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.308 ballistics

Discussion in 'Uncategorized Threads' started by amboy49, Nov 2, 2008.

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  1. amboy49

    amboy49 Well-Known Member

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    I know this is a trapshooting thread, but . . . . . .

    Sighting in a Kimber M84 in .308 this weekend prior to annual trip to OK for deer hunting.

    According to the Federal Premium ammo box if sighted in dead on at 100 yards, the bullet will strike - 3 inches low at 200 yards and 15.4 inches low at 300 yards.

    So, I sighted the gun in to shoot 3 inches high at 100 yards.

    Then, moved back to 200 yards and the point of impact actually RAISED to 8" high or five inches higher than at 100 yards ? !

    Factory loaded 168 Grain Federal premium match grade ammo - boattail spire points.

    This doesn't seem logical to me - that the bullet would strike higher at 200 yards than 100 yards. Especially considering the ballistic table printed on the box.

    Any explanationas to what I'm missing here ? I might also mention this was from a bench rest, 9 power scope, 68 degree temperature, no measuable wind.


    Thanks in advance
     
  2. timb99

    timb99 Well-Known Member

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    3" high at 100 yards should be just about dead-on zero at 200 yards.

    I sure can't explain it.
     
  3. Savage99Stan

    Savage99Stan Active Member

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    How high is your scope mounted above the bore line? If you have really high scope mounts, your bullet may have been still rising at 100 yards and not really representing "mid-range" trajectory. If that is not the case, you have me puzzled.
     
  4. Brian in Oregon

    Brian in Oregon Well-Known Member

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    Stan is right. I like high mounted scopes for this very reason.
     
  5. Shooting Coach

    Shooting Coach Banned User Banned

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    You are referring to the Rule of Three. This is the method savvy hunters use for sighting their game rifle in for achieving maximum point blank range. The idea is to be able to hit a vital area about 6" over or under the point of aim without hold over or under.

    Your firearm can be used on big game with this method to about 270 yards. The 30/06 will only add about 10 yards to this, a testament to the efficiency of the smaller 308. A Magnum rifle, with a good boat tail bullet launched at a speed of around 3200 fps, using the same method, will have a maximum point blank range of 325 yards.

    Using a premium boat tail bullet can deliver up to 30% more energy at 300 yards, along with a flatter trajectory. The Rule of Three assumes one WILL use a premium boat tail bullet.

    At 200 yards, you will be a little high. At around 240 yards, you will be on the money. At 270 yards or so, you will be a little low, but still in the vital area of deer sized game. At 300 yards, you would hold to the top of the back over the shoulder of deer sized game.

    I generally tell folks to limit their shots past 300 yards unless they really go to the range and WORK with their rifle at long range.
     
  6. halfmile

    halfmile Well-Known Member

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    Well, first things first.

    The only 168 grain load on Federal Premium's website is a Match King bullet.

    Are you sure you want to shoot deer with this bullet? Especially since they have much better bullets available for the purpose , accubonds, Nosler partitions, etc.

    That said, their ballistics chart for this load (see pitcher) with a 200 yard zero shows 2.1 inches high at 100 and 8.9 low at 300.

    You better set it at 2.1@100 and check again.

    If you want a drop chart, I can give you a url to an online ballistics calculator. (email me)

    HM
     
  7. Shooting Coach

    Shooting Coach Banned User Banned

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    Using the Hornady 165 gr Super Shock Tip at a muzzle velocity of 2750, the chart is as follows. This is one of the better long range premium boat tail game bullets. This velocity is attainable without loading to excessive pressure. Varget and Reloder 15 (their spelling) can get you where you want.

    50 yd +1.88"


    100 yd 3.01"


    150 yd +2.79"


    200 yd +1.11"


    250 yd -2.15"


    300 yd -7.11"
     
  8. Ahab

    Ahab Well-Known Member

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    Obviously your 1st sight in was at 100 yds instead of at 25 yds.

    The bullet crossews your line of sight twice...once as it rises out of the muzzle and the second time at the pint you have sighted in!

    Clear as mud?

    It is much better explained at:

    http://elkhunter2.tripod.com/sight_in.html
     
  9. setter

    setter Member

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    <a href="http://s248.photobucket.com/albums/gg171/jmoody1432/?action=view┬Ąt=target.gif" target="_blank">[​IMG]</a>

    Here's an example from the website Ahab posted. It illustrates bullet striking points. In this case the rifle was "sighted in" 3 inches high at 100 yds, and in this case the bullet strikes 5 inches high at 200 yds and is at point of aim at 300 yds and some 4 or 5 inches low at 360 yds.
     
  10. wolfram

    wolfram Well-Known Member

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    Amboy,

    I'm wondering if you scope rings are parallel to the bore. How about if you try this in reverse and see what happens. Zero at 250 then shoot the 100 yard group. If the group is way low then pull the bolt and look through the bore centered on a 25 yard target and compare that to where the reticle is centered.

    The other possibility is your shooting technique as in how well was the rifle supported and where. If you have pressure on the underside of the barrel bad things happen to your groups.
     
  11. amboy49

    amboy49 Well-Known Member

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    First of all, thank you to everyone who posted a response.

    Great information and certainly demontrates the amount of knowledge available to be shared for the asking on this forum.

    As a second chapter, I plan to go back to the range and shoot from 100 yards to see where the POI is now since the gun is zeroed at 200 yards. Hopefully it will prove to be 3 inches high and I'll be good to go.

    To Wolfram - thanks for the comments. I was pretty careful to keep the gunstock on the front rest - and the barrel off of it. One thing I didn't take into consideration was that the barrel was continuing to heat up - and it is a sporter weight barrel. However, I wouldn't think this would cause the groups to "walk" into a higher POI by such a significant degree.

    I'll give a final report after shooting at 100 yards.

    Agains, thanks all.

    Noel Kendall
     
  12. Chango2

    Chango2 Active Member

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    Barrel could be rising when shot fired; the dynamics of that particular gun? I always torqued my barrels into the stock (that has pillars) before taking the gun to the range. I used an inch pound wrench. I found dynamics, I assume, caused some of these "problems" with my varmint guns.
     
  13. timb99

    timb99 Well-Known Member

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    Wait about 4-5 minutes in between shots. More, if you have time.
     
  14. ron (vt)

    ron (vt) Member

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    Back about 11 years ago, Mike Venturnio in Shooting Times noticed the same thing when firing his 308 that the bullets impacted higher @ 200 yds than at 100. He could not find a reason for it.
    One year later he revisted it when testing a different 308 and observed the same thing one day but another day things were more normal in than bullets were impacting lower @ 200 than 100. Some of differences were that humidity was higher and the temperature lower.
     
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