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Old March 5, 2013, 06:31 PM   #1
Fire_Moose
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.308 light loads and scope zero

Been dabbling in 308 loading and shooting. I came up with a load that shoots very well at 100 yards.

Right 4 were first then adjusted scope and got the left 3.


I moved the target out to 200 yards and fired the same rounds and got a ~3" drop and ~3" right group that was not nearly as tight.

I attributed this to a light charge, used because of LR magnum primers. No chronological to check velocity either.

So I come here asking...
how much would a standard .308 plinking round drop from 100-200 yards?

And

Even at longer distance the same round should group as they did at the shorter distance, no?

And

My.scope is now zeroed for this one round at 100 yards....what should I do about that? Is it OK?

Thanks in advance.
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Old March 5, 2013, 06:43 PM   #2
Bart B.
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When the range is doubled, shot group sizes will at least double. For each 100 yards of range past 100 yards, groups get about 10% to 15% bigger due to velocity spread, subtle invisible air movements and bullet BC spread.

5" to 6" drop at 200 yards when zeroed at 100 is about normal for .308's. Without knowing what a "light" load is, it's hard to estimate.

What you've done at 100 yards is pretty good.
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Old March 5, 2013, 08:36 PM   #3
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Re: .308 light loads and scope zero

For clarification. The load is

34.8gr 2460
168gr amax
2.775

I'm surprised there is so much drop at a mere 200 yards. Thanks Bart very helpful.
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Old March 6, 2013, 08:29 AM   #4
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If your estimate velocity is around 2300fps & your sighted in for 100yds. I'm figuring anywhere from 6" to 6.5" low at 200yds.
If you were to bump it up to 2500 to 2550 it would probably be 5" low at 200.
Nosler has an app to figure this out & I believe you can go on Hornady's web page has a section has something to.
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Old March 6, 2013, 02:24 PM   #5
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I cannot say to the type of powder you have used,for I use 4064 and Varget for my 308 loads.Now with that said I load a 175gn SMK with 42gn of Varget,and my load comes in at around 2544 fps.My drop chart shows a -4.66 drop at 200 with a 100 zero.So I would say that your load is a little faster than mine.I am guessing but you load sounds to me that its in the 2700 fps range.

the choice of 100 yards is OK for a zero,but most who hunt like a 200 yard zero.All this will help with is that it will allow you to just point and shoot from 100-200 yards,without having to do anything.Me I too like a 100 yard zero,for I have the target knobs to move up/down when needed.Or I just use Kentucky elevation out to 300 if I need to.( for deer hunting only ) BUT one must know to what the bullet will do at a given range or its a waste of time.

and as Bart B. said,if your rifle will hold a 1/4" group at 100 it will grow to a 1/2" group at 200 in general,but seen a few to stay a 1/4" a time or two out to 200.Different combos in bullet and powder choices will do different things,even changeing the OAL will make a bullet change the group size.
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Old March 6, 2013, 03:26 PM   #6
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Re: .308 light loads and scope zero

I can't imagine them at 2700fps. The 34.8gr is 3.2gr lower then starting in my Lyman book. The starting listed velocity is 2336fps...perhaps I measured out to the wrong holes in the cardboard. Ill have to recheck these at 200yrds.
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Old March 6, 2013, 04:12 PM   #7
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Fire_Moose,

Actually, groups normally grow somewhat more with each additional 100 yards. as Bart was explaining. This is because aerodynamic drag is constantly slowing the bullet down. That means it takes a longer time to traverse each successive 100 yards than it did the previous 100 yards. The in-flight influences that open groups up are thus given more time to act on the bullet over each successive 100 yards. This includes wind, gravity, and any drift components to the flight, such as may be introduced by muzzle crown or bullet base imperfections, rifle disturbance during the bullet's barrel time, etc.

I don't know what length your rifle barrel is, but just to make an example with your bullet, QuickLOAD suggests that with a 24" barrel you'd get somewhere in the vicinity of 2200 feet per second from that load. QuickTARGET Unlimited, an associated exterior ballistics program, shows flight time of 0.1421 seconds to the target at 100 yards and 0.2968 seconds to 200 yards. That means it took 0.1547 seconds to cover the second 100 yards, and that's 9% longer for drift effects and other influences to act over the second 100 yards than over the first, and the effects of both must be added to one another for the total effect at the 200 yard target.

If the velocity estimate is close to what you actually have, I get 7" of drop between the 100 yard target and a 200 yard target. An extra 8 mph of 9:00 wind would move you 3" right at 200 yards, but I think you'd have noticed and remarked on that. If your barrel has any contact with the stock and you shifted your hold or its position on the bags any, that could account for both the missing 4" of drop and the rightward shift. So could a loose scope base or ring or other scope problem. For example, a scope with parallax set for 100 yards (common) and not adjustable for 200 could help throw you off. All speculation on my part, but about as much as I can suggest from behind a desk.
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Old March 6, 2013, 04:14 PM   #8
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After I double checked Ill bet your not even at 2300fps.
Speer has a load of 40gr. of AA2460 & a 168gr. bullet at 2369fps. & it says to use a mag. primer. And a max. of 44gr. At 2692fps.
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Old March 6, 2013, 05:39 PM   #9
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Using the 80.75 ft/s/grain from that data gets us to just under 1950 fps for a 22" tube. Closer to 2000 fps from a 24" tube. So it's suggesting QuickLOAD missed by 200 fps. Always possible until you've verified a powder lot match. QuickLOAD's data is all based on testing just one lot of powder at some point in the past, and if the powder distributor's manufacturing source has changed over time, the model can be off. The lower velocity brings the expected drop closer to slightly over 9", though.
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Old March 6, 2013, 05:52 PM   #10
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I was taking a big guess to what his load was/is running.His choice of powder is one I have never used before and never will.I got my numbers from putting a few numbers into my BC app,and a 168gn bullet running 2700 +/- fps will drop in the 3" range at 200.All I did was that I used his info given to use.

So can you give us all your info on that one load,then I can punch the numbers into QL for ya.

what I need from ya.1) OAL from tip to base 2) powder charge and powder type 3) type of bullet 4) length of barrel
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Old March 6, 2013, 06:10 PM   #11
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…And you want his fired case water overflow capacity. I don't know what brass he's using. But my result is from QL and doesn't quite match the Speer data. The powder model may need a bit of a tweak.
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Old March 6, 2013, 08:43 PM   #12
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Re: .308 light loads and scope zero

That's for the Info fellas. I think I got what I was looking for. But if yer wanting to plug some numbers....

1. 2.775"
2. 34.8 gr AA2460
3. 168gr Amax
4. 24" barrel
Think it was all Win brass
Win LR magnum
Brass trimmed to 2.003

Only reason I got this powder is because that's all the shop had that I had data for. I now have 4895 and n140 aswell

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Old March 7, 2013, 11:15 AM   #13
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OK without your cases overflow in H2o,but useing the rest of your info,I came or QL came up with 2194 fps.So now after punching in the numbers on my BC app I get a -6.94" drop at 200 with a 100 yard zero. ( this too was at the altitude I shoot ) so your altitude will change the fps in a +/- by so may fps

Now this doesn't mean a thing,your rifle may shoot faster than QL is giving us.For some reason in my rilfe many times I may say,my rifle shoots right along with QL.So I roll with what QL has to say and many times its been very close to what my rifle shoots from one distance to another.

could you post the other target,better yet next time out with this same load shoot at 300.( with your rifle set for a 100 zero ) then post that target with a ruler showing the drop from the center of target to the center of your group. ( 5 or more shots ) thats if you can shoot out to 300,if not then 200 should do.
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Last edited by 5R milspec; March 7, 2013 at 11:51 AM.
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Old March 7, 2013, 05:50 PM   #14
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Re: .308 light loads and scope zero

Ya ill do that. Using a fresh piece of cardboard. Ill do 200 and 300 yards. It's good to know the drop anyways for my rifle
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Old March 7, 2013, 06:40 PM   #15
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In addition to what Unclenick said about groups opening up as range increases, there's another cause the not too many people realize. Ballistic coefficients across a box of bullets are not all exactly the same.

With small dimentional differences in jacket thickness and core location, all bullets are unbalanced to some tiny amount. The more unbalanced they are, the more they nutate their points around the trajectory path; they tend to take microscopic flight paths. That means the more they nutate, the more drag or lower ballistic coefficient (BC) they have. A 2 to 3 percent BC spread in hunting bullets is normal. A 1 to 2 percent spread in match bullets is normal.

Go run your favorite ballistics software for a given bullet at a given velocity and change it's BC by 2%. Compare the drop for each 100 yards all the way out to 1000 yards. That'll show you how much the groups open up for the velocity you chose. Then add the velocity spread your bullets leave at and the results may surprise you.
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Last edited by Bart B.; March 7, 2013 at 10:03 PM.
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Old March 7, 2013, 07:55 PM   #16
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Oh! Good one! I've never had good data on that. Do you have any feedback on how much the meplat uniformers and pointing dies correct this?
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Old March 7, 2013, 09:19 PM   #17
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Unclenick, meplat uniformers only clean up the outside of the hollow tip of bullets. They do nothing to the inside. As hollow points formed by a pointing die swaging down and shaping the ogive from a straight wall lead cored jacket, they are not uniform in their dimensions all the way around. Sierra Bullets' HPMK meplats look like the mouth of a volcano and there are no perfectly-shaped volcanos on this planet. They show the imperfect properties of the jacket material as it's swaged down to the shape of the front half of a football. Cleaning up the outside does nothing to the inside. And everything behind where the uniformer removes metal is still like it was before. The lead core and the rest of the bullet jacket still have their dimensional variables. So, the rest of the bullet is still unbalanced as it's center of mass ain't exactly on the center of form. Cleaning up the meplat may reduce bullet unbalance as well as change the airflow around it a small percent, but I think it's hard to verify. But some people swear (with nice words, not dirty ones) by them.

One could measure the bullet unbalance like Mid Tompkins did back in 1971. In company with a machinist at Sierra Bullets, they made a well balanced collet to fit in a Dremel Moto Tool with a cup shaped to hold Lapua D46 185-gr. FMJRB match bullets. They'd already sorted several boxes of them by shape on an optical comparator; found four distinct shapes, obviously from four different forming dies. Spinning those bullets at 30,000 rpm and measuring current to the Dremel's motor showed more current needed for more unbalanced bullets, less for the well balanced ones. More current was needed to keep the motor spinning when the centrifugal forces of unbalanced bullets put more drag on the motor's bearings. In actual firing, those bullets would spin near 6 times as fast. A few were so unbalanced they flew out of the collet and bounced off the ceiling and walls. But they shot several groups at 600 yards all under 1.5 inches; some well under an inch down to .7 inch. .7 inch is a bit over 1/10th MOA at 600 yards. Today's benchresters don't shoot their stuff that well. And Mid used his Hart-barreled pre-'64 Win. 70 action in a solid wood stock shooting WCC58 .308 Win. full length sized cases with about 42 grains of IMR4064 powder in a SAAMI spec chamber.

Sierra Bullets' reloading manuals have published time-of-flight data showing how unbalanced bullets effect drag. Some bullets entering the timing screens had different times through them but entered at the same velocity. Therefore the slower ones had more drag. Wasn't much, but a few microseconds makes a difference. They calculated BC varied 1 to 2 percent because of the amount of unbalance the bullets had.
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Last edited by Unclenick; October 16, 2013 at 08:36 AM. Reason: fixed a typo to make the post easier to search for
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