Yesterday I did some shooting to sight in a new scope. I noticed an interesting event. I shot a few groups at 100 yrds. to get it sighted in and then moved back to 300 yards. The 100 yard groups were nothing spectacular. (1.5") When I moved back to 300 yrds. the groups didn't get any bigger. They stayed just under 1.5". According to my finguring, that translates back to about .5" groups at 100 yds. This is the second or third time I have noticed this with these bullets. They are 25 caliber Speer 100 grain SPBT. Does anyone have an explanation for this or have the same experience? Is it possible that I am just doing a better job of shooting at longer range? Thanks for your ideas. Oscar
In order to find out if thise is really true, or if you just shot better with the second group (or were luckier), place your first target at 100 yards, then place a second target 200 yards behind that one.
Then compare the groups...
Posts: 3282 | Location: Saint Marie, Montana | Registered: 22 May 2002
It is possible that your bullets are yawing after leaving the muzzle. The yaw damps out over time (i.e. distance) and the group size becomes smaller after.
I have seen this with long 6.5 mm bullets.
jim dodd
------------------ "if you are to busy to hunt, you are too busy."
Posts: 4166 | Location: San Diego, CA USA | Registered: 14 November 2001
Hunter Jim explained the phenomenon. High velocity center fire rounds will sometimes yaw or oscillate around an axis of flight until they stabilize. If they have not stabilized at 100 yds, they sometimes shoot the same group at 100 that they will at 300. I have a 270 that does this with 150gr bullets. I know that this sounds like hocum, but it is real. Ku-dude
you did'nt say what rifle twist or bullet weight your using. but I would think your bullets are not stabalizing enough at 100 yards (still got yaw on them)try a differant weight bullet for 100 yards and keep the other for 300 yards
Scott got it right. Bullets don't rotate off-axis that much. Eccentricity can allow this phenomenon, but they would rotate around an axis inside the bullet's diameter, not outside. Yaw due to eccentricity should never be larger than caliber. JMO, Dutch.
Posts: 4564 | Location: Idaho Falls, ID, USA | Registered: 21 September 2000
I have a 25-----shoots like yours-----I finally discovered it absolutely does NOT like boat tails-------I would suggest you try a flat base Speer # 1405 or Sierra #1620----------seat them 10 thou off lands-------the very best bullet for my 25 is a 115 gr Partition----BR guys have told me the following----for what I do I don't need a boattail----shooting under 500 yd-----a flatbase has more bearing surface in the case & in the barrel---therefore chances of being better aligned is much greater. Perhaps someone with more experience than I has a better explaination.
The yawing of the bullet may be slight but the effect on its flight can be significantly larger than the gyrations it makes. Imagine the shape of flight by an unstable bullet as compared with a stable one. The unstable bullet will fly with a sinusoidal or "porpoising" motion. The stable bullet will fly straight and true. When the oscillations of the unstable bullet damp out due to reduced velocity and air drag it will then fly straight and the group size may diminish.
If all the shots are taken with the same sight picture then parallax should not affect the group size. It may well affect where the bullet lands relative to your sight picture, but it doesn't distort the image or change the inherent accuracy of the rifle.
There is another possibility as well. It could be that the inherent accuracy of the rifle is much better than the 1.5 MOA at 100 Yards suggests and that almost all of the group size is due to human error. The inherent accuracy of the rifle is expressed in a dispersion from the muzzle and will magnify as a nearly linear function of range (subject to the aerodynamic effects already described), but human error sources may not be a linear function of range. You might be doing nearly as well at 300 yards as at 100 yards when focusing on a cross target for example, or a small spot. The error sources are combined (assuming no directional bias) using the root mean squares method, so if the rifle is capable of .25 MOA groups and you cause the rest of the error (lets say 1.5 MOA constant), then that is 1.52 inches total group size at 100 yards and only 1.68 inches at 300 yards. You see the point, being that a large constant error source can mask a small monotonically increasing error source if these are combined in the RSS method.
[This message has been edited by Harald (edited 02-05-2002).]
