exterior ballistics question

[dave gross]

I read with interest some of the ballistic info posted by Daryl on page 9 of this forum and am confused.  He shows that in a couple instances a shot was zeroed at 50 yards but was low at 25 and 75 yards. I can grasp how gravity would pull the ball down at 75 yards but how would it be low at 25 and then climb to zero at 50 yards. Doesn't gravity begin pulling the ball down as soon as it exits the barrel and if that is the case wouldn't the ball have to hit the 25 yard target at bit high, hit the bullseye at 50 and then hit lower and lower at further ranges.  Just asking.

Dave Gross

[Dphariss]

I read with interest some of the ballistic info posted by Daryl on page 9 of this forum and am confused.  He shows that in a couple instances a shot was zeroed at 50 yards but was low at 25 and 75 yards. I can grasp how gravity would pull the ball down at 75 yards but how would it be low at 25 and then climb to zero at 50 yards. Doesn't gravity begin pulling the ball down as soon as it exits the barrel and if that is the case wouldn't the ball have to hit the 25 yard target at bit high, hit the bullseye at 50 and then hit lower and lower at further ranges.  Just asking.

Dave Gross

Depends on the sight height and the velocity.
Sighted at 50 yards my 16 bore at 1650vfps would only be about .12" high at 25.
If you raise the height of the sights (these are about .75 above bore center) the ball will be lower at 25. With the sight 1" above the bore the ball is dead on at 25. But its 1" low at the muzzle.
At 2000 fps the ball will be low at 25 even with sights .75" above bore center.
At least thats what my ballistic program shows.

Dan

[dave gross]

I can see the logic of your posting.  I was picturing three targets at the distances referenced with their X-rings centered on a line.  Now for the rifle to be zeroed at 50 yards the muzzle must be elevated slightly above this line so how can the ball hit low at 25?  Lets suppose that the rifle is zeroed at 100 yards instead of 50.  Could the ball then be low at 25 yards and then climb to  zero at 100? I will set up some targets here with the x-rings at the same level and see what happens.  Now what did I do with that surveyor's transit?DaveG

[Larry Pletcher]

I think it's helpful to remember that the line of sight is straight while the path of the ball is a arc.  The path of the ball will cross the line of sight twice.  I used to shoot a .40 that  was zeroed at 50 yds.  I found that it also was on a 25.  Essentially it was climbing into the 10 ring at 25 yd and then falling back into the 10 ring at 50 yds.  Somewhere in between those yardages it was a above the line of sight.

Regards,
Pletch

[BrownBear]

The arc is the key.  The ball is starting below line of sight, while in fact the barrel is pointed up relative to the sight plain.  It travels up on its arced path, and in this example is still below line of sight as it passes 25 yards.  It doesn't hit line of sight till 50, but is back below by the time it reaches 75.  An alternate path with slower velocity might be for it to actually continue angling up above line of sight between 25 and fall back to line of sight at 50, continuing to arc down toward 75. 

In the second question regarding 100 yard zero with a low strike at 25, that easy to conceive if the ball is above line of sight between 25 and 100, and in fact coming down to line of sight at 100.  It would have to be going really fast or the sights be really high to stay below LOS all the way to 100.

In my experience though, the higher the sights, the more subject the rifle is to the detrimental effects of canting.  My hunting rifles have very low sights to help minimize that effect, due to the probability of very quick sighting on uneven terrain. 

[Daryl]

Yup.

[dave gross]

I now see how the conditions stated on the ballistics charts can exist.  I failed to take into account the heght of the sights above the bore center line and wrongly thought that the barrel axis was  parallel to the ground and centered on the target at 50 yards which I know not to be true.  I did not mean to imply that your info was faulty Daryl. I find you to be a valuable resource here.  My apologies if my comments were taken wrongly.

Dave G

[Roger Fisher]

The arc is the key.  The ball is starting below line of sight, while in fact the barrel is pointed up relative to the sight plain.  It travels up on its arced path, and in this example is still below line of sight as it passes 25 yards.  It doesn't hit line of sight till 50, but is back below by the time it reaches 75.  An alternate path with slower velocity might be for it to actually continue angling up above line of sight between 25 and fall back to line of sight at 50, continuing to arc down toward 75. 

In the second question regarding 100 yard zero with a low strike at 25, that easy to conceive if the ball is above line of sight between 25 and 100, and in fact coming down to line of sight at 100.  It would have to be going really fast or the sights be really high to stay below LOS all the way to 100.

In my experience though, the higher the sights, the more subject the rifle is to the detrimental effects of canting.  My hunting rifles have very low sights to help minimize that effect, due to the probability of very quick sighting on uneven terra

That iswhy the old ones have to strain their memory to remember to hold above that tack or tootpick or olive on a tootpick and such when shooting them at about 12 yds.

[Daryl]

Absolutely Roger.  How high depends on the rifle's zero and height of the sights. It varies with each rifle, of course.  Cutting horizontal strings has to be the worse target I can think of right now.

 Dave - no problem - I can understand your questioning the data. Normal question, and that's why we're all here.

[Dphariss]

I can see the logic of your posting.  I was picturing three targets at the distances referenced with their X-rings centered on a line.  Now for the rifle to be zeroed at 50 yards the muzzle must be elevated slightly above this line so how can the ball hit low at 25?  Lets suppose that the rifle is zeroed at 100 yards instead of 50.  Could the ball then be low at 25 yards and then climb to  zero at 100? I will set up some targets here with the x-rings at the same level and see what happens.  Now what did I do with that surveyor's transit?DaveG

You need to buy a book on ballistics or one that ILLUSTRATES bullet paths to understand this. I think Lyman's BP Handbook will help more than shooting excercises.

The ball starts out low. It must rise to be on with the sights the center of the bore is pointed above the target at the distance the rifle is zeroed for. The is called the line of departure (LOD). The higher the velocity the longer the projectile will be below LOS (line of sight) since higher velocity produces less drop going to the target the LOD is at a shallower angle. DEPENDING on how far out it is zeroed. Zero a round ball at 100-120 yards and it will likely cross the line of sight at 8-10 yards. Zero it for 25 yards and it may not come up to line of sight until 25 yards.
If you sight a 30-06 with a scope that ins 1.5" over the bore dead on at 25-30 yards it may not be on again until 200-275 it will be high to that point. But it will be low to the close range zero. At 10 yards it will be about 1" low since the sights are high.
A M-16 with its very high sights will be low to a considerable distance and is often sighted a couple inches low at 20-25 to give the best military trajectory. I would have to dig to find the exact numbers.
Kentucky rifles usually have very low sights. This coupled with 1600-2000 fps velocity will keep the ball very close to LOS to 75-100 yards. Thus they did not have to hold high at close range to make up for high sights as is the case with the M-16. It would be impossible to shoot a squirrel in the head with the M-16 or modern scoped rifle at 15-20 yards since the sights are so far from the bullet track. The shooter would have to hold well over the squirrels head to hit it.
With the low sighted longrifle you just put the sights in the middle of his head (depending again on the range the rifle is zeroed for, velocity etc etc).
Low velocity bullets need a higher LOD to get the bullet to the target at 50-100 yards so the bullet will cross LOS sooner.
This is not something that shooting through several targets is likely to solve since unless the paper is very thin it may cause the bullet to deflect.

Dan

[John A. Stein]

Daveg--Get your garden hose out and watch the stream. That stream of water obeys the same laws of physics that any free traveling missile does, baseball, bullet, howitzer shell or charge of shot. You can see what a load of shot does when you are swinging a shot gun on flying birds. John

[dave gross]

Yesterday I set out to prove to myself the phenomenon of the "climbing" projectile as we have discussed.  I set up a target at 50 yards and then using a surveyors transit level placed a sheet of paper at 25 yards with its top edge just at the center of the X-ring of the 50 yard target.  I then fired a shot at the 50 yard target using my rifle which is zeroed at that range and with as close to a center hold as these eyes will allow.  Guess what.  The ball went in the X-ring at 50 and just nicked the top edge of the paper at 25 yards. So it was ever so slightly low at that range.  Thanks to all for the education....and John I did try to do the garden hose thing....but the hose is froze up so I will try it in the Spring. 

Dave Gross