| DarylS |
| (.700 member) |
| 07/10/07 02:16 AM |
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Re: Building a big bore
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Quote:Quote:
To have that flat trajectory with a large ball, is a wonderful thing. Having the ball within 1 1/2" of the Line of sight to 120yds. makes pin point accuracy a normal & easily aquired result. With the large bore, accuracy merely improves, the more powder used and the trajectory becomes flatter, rather than giving any expra elevation to the ball. The sights must be low, on the barrel for this longer range Point blank sight to work properly. High sights merely add elevation to the ball with more powder, destroying it's point blank range.
This piqued my interest, so I looked at point-blank ranges over different sight heights, using the Sierra "Infinity" software. I've compared this software to several others, and they agree very well.
Using one bullet and velocity, I checked maximum point-blank range for several different heights of sights above bore, in particular 1.0, 1.5, 2.0, and 3.0 inches. I used +/- 1.5 inches as the trajectory criterion for calculating point-blank range. The range at which a bullet will be neither more nor less than 1.5" from line of sight increases as height of sights above bore centerline increases. That is, tall sights improve point-blank range. The software ignores the short period from muzzle exit until the trajectory approaches the line of sight.
Take care, Tom
; Point blank range and how it's achieved, is interesting in that it isn't an exact science. By this, I mean ballistic tables don't tell the whole story. With high sights, as you increase the powder charge, you get a corresponding increase in elevation, which destroys the point blank range. The ball, instead of rising only 1 1/2" above the line of sight, increases to 3 or 4 inches as the 'regular powder charge, say 100gr. is increased to 175gr. (in large bores)
: Low sights allow the 'plinking', deer/black bear/boar charge of around 90 to 110gr. to hit within the limit of 1" above or below the line of sight, yet increasing the charge doesn't add to that elevation more than 1/2" of so but the increased velocity gives an extended distance to the point blank range. High sights change point of impact too radically with the increased powder charges for shooting heavy game.
: Of course, having 'express-type' sights gives some leeway. My own rifle, with the point blank sight zero'd at 50 yards with 100gr., then gives a zero at 100 yards with the 165gr. charge, while keeping the mid-range zero within point-blank status. With higher sights, the heavy charge zero would be on the order of 130 yards, but at closer ranges, where the point-blank sighting is most necessary, as in a charging bear, the ball would strike too high to be useful. Military rifles and rifle muskets shooting minnie balls show this 'failing' in spades. With them, to have a 100 yard zero, the ball/bullet must rise over 5", maybe 8" or more above the line of sight, thereby eliminating the point blank sighting. How do you hold 5" to 8" low of where you want the ball/bullet to hit when the bear is bent on running you over and ripping you up and closing at 35 mph? It is best to have sights low, so the ball is close to the line of sight at all ranges - hense the importance of point blank range. You need not know what the range is within normal distance, merely take a fair sight and let go. You know your ball is within lethal impact of the sight.
; With express sights, you can have, as I do, a sighting with deer/b.bear/plink9ing loads, of 50, 100 and 150 yards, while with the heavy big game charges, those very same sights give 100, 150 and 200 yard sighting.