 308 drop over the course of distance? Some tests online are showing that a 20" does not create a significant loss of velocity....???
 
One other thing was what would be my effective range from minimum to maximum adjustment in yards and how does that change if one adds a 10 or 20 MOA base into the mix?
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: April/30/2010 at 04:47
Trays 7940 View Drop Down
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What type of bullet and weight will you be using?
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: April/30/2010 at 05:00
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Trays got it right, the bullet type and velocity determine the drop. The barrel means almost nothing, you may loose a little velocity witch in turn increases your drop.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: April/30/2010 at 09:34
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To get some quick ideas of bullet drop you can go to the Federal Premium ammo website and find your way to the rifle ammo selection section.  Pick .308 and then find a bullet that is close to what you are using.  You can the look at the tables for the cartridge and get an idea of the "drop." 
 
Note that the bullet "drop" will of course depend on what range you are sighting into.  For example I found that for a 150gr Nosler Partition, a rifle sighted dead on at 100 yds may result in a drop of almost 4" at 200yds, and 13" at 300yds.  The same set-up sighted in at 200yds has a drop of about 9" at 300yds.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: April/30/2010 at 11:31
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Elevation range on a scope is typically expressed in MOA (an angular measurement) rather than inches (although MOA or mils both translate to a dimensional measurement at a range, e.g 1.047" and 3.6" @ 100 yards, respectively). So you want to make sure that bullet drop expressed in MOA at the max. range you're shooting at doesn't exceed the usable elevation of the scope.

For example, I have a 20"-barreled .308 and a load for it that drops 36.5 MOA (381.2") @ 1000 yards, using a 100-yard zero. So I need at least that much usable elevation in the scope. By usable, that means after you have set your zero you still have enough adjustment left over to compensate for drops out to max. range. Typically, this is about half of the advertised total elevation since the scope starts out adjusted in the middle of its range of travel.

In the case of this rifle, its scope has a total travel of 100 MOA. Assuming half that is usable with my 100-yard zero, I can make shots out to 1000 yards with a zero-MOA base. However, I always buy 20-MOA bases because it really doesn't hurt and can be re-used on a rifle that might have worse ballistics. My 20-MOA bases are .44" in the back and .29" in front so the difference between that and a flat base is .15" and that doesn't enough to the scope height to make it any harder to get a good cheek weld.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: April/30/2010 at 15:43
paulsfin View Drop Down
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I'll be using Federal 168gr Gold Medals. their website advertises 2460 fps MV.
 
jonoMT, so using the 20 MOA base would allow me to "move" my zero lower in the scope thus allowing me to free up more effective elevation? According to Zeiss, I should have a total of 42 MOA adjustment (rounded down), halving that means I have 21 MOA to use. Or rather I could shoot out to where the bullet has dropped 21 MOA and no more. What happens to the scopes sight picture if its E knob is cranked all the way over? I ask because if I add a 20 MOA base and have 21 MOA adj range up or down, that means the scope will be working from one extreme end to the other. Is this bad or have any pitfalls?
 
Thank you for your patience, I'm not trying to get out to a 1000m but rather trying to figure out just how far I could go with this scope and rifle setup.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: April/30/2010 at 16:09
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Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: April/30/2010 at 16:38
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The Fed web page only gets you to 500y. The fun stuff like 400" bullet drop doesn't happen till you get out to 1000y. I'm looking at my dope for FGMM 175 and it drops 395" at 1000y. That is with a 2.7" high scope line, flat base, and 100y zero. What is your scope heigth? I will figure your scope limits base on the Fed GMM velocity and ballistic coeff.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: April/30/2010 at 16:50
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That 2460 is the amount of energy the bullet retains at 100 yards, your muzzle velocity is 2650 fps.
use JBM ballistics calculator to give you your dope.  use the (G1) data as a ball park because your velocity will vary from advertised due to barrel length and atmospheric conditions in your AO. 
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: April/30/2010 at 16:51
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