Bullet Trajectory different than computer model

[arthurrh]

It doesn't take any special math to see the problem. Draw a line to represent a level shot straight out of a gun, no drop or anything.

Then draw one parallel to it to represent a normal scope mount.

Then draw one almost parallel to it, with a degree or two of down angle. You'll see that it will cross the rifle line at some point, and then the further you go the further it moves away from the other two lines.

Meaning that if you have an inclinced base and your equations doen't take it into account, you will have increasing errors the further your target is.

[Zak Smith]

You've missed the whole purpose of the erector assembly. You are correct that the scope tube is fixed in modern scopes with respect to the rifle (inclined or not), however, when the reticle is adjusted via moving the erector assembly through dialing the windage and elevation knobs, the point of aim (which defines a ray going out in space) moves. In particular, with correct data, that point of aim will coincide with the trajectory at the distance the elevation knob has been dialed for.

If what you say is true, nobody could use even a straight base (scope tube parallel to bore) and then dial 30 MOA up elevation to hit at 1000 yards because of the opposite problem (IE-- it's the opposite case of having a 30MOA base and it dialed for 100 yards).

Feel free to respond:

I challenge anyone who thinks it makes a difference to either

a. explain how it makes a difference, beyond what I showed in posts #19 and #21, or

b. do an experiment which proves there is a difference.

[Unclenick]

Arthrrh,

I'm afraid Zak has it right here. A mount that inclines scope downward is no different than iron rear sight and mounting it up on a block to get more elevation. It's the same as using a ladder sight or a tang sight to be able to get the line of sight angle up higher.

In the case of the scope it just means that scope center zero isn't parallel to the bore line, same as its line of sight won't be parallel if you run the clicks up on the elevation knob. The reason for the down-angled mount is to prevent the scope from running out of elevation adjustment at very long range, where the conventional parallel mount wouldn't let it crank up far enough to hit the target.

Let's get the load specifics and see what we've got? (Mathematical hands rubbing greedily together).

Nick

[ethernectar]

Ok, here's the load:

Virgin Remington brass
Large Rifle Primers
77 g Reloader 25
Sierra 190 Matchking HPBT

Rifle: JP Custom Sendero

26" barrel w/ JP's brake
1 in 10 twist
Badger base (20 moa) and rings
Leupold Mark 4 from Premier Reticle (1/4 minute clicks)
McMillian A5 stock
Shot prone from a rear bag and front Harris bi-pod

Chrono was a buddy's. I believe it was a Chrony. Averaged 3006 fps.

I've ordered my own Chrono and it should be here this week. Maybe this weekend I can get out and give it a go.

More load details at: http://ballistics.ntinnovations.com/...cs.aspx?ID=577

[Zak Smith]

Edit: fixed SOB distance

_Bullet_           _BC_ _MV_         0     100     200     300     400     500     600     700     800 | YARDS
300WM 190gr SMK   0.53* 3006 >   -0.00    0.00    1.23    3.20    5.51    8.09   10.95   14.11   17.60 | drop (moa)

Drop values are in MOA (1MOA = 1.0472" @ 100 yards).
I recommend a 100 yard zero because it will be less affected by atmospheric change and they are more common for sighting in than 200 yard ranges.

[Unclenick]

Tabular trajectory data at Std.ICAO Atmosphere
-------------------------------------------------------------------------------------
Gun / Ammunition : .308 Win.
Bullet : .308, 190, SRA HP MatchK 2210
Bullet weight : 190 grains or 12.31 Grams
Muzzle velocity : 3006 fps
Crosswind speed : 10 Mph
Ballistic Coefficient(s) (G1):
C1=0.524@V>2100 fps;
C2=0.516@V>1600 fps;
C3=0.506@V>0 fps;


Optimum trajectory information :
Optimum sight-in range (X) = 210 Yds.
with max. ordinate above LOS at range (M)= 128 Yds.
and max. point blank range (P)= 243 Yds.
-------------------------------------------------------------------------------------
Sight-in clicks, 1 click = 0.665 cm/100 yd. or 0.262 in/100 yd.
Height of sight above bore axis = 5.0 cm or 1.969 inch
Gun is zeroed-in at 200 yds,
by sighting-in at level firing
-------------------------------------------------------------------------------------
Range Velo Time of Energy Path Deflection Total Sight correction Target
city flight to at crosswind drop for setting new lead
LOS of 10.0 Mph zero range 33 fps
-------------------------------------------------------------------------------------
·Yards fps s ft.lbs. in. in. MOA in. Clicks MOA yds ·
-------------------------------------------------------------------------------------
| 0 3006 0.0000 3812 -2.0 0.0 ----- 0.0 ------ ----- 0.00
| 100 2824 0.1029 3364 +1.2 0.6 0.53 2.0 -4.7 -1.16 1.13
M 124 2781 0.1288 3263 +1.3 0.9 0.68 3.1 -4.2 -1.04 1.41
X 200 2649 0.2132 2961 0.0 2.4 1.14 8.4 0.0 0.00 2.33
P 234 2591 0.2522 2833 -1.5 3.3 1.34 11.7 +2.5 +0.62 2.76
| 300 2481 0.3298 2597 -6.1 5.4 1.70 19.7 +7.8 +1.95 3.61
| 400 2319 0.4545 2268 -17.9 9.7 2.32 36.6 +17.0 +4.26 4.97
| 500 2163 0.5891 1974 -36.2 15.9 3.03 60.2 +27.7 +6.92 6.44
| 600 2012 0.7324 1707 -61.8 23.5 3.74 91.0 +39.3 +9.84 8.01
| 700 1866 0.8858 1468 -95.7 32.9 4.49 130.0 +52.2 +13.05 9.69
| 800 1728 1.0547 1260 -140.7 45.1 5.39 180.2 +67.2 +16.79 11.53
| 900 1598 1.2368 1078 -197.6 59.6 6.32 242.3 +83.8 +20.96 13.53
| 1000 1474 1.4304 917 -266.9 76.1 7.27 316.8 +102.0 +25.49 15.64
-------------------------------------------------------------------------------------
M = Peak vs. L.O.S, X = Set Zero, P = Max. Point Blank Range
Elevation above Angle of Site (0.0 deg.) = 0.0826 deg.

[Unclenick]

Hmm. Not too legible. Let me PDF it.

Nick


Edit: Check the added table and see if your ballistic software gave the same clicks off the 200 yard zero? That is the starting point. See if we match?

[ethernectar]

The info from the NT site isn't that much different; EG:

Drops from NT:
0 6.28 18.5 36.4 61.9 96.1 140.3 196.23 263.48
From UncleNick:
0 6.1 17.9 36.2 61.8 95.7 140.7 197.6 266.9

I'm beginning to wonder if my chrono reading is just low. How much velocity in FPS would it take to be as high as I'm running?

This is probably something that would be better solved by running to the range on an off day and running through a bunch of ammo at given distances...

Anyway, any other ideas? Thanks for the help so far. Definitely a new challenge to wrap my brain around.

matt

[Zak Smith]

If you are 5 MOA higher than you expect at 1000 yards, that works out to like 300fps FASTER than 3000.. no way in 300WM.

[Unclenick]

Ethernectar,

I ran this load on QuickLOAD. The link you gave listed the overall length at an impossible 3.925 (the bullet shank wouldn't even be inside the neck). I was guessing they meant 3.3925, but tell me your OAL? At 3.392 you are over 7% compressed and up against the pressure maximum. Predicted velocity is 3070.

Also, what altitude is your range above sea level? Are you shooting up or down slope, and if so, at what angle?

Nick

P.S. QuickLOAD is usually within 2% of predicted velocities, but it can get to more like 1% if I have specific bore information: number of lands, width of a land and height of a land, slugged groove diameter.

[ethernectar]

Will measure the load itself and see. Its not a + load, and it'd need 2 or 3 more g of powder to be at the max recommended load.

Altitude is probably a few hundred feet, but I'll confirm that. At the shorter distances 200-300 you're pretty even with the target, farther out, the berms are higher.

I don't have those specifics on the bore. Its a Remington 700 Sendero barrel.

[ethernectar]

This is interesting....

The load measures 3.52 but the OAL in the loading manuals is 3.36 or close to that (from memory).

I arrived at my OAL (3.52) by test-fitting the load in the chamber as instructed by the guy I'm learning this from.

What difference would this make in the bullet's flight?

Note we've checked repeatedly the primers and spent brass. No signs of too much pressure.

[Zak Smith]

None. You're barking up the wrong tree with the OAL.

Outside of freak things like too much bullet deformation or it getting jammed cockeyed into the lands (which would reveal itself in unstable flight and poor groups at 100)...

All we really care about that the OAL could affect is the muzzle velocity, and we measure that directly.

[ethernectar]

Thanks Zak,

I got my Chrony this week. So I'll head out to the range when I can and see what it shoot with this one...