280 rem trajectory doesnt seem right

[Big Tex]

hey guys new to forum and long range shooting....i have a semicustom 280 rem shooting 1/2 groups at 200yds with 168 gr berger at around 2800fps..... havent found zero yet but im 3" high at 100 and 4" high at 200...this doesnt seem right??? seems like i should start dropping by 200yds?? Could scope be mounted off plane?? or does the trajectory sound right? ...have somewhat the same issue with a 257 wby same exact scope/base/ring set up....leupold vxlll 4.5-14x50/one piece std base/30mm rings
thanks
both rem 700 actions

 

[BryanLitz]

Nothing strange about that.

You're 3 MOA high at 100, and only 2 MOA high at 200. The bullet is dropping, it just happens to still be above the aim point.

when you zero for long range, say 600 yards, you could be 12" high at 100 yards, and 22" high at 200 (for example) because the bullet will climb for about 300 yards before beginning to drop down to the zero at 600.

Come down 3 MOA to zero at 100 and you'll be about 2" low at 200.

-Bryan

 

[green 788]

Agree with Bryan... but I'll add this...

I've seen "weird" 100 versus 200 yard zeros more with light weight, sporter barrels than with heavy barrels.

There are dozens of explanations as to why this happens, so brace yourself.

In my opinion, it has to do with the whipping direction of the barrel when the bullet is released from the muzzle.

Check these animations from varmint al... Barrel Harmonics Mode Shape Movies

So... the position of the muzzle at bullet release has a direct effect on where the bullet crosses the line of sight. Your scope of course looks "dead straight" at the target. Your bullet, however, must rise to cross the line of sight (since the barrel is below the scope), then begin its fall toward the zero point of the scope.

If the bullet crosses the line of sight closer to the 100 yard point, it may still be rising at the 100 yard point. The bullet is likely continuing to rise, even past the 100 yard point, then it will begin to fall... and by the 200 yard point, it's higher than you'd think it would be.

Check your 150 yard zero... we've seen seasoned long range shooters at our rifle matches who can ring the plate at 800... 900... and 1000 yards darned near every time.... but a playing card at 165 yard causes them all sorts of grief. Most folks never plot their 165 yard zero, so they really are often surprised to find out just where it really is (ballistics drop charts don't generally agree with reality in this range)...

If you zero your rifle for a 200 yard zero (drop the scope elevation 8 clicks)... that would put your 100 yard point of impact only an inch above the crosshairs, and you'd have a "point blank range" of 275 yards, perhaps a little more. Meaning just hold right on and shoot, all the way to the extent of your point blank range.

In short, it doesn't seem that your rifle is doing anything that much out of the ordinary. I've seen far more exaggerated examples of what you're describing--which is why I offered my explanation/theory as to what's causing it.

Dan

 

[Big Tex]

hey thanks guys...makes me feel better

 

[Big Tex]

one more question...when i plug in my #'s to the ballistics calculator if im 3" high at 100 yds it says i should be under 3 at 200yds...according to it ,to have my trajectory
(3 at 100/ 4 at 200) id have to be shooting 4700fps( im new but i know thats wrong)....am i reading the trajectory wrong? sorry guys like i said im new...what exactly does drop on ballistics charts represent? is it the drop from point of aim or point of barrel plane?

 

[green 788]

The drop chart cannot anticipate the point at which your bullet crosses the line of sight, therefore it doesn't know with certainty whether the bullet is still rising at 100 yards or whether it has begun its descent.

The chart assumes that the bullet has begun its descent by the 100 yard point, falling back through the line of sight at or very near 100 yards (after first rising thru the line of sight sometime around 40 to 60 yards or so).

This is why you have to "true" your chart. You'll likely not find a perfect program that gives you accurate drops for all ranges...

What you can do is set your zero for 200 yards, by which time the bullet is definitely falling, and plot your drops from there on out. Then make some notes on the data card indicating your 100 yard to 175 yard "hold under"...

Dan

 

[BryanLitz]

The drop chart cannot anticipate the point at which your bullet crosses the line of sight, therefore it doesn't know with certainty whether the bullet is still rising at 100 yards or whether it has begun its descent.

If you output your chart in small enough range increments, you can tell if it's rising or falling at the 100 yard zero. Consider the output below in 10 yard increments, you can see that the trajectory rises slightly above the line of sight before 100 yards, then falls down to the zero point at 100 yards, and continues to fall beyond that. With a faster MV, and a higher sight height, the bullet could very well still be rising at 100 yards, which the drop chart would show if the range increment is small enough.

      Range    Velocity    Energy     Trajectory         TOF          Drift
      (yards)     (fps)     (ft-lb)     (inches)         (sec)       (inches)
          0      2600        2326          -1.50         0.0000         0.00
         10      2581        2292          -1.11         0.0116        -0.01
         20      2561        2258          -0.76         0.0232        -0.03
         30      2542        2224          -0.47         0.0350        -0.07
         40      2523        2190          -0.24         0.0469        -0.12
         50      2504        2157          -0.06         0.0588        -0.20
         60      2485        2124           0.07         0.0708        -0.28
         70      2466        2092           0.14         0.0829        -0.38
         80      2447        2060           0.15         0.0951        -0.50
         90      2428        2029           0.10         0.1075        -0.64
        100      2409        1998          -0.00         0.1199        -0.79
        110      2391        1967          -0.16         0.1324        -0.96
        120      2372        1937          -0.39         0.1450        -1.15
        130      2354        1907          -0.68         0.1576        -1.35
        140      2335        1877          -1.03         0.1704        -1.57
        150      2317        1848          -1.44         0.1833        -1.81
        160      2299        1819          -1.92         0.1963        -2.07
        170      2281        1790          -2.46         0.2094        -2.35
        180      2263        1762          -3.07         0.2226        -2.64
        190      2245        1734          -3.75         0.2360        -2.95
        200      2227        1706          -4.50         0.2494        -3.29
        210      2209        1679          -5.32         0.2629        -3.64
        220      2191        1652          -6.21         0.2765        -4.00
        230      2174        1626          -7.17         0.2903        -4.39
        240      2156        1600          -8.20         0.3041        -4.80
        250      2139        1574          -9.31         0.3181        -5.23
        260      2121        1548         -10.50         0.3322        -5.68
        270      2104        1523         -11.76         0.3464        -6.15
        280      2087        1498         -13.10         0.3607        -6.64
        290      2069        1474         -14.52         0.3751        -7.15
        300      2052        1450         -16.03         0.3897        -7.68
        310      2035        1426         -17.61         0.4044        -8.23
        320      2018        1402         -19.28         0.4192        -8.81
        330      2002        1379         -21.03         0.4341        -9.41
        340      1985        1356         -22.87         0.4491       -10.02
        350      1968        1333         -24.80         0.4643       -10.67
        360      1951        1311         -26.82         0.4796       -11.33
        370      1935        1288         -28.93         0.4951       -12.02
        380      1918        1266         -31.14         0.5106       -12.73
        390      1902        1245         -33.43         0.5263       -13.46
        400      1885        1223         -35.83         0.5422       -14.22
        410      1869        1202         -38.32         0.5582       -15.00
        420      1853        1182         -40.91         0.5743       -15.81
        430      1837        1161         -43.60         0.5905       -16.64
        440      1821        1141         -46.39         0.6070       -17.50
        450      1805        1121         -49.29         0.6235       -18.38
        460      1789        1101         -52.30         0.6402       -19.29
        470      1773        1082         -55.41         0.6570       -20.22
        480      1757        1062         -58.64         0.6740       -21.18
        490      1741        1043         -61.98         0.6912       -22.17
        500      1725        1025         -65.43         0.7085       -23.19

The chart assumes that the bullet has begun its descent by the 100 yard point, falling back through the line of sight at or very near 100 yards (after first rising thru the line of sight sometime around 40 to 60 yards or so).

The chart assumes no such thing. If you only look at the output in 100 yard increments, YOU can't tell if it's rising or falling, but the ballistics program that generated the chart *must* know the state of the bullet (rising or falling) on a continuous basis thruout the trajectory, even if it's just outputting every 100 yards.

This is why you have to "true" your chart. You'll likely not find a perfect program that gives you accurate drops for all ranges...

The reason for ballistic program predictions not lining up with observed drop can almost always be attributed to inaccurate inputs. If you input accurate data into a good ballistics program, it will calculate an accurate trajectory at least over the supersonic range of the projectile. This has been documented many times. Admittedly it's difficult to get all the variables truly accurate, but if you do, the predictions are valid without correction.

What you can do is set your zero for 200 yards, by which time the bullet is definitely falling, and plot your drops from there on out. Then make some notes on the data card indicating your 100 yard to 175 yard "hold under"...

Dan

You could do that. Typically I advise shorter range zeroes because it makes the zero less susceptible to conditions, wind, etc. If you want a long PBR for quick engagements, it makes sense to consider a longer range zero.

Dan,
I'm not attacking you or trying to be contrary for the fun of it. Just offering a new guy multiple perspectives to consider. No offense intended.

-Bryan

 

[green 788]

Thanks Bryan, no offense taken.

I think if one does the chart as you show it, and inputs enough data to steer the results closer to the actual trajectory (which is what we should do, as I know you'd advocate)... then things look much better.

But if you take a chart from the back of your handloading manual... or just do a quickie on JBM without adding enough pertinent data... you can get what appear to be strange results.

I do think that bullet crossing line of sight has got to have some impact on the final, perfected chart. There's just too many odd things happening out there in the field to explain it any other way...

Dan

 

[Joe King]

Whats your scope height? Get that input wrong and you can run around in circles for a while wondering just WTH is going on. I know this I ran your bullet and vel across Bryan's calc and with your scope I set scope height at 1.8". At 3" at @100 you should see 3" high @200 but as Green788 pointed out it doesn't always work out that way. could be a parallax issue (you may not have it adjusted even though your sure you did). Could be and adjustment you made to your shooting position between yardages. 1" at 200 is a pretty damned small amount to be off and most anything can cause it to show up, just the nature of the beast if your not very very consistent with how you shoot. At any rate keep after it and you'll get it figured out.

Bryan's suggestion on a 100yrd Zero, is how I would go about it personally, or at least go dead on at 200.

 

[Big Tex]

thanks guys....ill keep shootin and let it do what it do

 

[JeffP40]

I believe Brian is on the money in a couple of ways. First, make sure your scope height is very accurate. Then, as Brian suggests, use a hundred yard zero. I like it because all of your dope is in positive numbers, you are not starting with negative drops at short range and changing to positive at longer. That will give you an idea of what the bullet is doing as it goes downrange. With a 3" high "zero", (it isn't a zero if it is 3" high), there are a lot of variables to deal with. Namely, are you really 3" high, or almost? Get an absolute zero at 100 and go from there. I do mean absolute, no "close enough". Once you can smack a small dot at 100, then check your actual drops at distance to see how it line up with a chart. Tweak your input numbers to get them to match you actual drops and then go smack things.
If you are looking for a maximum point blank range for hunting, then a 2 or 3" high zero at 100 will do fine, although your real zero will be set a longer distance.