An “aha” moment, or no?

igotta40 said:

The target is sitting too high in relation to the bench I’m sitting at and I’m pointing my muzzle up.

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This ain’t it.

The real deal: when you boresight your rifle like that at 25 yards, you’re adding WAY too much elevation to the scope. If your bore centerline and your optic centerline were extended out straight, they’d make an X. Let’s say your optic height is 2”. Crossing at 25yrds, using common builder’s vernacular, that’s 2” of rise over 25yrds of run to bring the bore line to meet the optic line at 25yrds… so then extending that out to 100yrds, you’ll get the same 2” rise per 25 yards of run, so we’d have 6” of rise for those 75yrds of run past the intersection.

Installing optics can be much easier than that. Optically center the scope, dial down 2moa for the optic height and 1.5moa for the absolute gravitational drop compensation at 100yrds (fast bottleneck cartridges). You’ll be on paper at 100yrds.

Yes and no, because your scope slopes down a bit to correct for off bore parallax.
In practice your rifle always points straight ahead. It can’t do anything else. Everything else is relative to the bore.
Remember that the instant the bullet leaves the muzzle it starts falling at 32ft/second/second. Gravity makes it do this and there is nothing anybody but God can do about it.
Common practice for hunting firearms is to set the sights/optics so the bullet hits a target at half the intended maximum range high by half the drop at the intended range. This gives you the closest you can get to dead on hold at anything from point blank to maximum range. POI will be a little high to 3/4 range then a tiny bit low at max range.
As an example let’s say we are setting up a .22lr to shoot Thunderbolts out to 100 yards. Thunderbolts will drop about 6" in 100 yards. If we adjust the sights so it hits dead on at 25 yards, 3" high at 50 yards (having already dropped 3" from the bore axis) it should drop another 3" and hit almost dead on at 100 yards.
If at the same time we shot a laser down the bore it would show at 6" high on the 50 yard target and 12" high on the 100 yard target.
To be really precise one would need to factor in ballistic coefficient of the bullet to get speed at and time to various distances, but this will get you close enough for most hunting uses. The bullet will be traveling slower and falling faster so will drop further in the second half of the range.
Trying to figure this stuff out will twist your brain. Divide the numbers in the graphic below by two to fit this explanation. I was too lazy to edit it.
Adjust the high/low numbers for the ammo you will be shooting from an appropriate ballistics chart.

Great explanation.

Atavar said:

Thunderbolts will drop about 6" in 100 yards.

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Thunderbolts - a .143G1 leaving the station at 1255fps - have 13” of absolute gravitational drop to be compensated at 100yrds. Spitballing an average velocity of around 1125fps over the first hundred yards, decelerating from the 1255fps muzzle velocity rather quickly, we get (300ft/1125ft/sec=) .26 flight time to 100, and .26 sec at 32.17ft/sec^2 gravitational acceleration (32.17/2*.26^2=) 1.08ft = 13”. Applied Ballistics and StrelokPro confirm.

Dialing 13” compensation (17” for standard velocity/sub-Sonic ammo) + 2” optic height from optical center will put a 22LR on paper.

Remember my numbers started with a 50 yard zero. It’s sorta hard to dial in MOA with iron sights.
Most shooters find it easier to start from a known zero distance.

Correct posts above, just restating the same info in another way. The sights are above the barrel. That means the muzzle is always pointed upwards at a slight angle. The higher the sights above the barrel, the greater the angle. When the bullet leaves the bore it will always be traveling up at a slight angle. This is true with handguns too.

You have 2 zeros. The 1st zero is when the bullet crosses the line of sight going up. At some point the bullet will reach the apex and start falling back down, the 2nd zero is when the bullet crosses line of sight going down. Most guys zero handguns at the 1st zero when the bullet is still rising. Most rifles are zeroed at the point where the bullet is coming back down. With a 25 yard zero your 2nd zero will be somewhere around 300 yards.

I start at 50 yards and don't even try to get my rifles zeroed there. Just want it close enough to know I'll be on paper at 100. I like to zero at 100 yards. With all of my centerfire cartridges a 100 yard zero means my bullet will be no more than 1" high or low between the muzzle and 100 yards. And no more than 1' low between 100 yards and around 130 yards. Only about 2" low at 200. Even at 300 yards I'll be no more than 10" low which isn't hard to compensate for. Beyond 300 yards you need accurate range estimates and optics designed for the longer shots.

Some guys like to zero at 300 yards, but that can cause issues at closer ranges. As you have discovered you will be dead on at around 25 yards, but high enough at 100 or even 200 yards to easily shoot over a game animal if you don't hold low.

To me it is a lot easier to remember to hold high at 300, than to remember to hold low at 100. And since most shots will be under 300 yards I don't recommend a 300 yard zero. Unless of course you anticipate a 300 yard shot to be the closest shot you'll get.

Varminterror said:

This ain’t it.

The real deal: when you boresight your rifle like that at 25 yards, you’re adding WAY too much elevation to the scope. If your bore centerline and your optic centerline were extended out straight, they’d make an X. Let’s say your optic height is 2”. Crossing at 25yrds, using common builder’s vernacular, that’s 2” of rise over 25yrds of run to bring the bore line to meet the optic line at 25yrds… so then extending that out to 100yrds, you’ll get the same 2” rise per 25 yards of run, so we’d have 6” of rise for those 75yrds of run past the intersection.

Installing optics can be much easier than that. Optically center the scope, dial down 2moa for the optic height and 1.5moa for the absolute gravitational drop compensation at 100yrds (fast bottleneck cartridges). You’ll be on paper at 100yrds.

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Perfectly explained

Varminterror nailed it.I usually bore sight at 100 yards and shoot at a fairly large sheet of paper with a dot in the center of it to get away from the 25 yard equations,but what he said is how it is.

Atavar said:

Remember my numbers started with a 50 yard zero. It’s sorta hard to dial in MOA with iron sights.
Most shooters find it easier to start from a known zero distance.

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I don’t need the tables, I have a trued ballistic solution for the Thunderbolts out to 450 yards. I’ve bought tens of thousands of them in bulk for years, and sent the majority of them out past 200 yards, because THAT is how I shoot my 22’s the most. Thunderbolts, Golden Bullets, CI SV’s & MiniMag SP’s and HP’s, these are data I don’t have to speculate about.

Zero distance doesn’t affect time of flight or absolute gravitational drop. Your 6” at 100yrds isn’t absolute drop, it’s drop below a 50yrd zero. Considering the zero, especially with 3” high over here and 3” under there, just complicates things - to hit our targets, we have to compensate for optic height offset and gravitational drop based on the time of flight.

Not sure why you’re fixated on Rimfire ammo, when it seems pretty clear the OP is shooting something with far less drop.

Atavar said:

from an appropriate ballistics chart.
View attachment 1086821

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This is such an excellent graphic.

The number of people who are convinced that bullets magically rise on leaving the barrel are legion (and may explain the "no recoil until the bullet leaves" myth, too)

Line-of-sight is relentlessly linear, but, we are using that linear thing to impose positions along an arc, whose curve depth is "invisible" to our apprehension. (This devolves into the complexity of 3rd & 4th order curve mathematics being obscure, and that spatial perception is not universal in humans.)

CapnMac said:

(and may explain the "no recoil until the bullet leaves" myth, too)

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I've had that argument several times and even made a slow motion video to prove it lol. Whats interesting is if you take a magnum revolver zero'd at 25 yards or whatever and put a wood down down the barrel, and a straight edge on the sights, you will find that the sights are pointing up. They have to be to compensate for how much the gun rotates while the bullet is in the bore.

While y'all are at it: If your 22 LR is shooting to Point of Aim at 100 yards here on Earth, how high will they be at 100 yards on the Moon?

(BTW, this is similar to an extra point problem I had on my 1st semester Calculus final in 1970.)

Dr T said:

While y'all are at it: If your 22 LR is shooting to Point of Aim at 100 yards here on Earth, how high will they be at 100 yards on the Moon?

(BTW, this is similar to an extra point problem I had on my 1st semester Calculus final in 1970.)

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Well the moon is 238,900 miles away so I'll say it will land 238,900 miles low.

Dr T said:

While y'all are at it: If your 22 LR is shooting to Point of Aim at 100 yards here on Earth, how high will they be at 100 yards on the Moon?

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I should have also stated that you should assume that the gun, the bench, and the target are located in a level spot in the Sea of Tranquility. Also, assume that the acceleration due to gravity is 1/6 that on earth.

It should not matter if it is rimfire or centerfire. The mathematics remains the same.

In my little postulated (and somewhat ridiculous) scenario, the elegant chart in Atavar's post is just almost enough. The first thing you need is how much higher than the point of aim the bore axis intersects the target plane. Since the moon has no atmosphere, we may assume that the velocity remains constant and equals the muzzle velocity (Also, cumbersome things like the ballistic coefficient don't matter). Once you determine the time it takes the bullet to fly one hundred yards, you can calculate how far the bullet will drop in that time given the acceleration due to gravity on the moon. The height of the intersection of the bore axis less this lunar bullet drop is how high the 22 LR will shoot on the moon.

The real question is: Why would I remember that Calculus final after 51.5 years of not worrying about it?

someguy2800 said:

I've had that argument several times and even made a slow motion video to prove it lol. Whats interesting is if you take a magnum revolver zero'd at 25 yards or whatever and put a wood down down the barrel, and a straight edge on the sights, you will find that the sights are pointing up. They have to be to compensate for how much the gun rotates while the bullet is in the bore.

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7:40 you can clearly see the tube recoiling before the bullet exits.

Walkalong said:

The OP makes no mention of caliber, so I assumed centerfire. Did I miss something?

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No idea why the 22LR diversion began.

It’s pretty clear from the OP’s description of the observation that the cartridge in question cannot be a 22LR; the trajectory described is only possible with a centerfire trajectory - a 22LR wouldn’t shoot flat enough.

The math problem was presented - “why is a 25yrd boresight alignment yielding 6-9” high at 100yrds instead of producing zero?” A 22LR would not produce that trajectory - not possible. Distracting with 3” high at 50 and 3” low at 100 isn’t pertinent. The explication of 25yrd vs. 100yrd intersecting LOS lines has been described. Dallying with 22LR, revolver sights, and moment of inertia is just dallying…

Varminterror said:

No idea why the 22LR diversion began.

It’s pretty clear from the OP’s description of the observation that the cartridge in question cannot be a 22LR; the trajectory described is only possible with a centerfire trajectory - a 22LR wouldn’t shoot flat enough.

The math problem was presented - “why is a 25yrd boresight alignment yielding 6-9” high at 100yrds instead of producing zero?” A 22LR would not produce that trajectory - not possible. Distracting with 3” high at 50 and 3” low at 100 isn’t pertinent. The explication of 25yrd vs. 100yrd intersecting LOS lines has been described. Dallying with 22LR, revolver sights, and moment of inertia is just dallying…

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Still good conversation though....

adcoch1 said:

Still good conversation though....

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We could also have a good conversation about cam timing of a ‘64 and a half mustang or about the surrealist art of Salvador Dalí, but it won’t help the OP with the specific issue he was described. As much a fan of evolution of discussion as I am, the divergences here were just weird.

lysanderxiii said:

7:40 you can clearly see the tube recoiling before the bullet exits.

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As I always say, there is no delay in physics!