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I took a new to me rifle to the range this morning. (I have already been several times finding a load the gun likes.) It's a 308 that likes the Sierra 165 HPBT Gameking seated out as far as I can. 47 grains R15 is giving me 2770 with a low spread. I also shot another known load from a different rifle. The chrony is on.
Anyway, now I found a load it likes, today was the day to check drop. At one hundred yards, I'm two inches high. At two hundred yards I am right on. The computer says with this bullet, this temp., this altitude, and at this speed, I should be about 9 inches low at three hundred. I shot a good group at three, but it was 20 inches low instead of nine!
(I'm shooting pretty level, and at three hundred it wouldn't make that much difference anyway.) The group from my other rifle was right on drop wise.
I tried again. My second group was in my first group, still 20 inches low instead of 9. These were on a fresh target. I'm not confusing groups or aiming references.
Any ideas? If it was a scope problem, it wouldn't be grouping so well at 100 and 200, right? (Both rifles have 2.5-8 Leupolds.)
The scope on this rifle has less than two hundred rounds through it and it seems to track well. Could it be a paralax problem? That seems way too big of a change to attribute it to paralax to me. Plus, the group sizes are still good.
The barrel is floated. I'm trying to keep it in the bags with the same pressure (death grip is what this rifle prefers) each shot. Whenever I've tried a different grip, my groups get much bigger.
This rifle was really finicky and hard to find a load for. I finally found one, and it is grouping well, it's just 11 inches low! I didn't even try to go out farther. The wind was picking up and I was out of time.
I don't just want to live with 20 inches of drop with a two hundred yard zero. I want the rifle and load to line up with the Boone and Crockett reticule in that scope, so I need to figure out what is going on. (The scope was painted to match the rifle.) I shot with the center of the reticule to measure drop. For the load to line up with the scope reticule, I need it to be at the nine inches low it should be at at three hundred. (I'll be using the small traingle setting with this load.)
I've shot a lot at longer ranges and I've never seen this before. A couple inches off, sure, all the time. Eleven off the chart at 300, never.
I had planned on taking this rifle on a pack in bear hunt this weekend. I can take the other rifle so it isn't a big deal, I've just never seen a load be so far off before. Before this, the biggest difference I've seen was three inches from the chart at 300 yards.
I don't have a clue what might be causing this. Any help would be appreciated.
The never-ending flight
Of future days.
Paradise Lost. Book ii. Line 221
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I have NO idea what is going on there! I am pretty sure that the bullet is NOT actually dropping 20" below line of sight between 200 and 300 yards, however. 9" to 10" would be about right based on my experience with a .308 and the .300 Savage. With 150s at 2600fps in my .300 Savage and a similar sight in to yours (2.5" high at 100 yds), I get around 12" of drop at 300 yards. Something is not what it seems....I'll be interested in the answer when you do figure it out! It is challenges like this that make the hobby interesting.
Last edited by 300_savage; 04/26/10.
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Campfire 'Bwana
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Campfire 'Bwana
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damned if I can explain that.
Proudly representing oil companies, defense contractors, and firearms manufacturers since 1980. Because merchants of death need lawyers, too.
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Campfire Kahuna
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The only thing that immediately occured to me is that 20" would be about the drop below POA at 400 with that load and sight-in. But it was shot at 300....
Apparently the mysteries of the universe are uending.
“Montana seems to me to be what a small boy would think Texas is like from hearing Texans.”
John Steinbeck
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Campfire Kahuna
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Dang! Where is that boy when you really need him?
He's probably off hunting brown bear with his South Carolina .375. Or maybe competing on the Swedish bikini team.
“Montana seems to me to be what a small boy would think Texas is like from hearing Texans.”
John Steinbeck
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Campfire Ranger
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I think he hunts them with his bare hands...
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Campfire Tracker
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Dang! Where is that boy when you really need him?
He's probably off hunting brown bear with his South Carolina .375. Or maybe competing on the Swedish bikini team. Some times the imagination can be a horrible thing. Thanks, John.  Bear
Bear
Life is what happens to you as you are making other plans.
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I was thinking along with MD about 400 yards. That is extremely curious.
The view one sees is his own
Practitioner of the ancient art of skank fu
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Bullets got mixed up. You're shooting ones heavier than you think?
That's an interesting problem you have there.
Ella
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I have the Federal Ballistic chart on my computer. I looked it up, the .308 with 165 bullets at 2700 FPS. With a 200 yard zero, POI is -8.8 inches at 300 and -25.8 at 400 yards.
This is comparing two seperate trajectory charts, yours and Federals, and they are almost identiclal. Actually, all the 165-168 grains bullets are within an inch or two of your calculations.
I do not know the answer, either. Did you do the cronographing on the same day as the drop tests? Is the ammunition the exact same thing for both tests.
I think I would start by remeasuring the 300 yard distance, and also testing the loads again over the chonograph, and doing the drop test at the same time. Also, re-recheck your 100 and 200 yard zero.
I have used these trajectory charts for many different calibers, out to 500 meters, to figure drop, and they are always pretty close.
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Campfire 'Bwana
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I agree. I shoot a lot of 308 rounds at 300 yards and 9" drop relative to a 200 yard zero is mighty close for a variety of loads. The extra drop should be way too much to attribute to parallax as well. Unless the OP accidentally went to the 400 yard berm I don't have an explanation. 
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Seen Nosler Partitions shoot great at 100yds and groups fall apart at 200 and 300yds. I have shot that bullet in a number of 30/06s, 300WM and 300rums and did not have the same problem. According to the Sierra data you are about four grains above max for that bullet in a 308Win. with RL 15. Not saying that is the problem but I would try a different powder. If the problem continues, switch bullets.Rick.
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Am I understanding correctly that you did not turn the vertical adjustment on the scope between the time you sighted in at 200 yards and the time you shot at 300 yards (you just shot higher on the target and the impact was 20" below point of aim)?
I would re-zero the scope for 100 yards and then see what I got for drops at 200 and 300 yards without touching any adjustments on the scope. I know that checking the trajectory at 100 yards for a 200 yard zero should be doing the same thing, but if I can't figure something out, I change something for which the effect of the change should be well defined to see if that sheds any light on the problem.
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Dang! Where is that boy when you really need him?
He's probably off hunting brown bear with his South Carolina .375. Or maybe competing on the Swedish bikini team. As well he should be, after all he did invent it.
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Campfire Kahuna
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Is it really 300 yards? Might the BC you ran on the calculator be a...cough...fantasy? In the absence of anything else concrete I'd be suspicious of the advertised versus real BC. Marketing and truth are seldom found in the same bed.
I am..........disturbed.
Concerning the difference between man and the jackass: some observers hold that there isn't any. But this wrongs the jackass. -Twain
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Campfire Kahuna
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How high is your scope above bore center line?
I am..........disturbed.
Concerning the difference between man and the jackass: some observers hold that there isn't any. But this wrongs the jackass. -Twain
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Campfire 'Bwana
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I shoot that same bullet at lower speeds and don't get anywhere near the reported extra drop. It's not the BC of the bullet.
Last edited by mathman; 04/26/10.
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DigitalDan - That's one reason I suggested zeroing at 100 yards and seeing what the drop is at 200 and 300 yards. A quarter or even half inch difference in trajectory at 100 yards might be lost in the group size (even with a good three-shot group), but the difference will be more noticeable if you shoot at 200 and 300 with a 100-yard zero.
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Campfire Outfitter
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I took a new to me rifle to the range this morning. (I have already been several times finding a load the gun likes.) It's a 308 that likes the Sierra 165 HPBT Gameking seated out as far as I can. 47 grains R15 is giving me 2770 with a low spread. I also shot another known load from a different rifle. The chrony is on.
Anyway, now I found a load it likes, today was the day to check drop. At one hundred yards, I'm two inches high. At two hundred yards I am right on. The computer says with this bullet, this temp., this altitude, and at this speed, I should be about 9 inches low at three hundred. I shot a good group at three, but it was 20 inches low instead of nine!
(I'm shooting pretty level, and at three hundred it wouldn't make that much difference anyway.) The group from my other rifle was right on drop wise.
I tried again. My second group was in my first group, still 20 inches low instead of 9. These were on a fresh target. I'm not confusing groups or aiming references.
Any ideas? If it was a scope problem, it wouldn't be grouping so well at 100 and 200, right? (Both rifles have 2.5-8 Leupolds.)
The scope on this rifle has less than two hundred rounds through it and it seems to track well. Could it be a paralax problem? That seems way too big of a change to attribute it to paralax to me. Plus, the group sizes are still good.
The barrel is floated. I'm trying to keep it in the bags with the same pressure (death grip is what this rifle prefers) each shot. Whenever I've tried a different grip, my groups get much bigger.
This rifle was really finicky and hard to find a load for. I finally found one, and it is grouping well, it's just 11 inches low! I didn't even try to go out farther. The wind was picking up and I was out of time.
I don't just want to live with 20 inches of drop with a two hundred yard zero. I want the rifle and load to line up with the Boone and Crockett reticule in that scope, so I need to figure out what is going on. (The scope was painted to match the rifle.) I shot with the center of the reticule to measure drop. For the load to line up with the scope reticule, I need it to be at the nine inches low it should be at at three hundred. (I'll be using the small traingle setting with this load.)
I've shot a lot at longer ranges and I've never seen this before. A couple inches off, sure, all the time. Eleven off the chart at 300, never.
I had planned on taking this rifle on a pack in bear hunt this weekend. I can take the other rifle so it isn't a big deal, I've just never seen a load be so far off before. Before this, the biggest difference I've seen was three inches from the chart at 300 yards.
I don't have a clue what might be causing this. Any help would be appreciated. .............ID,,,,,,,That is highly damned peculiar and I can certainly understand your concern. If your chrony is correct, the bullet weights are correct (no mix ups) and the bullet BCs are correct for the bullets you are using, then you shouldn`t have an 11" difference in the bullet drop with a MV of 2770 fps. Something is going on here between the 200 and 300 yard mark. It seems as though, like your shooting into a down range wind shear or into a substantial headwind that you don`t feel, but that is nevertheless downrange from your bench??? Possible ogive defects with this particular lot of bullets you`re using, which in turn may cause excessive bullet drag between 200 and 300 yards? The first thing I`d do if it were me, would be to try another 165 gr bullet brand using the same identical charge, powder, COAL etc and etc and then see what happens. If the results are NOT the same, and you do get the normal 9" drop between 200 and 300 yards, then that would mean there was something wrong with the lot of Sierras you were using. Imo, the first thing you need to do is look for consistencies, or hopefully in this case an inconsistency by changing the brand of bullet while maintaining the same bullet weight and roughly the same BC. I`ll be very curious too see what you find out! But I`d start with a bullet change first.
28 Nosler,,,,300WSM,,,,338-378 Wby,,,,375 Ruger
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