scope/distance/drop question

[MyRoad]

A pretty knowledgeable person assures me that if I zero in a rifle at 50 yards, since that's the peak of the arc, I'll be shooting HIGH at 100 yards. When I draw it out on paper, it seems the opposite would be true: If I've zero'd at 50 - a high point on the trajectory - then I'm forcing the barrel lower when I shoot, and at 100 yards after the arc begins to fall, I'd be shooting LOW.

Who's right?

Edit: as a side note, there are only two rifles that I own that I would sight in at 50 yards - an AK and my PC4. This is actually more relevant for the PC4 because I had an old scope lying around so I scoped it. I dialed it in at 50 yards, and then went out to 100. At 100 I was getting a 5-7" drop (depending on ammo). Since it's .40SW coming out of a 16" barrel, one would expect there to be some drop at 100 yards. But, if the gun is shooting high or low based on the 50 yard sight-in, then that may have impacted the actual drop amount.

 

[Iamsniper1]

If you are sighted in dead on @50yds you'll be shooting [email protected]out getting into a lot of high tech crap and a pissing contest with some others on this site heres what to do-Depending on the caliber, sight you weapon in @1and1/2"high@6oclock@100yds,you are now good to go beyond 200yds with no holdover,If your target is closer than 100yds just drop your sight/scope a few hairs.If you are shooting for the perfect zero then you'll be sighting in @dead on no matter what the range.heres a bit of advice-It only takes 9(nine) rounds to sight in a weapon-3 rounds on paper-3 rounds adjust sight/scope-3 rounds on target-Done.Look for patterns,NOT BULLSEYES.Anymore than 9 rounds-either the weapon or the shooter has a problem.

 

[USSR]

MyRoad,

In all likelyhood, your will be shooting high at 100 yards with a 50 yard zero. The POI at 100 yards with a 50 yard zero is a function of two factors: height of sights above bore and exterior ballistics. Look at it this way: If your have your scope mounted 1.5" above your bore and both the bore and scope are exactly perpendicular to each other, your POI at the 50 yard target will be 1.5" minus whatever the bullet drop over 50 yards is (very little). So, to get a 50 yard zero, you must adjust your scope down so that the POA meets the POI on the 50 yard target. You now essentially have your scope looking down and your barrel pointing up, with both converging at 50 yards. Past 50 yards, the bullet continues to rise until exterior ballistics takes it's toll on the bullet and it falls back through the POA. I have simplified this somewhat, but I hope you get the drift (no pun intended).

Don

 

[One of Many]

The exterior ballistics depends on the cartridge, and on the barrel length, as well as other factors. A .22 will have greatly different exterior ballistics from a .308 caliber.

As an example, my .308 with a 22 inch barrel, will have a point of impact (POI) just about at point of aim (POA) at 40 yards, about 1/2 inch high at 50 yards, and about 2-1/2 inch high at 100 yards. By 125 yards it has reached its peak elevation above line of sight and is dropping, reaching POA again between 200 and 220 yards. It drops to about 3 inch below POA around 250 yards. Considering a POI +/- 3 inch from POA (a 6 inch kill circle) with one sight setting, you can usually sight in a bit high at 100 yards and just hold dead on the target at usual hunting distances.

Sighting in at 25 or 50 yards is used to get on paper from an unknown sight setting - actual hunting adjustment should be done at 100 yards for medium range guns, and at 200 or 300 yards for a long range gun used on the open ranges where shots may occur at 300 to 500 yards. The farther away from the muzzle the impact occurs, the greater the drop, - and it isn't a straight line drop, it drops faster the farther the distance from the muzzle.

I suggest that you do a search for a ballistics program (I think Remington may have a free program) and see how much the drop changes with distance when sighted in at any given range.

 

[Legionnaire]

Simple way to think about it is that the bullet will travel in an arc. Only under a very precise trajectory will the bullet actually touch the line of sight once; that would require line of sight to be a true tangent to the arc of the bullet's flight.

Much more typically, the bullet will cross the line of sight twice: once on the way up, and once on the way down. For most rifle cartridges to be "zeroed" at 50 yards means that the bullet is still on the way up. It will "zero" again on the way down. Which is why one can calculate a short-range zero and know you'll be pretty close to zero at 100 yards. The specific short-range zero will, of course, depend on the cartridge.

So it is quite possible for a gun that is zeroed at 50 to be either high or low at 100 yards. Depends on how quickly the bullet reaches the apex of its arc and starts back down again. I suppose a heavy handgun cartridge could, in fact, drop below line of sight again at 100 yards if zeroed at 50. But that would be unlikely for a high velocity bottle-necked cartridge.

 

[rockstar.esq]

So my first instinct is to say that either the person mispoke or you misheard becuase 50 yds is not the peak of the arc for any cartridge I know of (excepting black poweders of some sorts). This topic is closesly related to maximum point blank range which depends on your acceptable shot stringing. On deer the target area is about 18" so 9" high or low would still be lethal. The max point blank range would be the distance out to which a center hold on your target gets a hit exactly 9" low. With a fast cartridge like a .223 Remington you'll find that the vertical difference between a 50yd and a 100yd shot is fairly small. This is testimony to the cartridges intended application. It's easier to make a hit using a flatter trajectory because range finding is less important. Should you wish to investigate exterior ballistics further you might be intrigued by the nuances associated with shooting uphill and downhill. A search of this forum will turn up some good information.

 

[MyRoad]

Very interesting replies - thanks all.

The PC4 is sort of a curiousity, like lots of folks I've wondered how handgun calibers do over distance coming from a carbine. If I have lots of time on my hands one day, maybe I'll play with that some more (like sight it in at 100 this time, and see how it effects the POI at 50).

As far as my .308 goes, I'll take this all more seriously, but I'll find out on paper exactly what I need to know. I will sight it in at 100, and then test it at 50 so that I can anticipate how the POI will change. This season I don't expect to have any shots over 100 yards.

 

[roscoe]

A lot would depend on how high your scope is relative to the bore and how fast the round is going. If your view is high above the bore and the round is fast, it should still be ascending at 100 yards, I would think. If you are looking through a low peep sight at a subsonic .22, the round would be dropping fast at 100 yards.