PDA

View Full Version : 308 Trajectory, etc.


ripcurlksm
05-19-2009, 11:03 AM
Spent some time this past weekend shooting my M1A Scout at Angeles Range with iron sights and following the instructions in the M1A/M14 manual to zero the rifle at 200 yards.

I was shooting a mix of:
- Portuguese .308 surplus 147 grain FMJ bullet
- Federal 308 Win. 150 grain Soft Point

Here are my come-ups from the bottom:
200 yard - 14 clicks ( zeroed on paper)
300 yard - 12 clicks (steel)
400 yard - 14 clicks (steel)

Now I found this trajectory chart from Federal 308 Win. 150 grain Soft Point

Long Range - 200yd zero
http://www.calguns.net/calgunforum/attachment.php?attachmentid=23616&stc=1&d=1242759062


So based on my clicks and the chart above and with my rifle zeroed at 200 yards (14 clicks):
1) 14 clicks is dead zero at 200 yards (fine)
2) 12 clicks at 300 yards, means that I had to adjust my rifle -2 MOA, to make it shoot 6 inches lower (2MOA @ 300yds = 6 inches) -- (according the the chart above it should be closer to 9 inches, but i suppose its close)
3) 14 clicks to come back up to 400 yards (this does not make sense to me, shouldnt I have had to adjust it - 26 inches according to the chart?)

So with this said, is the trajectory really peaking at ~300 yards? The part I am fuzzy about is why I am back at 14 clicks for 400yards when the bullet should technically be dropping ~26 inches??

grahlaika
05-19-2009, 12:54 PM
Those graphs are very much just a guideline of what sort of performance you'll get from that kind of ammo. Don't think of it as gospel. Those charts were developed using a very specific set of conditions, which you're very likely not meeting.

You actual bullet performance depends on the actual muzzle velocity, your rifle twist, your rifle accuracy, as well as local range conditions (temperature, density, incline, etc.). The ammo you're using may have significant variances in muzzle velocity, which may explain your variance in expected drop (you're comparing surplus ammo with hunting ammo, which is a problem as well). It also depends on how many shots you took with each to constitute a good measurement. Moreover, if your rifle is 1-2 MOA accurate (as the M1A is), then you're looking at a variance of +/- 8 inches at 400 yards!

Your best bet if you really want to be accurate is to actually get a chrony and measure your MV, then enter the data into a ballistics computer. Otherwise your technique will be approximate at best, which may be good enough for you.

fairfaxjim
05-19-2009, 3:02 PM
I am a little confused by your computations. I was under the impression that the M1A (or at least the M4) iron sights were either 1 moa or for NM sights, 1/2 moa per click. I may not be remembering as good as I thought.

1 moa @ 100 yards is 1.047". Your table shows about 9" (my data strip for my 165 gr. BTHP shows 9.2" drop at 300 yd with a 200 yd zero) With your sights zero'd at 200, and you drop at 300 = 9", you would need (9/1.047/3) = 2.87 moa (round to 3) or either 3 or 6 clicks up from your 200 yard setting. Your 400 yard drop is about right, 26" (my data strip is 26.5".) From your 200 yard zero setting you would need (26/1.047/4)= 6.2 moa or either 6 or 12 clicks (not from your 300 yard setting.)

Once you have the zero set, and know the moa/click, it is a simple equation:
(Bullet Drop from zero/1.047"/(range yds/100)/moa per click)= # of clicks
i.e. 400 yards (400/100=4), 26" drop from 200 yard zero, .5 moa per click
(26/1.047/4/.5)= 12.4 clicks

ar15barrels
05-19-2009, 3:23 PM
So with this said, is the trajectory really peaking at ~300 yards? The part I am fuzzy about is why I am back at 14 clicks for 400yards when the bullet should technically be dropping ~26 inches??

Sounds like you are using your near zero as a far zero and then being suprised when your far zero is further away than your near zero.

ripcurlksm
05-19-2009, 4:07 PM
grahlaika, thanks-- I know shooting out of a shorter 18" barrel is shooting ~120fps slower than a 22". I understand that the chart is a guideline, but I thought it odd vs. the results I was getting in that my 200 & 400 were the same adjustment.

fairfax, yes I have 1/4" clicks and @400 yards the bullet should drop ~26", however it doesn't seem that way, perhaps because of what ar15barrels has said?

ar15, glad you're here -- I understand what you are saying (I think)... but for example the near zero in the chart is at ~25 yards -- With me shooting paper at 200 yards, wouldn't I be at the far zero range?

also to note, most of the ammo I shot was the surplus

ocabj
05-19-2009, 6:28 PM
You have your rifle zeroed at 200 yards, and actually have to take *off* elevation for the 300 yard zero?

What hold are you using at 200? What hold at 300?

Assuming the same hold at 200 and 300, on targets with equivalent size in MOA (a factor if not using a center hold), a 200 to 300 zero would be around 2.5 MOA difference, give or take a half to 3/4s of a minute.

ar15barrels
05-19-2009, 8:33 PM
ar15, glad you're here -- I understand what you are saying (I think)... but for example the near zero in the chart is at ~25 yards -- With me shooting paper at 200 yards, wouldn't I be at the far zero range?

also to note, most of the ammo I shot was the surplus

Were you mixing ammo types during this?
Each ammo type may have significantly different POI at similar ranges simply due to barrel harmonics.
Gather your dope with only one load.
The other load may have a significantly different zero.
This could be screwing you up.

ripcurlksm
05-19-2009, 9:15 PM
it was mostly the surplus. i only shot a little of the Federal. Federal was the only chart I found, so I posted it as a reference I found

ripcurlksm
05-20-2009, 9:52 AM
Randall, do you still think it could be my near zero?

ocabj
05-20-2009, 10:03 AM
Didn't see any response to my post, but I'm going to hint that if you have your rifle zeroed for 200 yards, it should continue to drop after that at 300, 400, 500, and so-on, and you'll need to come up on your elevation for each distance after 200 yards.

If you are saying you have a 200 yard zero, and you had to come down on your rear sight elevation for 300, then:

1. You are not using center hold (point of aim = point of impact), or

2. You are using a relative hold, such as 6 o'clock, and the targets you are using at 200, 300, and 400 do not have the same size in units of MOA (for example, use a 12" target black at 200, 18" target black at 300, and 24" target black at 400), or

3. You're not using the same ammunition on all distances.

ripcurlksm
05-20-2009, 10:29 AM
apologies for not addressing your post ocabj -- yes I am using a 6-oclock hold @ 200yd zero. I think it is the size of the targets... I was aiming at the steel pig @ 300 yards at angeles range, then at the red square steel @ 400 yards

ocabj
05-20-2009, 10:37 AM
That answers everything. You need to be using the same sight picture and hold across all distances. If not using center hold, then the size of the targets at each distance should be the same size in units of MOA.

eighteenninetytwo
05-20-2009, 11:35 AM
This is really good information. Thank you to all who have assisted here. I have a question regarding the Far vs Near zero. I have a Garand in .308 and I took it to the range for the first time last weekend (heaps of fun but that's irrelevant). So the question is. If I'm zeroing at 100 yards - is this going to be the NEAR zero I'm using - is the trajectory flat enough over this minimal distance that this could be the first point of intersection between POA and POI (actually not intersection as I use a 6 o clock hold on a black about 6 3/4Inch). Or is this a far zero? Rear sight is pretty much as far down as it will go.

Shigaboo
05-20-2009, 12:21 PM
You mentioned that you were shooting at Angeles. The rifle side at Angeles gradually goes uphill about 15 or so feet after every 100 yard increment. When shooting up at an elevated target you're going to get different bullet drop, namely, less at an elevated target and more at a target below you.

eighteenninetytwo
05-20-2009, 3:13 PM
Thank you. Question still stand though, given the trajectory of the .308 is fairly flat out to 100yards am I right thinking that the changes of that being a 'Far zero' as opposed to teh near zero are slim. Is the bullet is not going to rise and fall enough in the first 100 yards that it would also be zeroed at say 30 yards also? I mentioned my 6 o clock hold on a 6 3/4 inch black as that means (I think) that the bullet is probably still rising at that point and crosses point of aim somewhere in front of the target. Am i missing somethign here. sorry for teh dumbass questions. Just trying to get to grips with the concepts

ar15barrels
05-20-2009, 3:24 PM
given the trajectory of the .308 is fairly flat out to 100yards am I right thinking that the changes of that being a 'Far zero' as opposed to teh near zero are slim.

Is the bullet is not going to rise and fall enough in the first 100 yards that it would also be zeroed at say 30 yards also?

Yes.

Yes.

As your far zero gets farther away, your close zero also gets closer.

Using an AR-15 as an example, you can zero for 50/200 or 25/300.

Your problem is definetly related to the inconsistency of your sight picture.

grahlaika
05-20-2009, 10:07 PM
In an ideal situation, you can get your near zero with absolute certainty if you boresight it at 25 yards (boresight lasers are made for exactly that purpose), then go from there. If you look down the barrel, the bullet has nowhere but down to go from there.

ripcurlksm
05-20-2009, 11:03 PM
thanks everyone for your time... im off to angeles this weekend again (happily) -- thanks

eighteenninetytwo
05-21-2009, 7:43 AM
Yes.

Yes.

As your far zero gets farther away, your close zero also gets closer.

Using an AR-15 as an example, you can zero for 50/200 or 25/300.

Your problem is definetly related to the inconsistency of your sight picture.

I'm certain that the .223 has a more curved trajectory than the .308 so if that has a 25 /300 zero and a 50/200 zero - it would seem to bear out my thought that the 100yards is teh near zero. Does anyone know the figures for .308 in respect of the near and far with 100yards being the near zero?

ar15barrels
05-21-2009, 8:15 AM
I'm certain that the .223 has a more curved trajectory than the .308 so if that has a 25 /300 zero and a 50/200 zero - it would seem to bear out my thought that the 100yards is teh near zero. Does anyone know the figures for .308 in respect of the near and far with 100yards being the near zero?

I don't think you could get a near zero to 100yds with a 308 and iron sights.
If you did, it would just touch the line of sight and then fall away again.
It would not cross over twice.

rksimple
05-21-2009, 8:43 AM
I'm certain that the .223 has a more curved trajectory than the .308 ?

Not within 300 yards, given the typical bullets and velocities that are commonly encountered.

ripcurlksm
05-21-2009, 9:11 AM
I don't think you could get a near zero to 100yds with a 308 and iron sights.
If you did, it would just touch the line of sight and then fall away again.
It would not cross over twice.

Here is the trajectory for a 100 yard zero with .308 federal 150 grain soft points, for example

http://www.calguns.net/calgunforum/attachment.php?attachmentid=23765&stc=1&d=1242925838

ar15barrels
05-21-2009, 10:01 AM
Here is the trajectory for a 100 yard zero with .308 federal 150 grain soft points, for example

http://www.calguns.net/calgunforum/attachment.php?attachmentid=23765&stc=1&d=1242925838

That shows a 1.5" sight height.
Iron sights are more like 3/4".

ripcurlksm
05-21-2009, 10:16 AM
right and so doesn't this chart generally illustrate what you said about the path just touching the line?

also, for using iron sights, we would subtract ~0.75" from the chart? So on the 100 yard zero chart the 200 yard zero would be 3.25" drop instead of a 4" drop?

ar15barrels
05-21-2009, 10:27 AM
right and so doesn't this chart generally illustrate what you said about the path just touching the line?

also, for using iron sights, we would subtract ~0.75" from the chart? So on the 100 yard zero chart the 200 yard zero would be 3.25" drop instead of a 4" drop?

You are thinking about this WAY too much.
When you change the sight height, you change the bullet's departure angle.
Drop is what the bullet actually falls from the angle of departure.
That does not matter because what you are really concerned with is where the bullet is in relation to the sight line.
Go measure your sight height and give me your bullet and velocity and I'll run you a chart in quicktarget when I get home tonight.

ripcurlksm
05-21-2009, 10:49 AM
that makes sense, so its just the line of sight that is shortened by ~0.75", and so the "0" on the chart would really be -0.75" instead of at 0?

Like this?
http://www.calguns.net/calgunforum/attachment.php?attachmentid=23771&stc=1&d=1242931742

I found this diagram with an exaggerated barrel angle
http://www.recguns.com/Ballistic/sightLine.gif

So I should measure from center of bore to tip of front sight?

eighteenninetytwo
05-21-2009, 2:12 PM
That green line can't be right, can it? because with a shorter sight height, to be zeroed at 100yards the angle of the bullet - would be less - in fact it should be 50% of the angle required for a 1.5 inch sight height should it not?

ripcurlksm
05-22-2009, 12:40 AM
yea its wrong. The green is the new line of sight, which means the angle needs to be raised to be brought back on the target?

Randall, if you get the chance to run those numbers:

Rifle: M1A Scout w/ 18" Barrel
Front Sight (from center bore): ~1.21" or 31mm
Rear Sight (from center bore at 200yd zero): ~1.34" or 34mm
Ammo: Federal 308 Win. 150grain Full Metal Jacket Boat-Tail (http://www.federalpremium.com/products/details/rifle.aspx?id=74)
Ammo Velocity (not taking into account im shooting from a 18" barrel): 2820 (muzzle), 2597(@100yds), 2385(@200yds), 2183(@300yds), 1990(@400yds), 1808(@500yds)

Another question Randall-- in the velocity loss between a 22" M1A and 18" M1A -- I have seen some folks post a velocity drop of 120-180fps for losing the 4" off the barrel. (1) do you know what the drop off is, and wouldn't it really be a percentage of total muzzle velocity based on the hotness of the ammo you are shooting instead of a number? (2) Does that velocity drop really just get applied to the muzzle velocity, and as the bullet travels it slows at an exponential rate relative to the muzzle velocity lost?

ar15barrels
05-22-2009, 6:47 AM
yea its wrong. The green is the new line of sight, which means the angle needs to be raised to be brought back on the target?

Randall, if you get the chance to run those numbers:

Rifle: M1A Scout w/ 18" Barrel
Front Sight (from center bore): ~1.21" or 31mm
Rear Sight (from center bore at 200yd zero): ~1.34" or 34mm
Ammo: Federal 308 Win. 150grain Full Metal Jacket Boat-Tail (http://www.federalpremium.com/products/details/rifle.aspx?id=74)
Ammo Velocity (not taking into account im shooting from a 18" barrel): 2820 (muzzle), 2597(@100yds), 2385(@200yds), 2183(@300yds), 1990(@400yds), 1808(@500yds)

Another question Randall-- in the velocity loss between a 22" M1A and 18" M1A -- I have seen some folks post a velocity drop of 120-180fps for losing the 4" off the barrel. (1) do you know what the drop off is, and wouldn't it really be a percentage of total muzzle velocity based on the hotness of the ammo you are shooting instead of a number? (2) Does that velocity drop really just get applied to the muzzle velocity, and as the bullet travels it slows at an exponential rate relative to the muzzle velocity lost?

I ran a quickload pass with 45gr of IMR4895 and got 2845fps from a 24" barrel, then reduced the barrel length to 18" for that load and came up with 2642fps.
I averaged the sight height.

Tabular trajectory data at Std.ICAO Atmosphere
-------------------------------------------------------------------------------------
Gun / Ammunition : .308 Win.
Bullet : .308, 150, Hornady FMJ-BT 3037
Bullet weight : 150 grains or 9.72 Grams
Muzzle velocity : 2642 fps
Crosswind speed : 10 Mph
Ballistic Coefficient(s) (G1):
C1=0.398@V>0 fps;


Optimum trajectory information :
Optimum sight-in range (X) = 149 Yds.
with max. ordinate above LOS at range (M)= 92 Yds.
and max. point blank range (P)= 171 Yds.
-------------------------------------------------------------------------------------
Sight-in clicks, 1 click = 1.0 cm/100 m or 0.394 in/100 m
Height of sight above bore axis = 3.251 cm or 1.28 inch
Gun is zeroed-in at 100 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
-------------------------------------------------------------------------------------
ˇYards fps s ft.lbs. in. in. MOA in. Clicks MOA yds ˇ
-------------------------------------------------------------------------------------
| 0 2642 0.0000 2325 -1.3 0.0 ----- 0.0 ------ ----- 0.00
M 75 2476 0.0876 2042 +0.2 0.4 0.56 1.5 -0.7 -0.24 0.96
X 100 2422 0.1180 1954 0.0 0.8 0.75 2.6 0.0 0.00 1.29
P 138 2341 0.1658 1826 -1.0 1.6 1.11 5.1 +2.0 +0.70 1.81
| 200 2213 0.2479 1631 -4.7 3.7 1.75 11.2 +6.5 +2.24 2.71
| 300 2014 0.3898 1351 -16.4 8.7 2.76 26.8 +15.2 +5.24 4.26
| 400 1826 0.5453 1111 -36.6 16.0 3.83 50.9 +25.5 +8.75 5.96
| 500 1652 0.7201 909 -67.9 26.8 5.12 86.1 +37.8 +12.98 7.88
| 600 1491 0.9104 741 -111.4 40.3 6.42 133.6 +51.6 +17.74 9.96
| 700 1348 1.1222 605 -171.1 57.6 7.86 197.2 +67.9 +23.36 12.27
| 800 1226 1.3556 500 -250.3 78.7 9.40 280.3 +87.0 +29.90 14.83
| 900 1128 1.6112 424 -353.6 103.7 11.00 387.5 +109.2 +37.53 17.62
| 1000 1054 1.8868 370 -484.6 132.2 12.63 522.4 +134.6 +46.29 20.63
-------------------------------------------------------------------------------------
M = Peak vs. L.O.S, X = Set Zero, P = Max. Point Blank Range
Elevation above Angle of Site (0.0 deg.) = 0.0619 deg.

Here's the short range trajectory:


-------------------------------------------------------------------------------------
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
-------------------------------------------------------------------------------------
ˇYards fps s ft.lbs. in. in. MOA in. Clicks MOA yds ˇ
-------------------------------------------------------------------------------------
| 0 2642 0.0000 2325 -1.3 0.0 ----- 0.0 ------ ----- 0.00
| 5 2631 0.0057 2305 -1.1 0.0 0.09 0.0 +60.6 +20.85 0.06
| 10 2620 0.0114 2285 -0.9 0.0 0.13 0.0 +25.4 +8.75 0.13
| 15 2608 0.0172 2266 -0.8 0.0 0.16 0.1 +13.9 +4.79 0.19
| 20 2597 0.0229 2247 -0.6 0.0 0.20 0.1 +8.4 +2.88 0.25
| 25 2586 0.0287 2227 -0.5 0.1 0.23 0.2 +5.2 +1.78 0.31
| 30 2575 0.0345 2208 -0.3 0.1 0.27 0.2 +3.2 +1.08 0.38
| 35 2564 0.0404 2189 -0.2 0.1 0.30 0.3 +1.8 +0.63 0.44
| 40 2553 0.0462 2170 -0.1 0.1 0.34 0.4 +0.9 +0.31 0.51
| 45 2542 0.0521 2152 0.0 0.2 0.37 0.5 +0.3 +0.10 0.57
| 50 2531 0.0580 2133 0.0 0.2 0.41 0.6 -0.1 -0.05 0.63
| 55 2520 0.0639 2115 +0.1 0.3 0.44 0.8 -0.4 -0.15 0.70
| 60 2509 0.0698 2096 +0.1 0.3 0.47 0.9 -0.6 -0.21 0.76
| 65 2498 0.0757 2078 +0.2 0.3 0.50 1.1 -0.7 -0.24 0.83
| 70 2487 0.0817 2060 +0.2 0.4 0.53 1.3 -0.7 -0.25 0.89
M 75 2476 0.0876 2042 +0.2 0.4 0.56 1.5 -0.7 -0.24 0.96
| 80 2465 0.0936 2024 +0.2 0.5 0.59 1.7 -0.6 -0.21 1.02
| 85 2455 0.0997 2006 +0.2 0.6 0.63 1.9 -0.5 -0.18 1.09
| 90 2444 0.1058 1989 +0.1 0.6 0.67 2.1 -0.4 -0.13 1.16
| 95 2433 0.1119 1971 +0.1 0.7 0.71 2.4 -0.2 -0.07 1.22
X 100 2422 0.1180 1954 0.0 0.8 0.75 2.6 0.0 0.00 1.29
-------------------------------------------------------------------------------------
M = Peak vs. L.O.S, X = Set Zero, P = Max. Point Blank Range
Elevation above Angle of Site (0.0 deg.) = 0.0619 deg.

ripcurlksm
05-22-2009, 9:32 AM
awesome info Randall, thanks! Ive been using a 200 yard zero for 308, may I ask what you zero at? 100? 200?

ar15barrels
05-22-2009, 10:00 AM
awesome info Randall, thanks! Ive been using a 200 yard zero for 308, may I ask what you zero at? 100? 200?

I always zero my 308 at 100.
My dope cards are built from there.

ripcurlksm
05-22-2009, 10:42 AM
have you ever seen the standard rear sights of an M1A? It doesn't have a mark for 100 and as you go higher in clicks, the numbers are spread out further

- 8
-
-
- 7
-
-
- 6
-
-
- 5
-
- 4
-
- 3
-
- 2
-


...
- 11


i guess I should just zero it at 100 yards, then get my 200 yard setting. calibrate my 200 yard setting to my 200 yard click count, then come down to my 100 yard click count, and mark the rear sight as a reference. WWRD?

stiff neck
05-22-2009, 4:07 PM
In an ideal situation, you can get your near zero with absolute certainty if you boresight it at 25 yards (boresight lasers are made for exactly that purpose), then go from there. If you look down the barrel, the bullet has nowhere but down to go from there.

Only in theory. Lasers will get you close, but rarely dead on. Bullets do not behave like lasers. One type of bullet/ammo might shoot high and wide at 100 yards, while another is dead on. You have to put holes in paper to confirm your zero for the ammo you're shooting. The different points of impact with different ammo may be minor at 100 yards, but significant at med/long range. Different bullets will have different points of impact. It might be minor at close range, but quite significant at med/long range.

grahlaika
05-22-2009, 4:41 PM
Only in theory. Lasers will get you close, but rarely dead on. ... Different bullets will have different points of impact. It might be minor at close range, but quite significant at med/long range.

...which is why I wrote that you'd only use that at 25 yards... as a starting point only... Clearly you'd get your scope/sight adjusted to the paper based on where the laser beam hits, then start shooting. Not sure if you're reading more into what I wrote than is actually there.

aplinker
05-22-2009, 5:20 PM
Just a simple way to think about any trajectory chart...

Your 0yd distance is set by your sight height over your barrel. The bullet trajectory is then set by the velocity and ballistics of the round. As you adjust your sights you can think of this (from a simplistic view) as grabbing that curve and changing the angle up or down while the 0yd distance remains fixed. The arc shape remains the same, but the relative heights will change.

ripcurlksm
05-22-2009, 5:48 PM
thanks