funny trajectory or scope problems?

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jcooper9099

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I have a rem 700 in 308, varmint barrel 26 inches, 20moa base Nikon Buckmaster 4.5-14 x 40 on top 2 inches above the bore center.

I know my scope is high, but I had the same problem with a Nikon 3-9 x 40 that sat very low. I am shooting either 147 grain pmc bronze (bc .401)or 168 grain SMK BTHP (.477 bc). No measurement of muzzle velocity, I use the factory data plus 40fps. I shoot dead on at 100 yards when the target is on the same plane as I am, submoa groups with the best being one ragged hole less than .6 inches in diameter.

Then I go to shoot at 170. At this distance I get up on the hill a bit and the target is 170yds away but 28 feet in elevation below me. This is only a 3.16 degree downward slope. However the bullets hit 2+ inches high at this distance (either cartridge) and I have to aim low to get on target. Cant figure out why. Could it be possible that the bullet is still on it's way up at 100 yards?
 
With a 20 MOA base, the bullet is definitely not on its way up at 100 yards.

Unless you are going to be shooting great distances, a 20 MOA base is unnecessary.
 
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20 moa base

What should I put in my ballistic calculator to compensate for the 20moa base? I do plan to shoot out to 100 yards soon with this rifle and the buckmaster did not not have enough internal adjustment for me to get on target out that far. I would have had nothing left.

For whatever reason I can't find any trajectory charts which confirm the bullet is on the way up at 100 but it would seem that way to me just from looking a the the relationship of the scope to the bore.
 
I do not use a ballistic calculator. I have always kept a shooting log with a trajectory table out to 500 yards. You should really learn your rifle first before relying on a ballistic calculator.


And I'm guessing you meant out to *1000* yards
 
RE:

Yes definitely meant 1000 yards. was just having fun with the new setup at the max distance at the range before I take it out to 300+. There really is no substitute for actual DOPE, but I can't afford to shoot as much as I would like to to get said DOPE. :D
 
There really is no substitute for actual DOPE
Click to expand...

This is very true. But also understand, that learning to judge wind is something the ballistic calculator won't do for you. If there is zero wind, then it can do well, however, I have never shot at over 300 yards where wind was not a factor. Learning how to read wind is something that can only be learned through a lot of shooting practice.

And when shooting at 1,000 yards....expect to have to judge multiple wind speeds and their directions


Good luck with the ballistic calculator, and keep us updated with how it works for those longer shots! Sorry I can't be of any help in that category....I'm just a pencil and paper type of person when it comes to my shooting haha (at least for now ;))
 
I really haven't messed with wind much except with a .22. Plenty of DOPE on that rifle but with the ever varying velocity of the .22 and shooting whatever is on sale, I haven't learned much. for the .308 (1 in 12 twist) do you have a recommendation on bullets to reach way out there and cut through the wind?
 
If you have an IPhone, down load this app. IStrelok, it is free. It will give you all the data you will need for any range shot.

Jim
 
jcooper9099 said:
What should I put in my ballistic calculator to compensate for the 20moa base? I do plan to shoot out to 100 yards soon with this rifle and the buckmaster did not not have enough internal adjustment for me to get on target out that far. I would have had nothing left.

For whatever reason I can't find any trajectory charts which confirm the bullet is on the way up at 100 but it would seem that way to me just from looking a the the relationship of the scope to the bore.
Click to expand...
You don't account for the base in the calculator. The base only gives you more available elevation in the scope because you have to dial down to get it zeroed at your reference zero range. For example, if your scope has a total of 60MOA of adjustment with a 0MOA base, a perfect world would give you 30MOA available to dial for longer ranges. The 20MOA base allows the zero to fall in the lower portion of the scopes adjustment range and leaves you 50MOA to dial for longer ranges.

That is all a tapered base does. It isn't cosmic. It just gives you more effective elevation adjustment which allows you to shot longer distances without having to resort to holding over the target.

Any ballistics program will tell you where the high point on the trajectory is based on the zero range.
 
This is why as good range finder will compensate for angle. Angle must be accounted for in order to obtain the true ballistic range.
 
Sighted in at 100 yards, that should be about the high point of the trajectory. At 170, the numbers say it should hit low.

The 28 foot elevation change will account for less than 0.1" difference in drop at 170 yards.

Altitude, wind, barometric pressure, temperature will affect trajectory.

These things happen. Might be interesting to sight in at 170 and see where that puts you at 100.
 
I am disappointed that the base is not cosmic. Because the base points down I have to bring the muzzle up by a given angle to be on target. so the trajectory is not coming out of a barrel level with the ground. I would think that a ballistic calculator would account for launch angle.

just think about putting a rifle at 45 degrees. Your high point in the trajectory will be hundreds of feet in the sky. I agree about the slope angle of 3 degrees not meaning much downward. But the muzzle angle can mean a lot.
 
If you're going to hit a target at a specified distance, the barrel has to be pointed at the same specific angle, no matter where or how the sight is mounted.
 
The angle can't be the same for every shot. The differnce may be neglible but when you dial up elevation in on your scope the rectice goes down thus you point your muzzle up.

Anyway. I just tested the rifle and scope at the same distance on a flat shot. It shoots 3 and 1/4 inches low. I guess I am dealing with some kind of optical illusion at that distance or parralax not being set properly. IDK.
 
The bore and the scope are on 2 different planes. When the bullet leaves the barrel, it will be travelling upward, in order to intersect the plane of the scope at the range the scope is zeroed at.

How many MOA are built into your mount is irrelevant. If the height above the bore remains constant, a 100yd zero is a 100yd zero. The bullet travels upward the same distance regardless of mount angle. The only dimension that could throw off p.o.i. like that is height above the bore.
 
I think you're right. when I shoot flat it shoots where it should at 170. I do have adjustable parallax. Could that affect my ling of sight that much? The angle of the shot down to the target is not enough to affect it that much
 
With a 20 MOA base, the bullet is definitely still on its way up at 100 yards.
Click to expand...

I recently attended a 4 day precision rifle course in December. My .308 Savage runs a 20 MOA base under the scope. If I don't dial in more elevation, any shot taken past 100 yds (where I zeroed the rifle) has POI lower on the target the further out the target is. I have actual dope supporting this, for every 50 yds, out to 600 yds.

Please explain your comment. It isn't making sense to me.
 
20 moa base

I put one on my 25/06 a couple of years ago and it shot 34" high at 100yrd's. So went to a 600yrd range and at 500m it shot over the 600yrd. berm. Had to take 15moa out to get it sighted in at 500m and was still 9" high. Al
 
Then your zero before the base was 14" high. Assuming that nothing else changes and you remove a scope from a rifle and remount it perfectly on a 20MOA base, if you do nothing and it was zeroed when you removed it, it will shoot 20 MOA high at what ever range it was previously zeroed. Assuming that you have enough scope adjustment available, you simply dial out 20MOA of elevation to account for the base and from then on it is exactly same as any other base.
 
20moa base

What happened when I tried sighting it in after installing the 20moa base it was hitting 34'' high at 100yrds. with scope crosshairs adjusted all the way up. That's when I went to the 600yrd. range and it still shot over the berm. Then I installed Burris Signature rings with the adjustable inserts and removed 10moa. Then I was able to sight it in at 500m, 20moa was too much for my rifle. I could have saved money by buying the Burris Signature rings to begin with. Live and learn. Al
 
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