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Grouping at different ranges

 
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  #1  
Old 11-08-2005, 08:45 AM
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Join Date: Mar 2005
Location: Oklahoma
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Grouping at different ranges

Gentlemen, first let me say that I really enjoy reading this forum. I have picked up several tips, ideas and a wealth of info. here. There has been an issue that has just itched my gray matter for a while. I have read several times (not necessarly on this site) statements like... "My gun won't group that well at 100 yds. but will shoot a 2" group at 300". Or conversly, it will shoot bugholes at 100 but won't group at 300, 400, etc. Could you guys enlighten me on how this can happen, the physics, the external factors, or other contributors? I just can't see how this could be. Thanks for your input in advance.
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  #2  
Old 11-08-2005, 09:28 AM
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Re: Grouping at different ranges

Okiehunter,

Well, there could be a book written about such things easily, pretty broad topic to cover all the whys and hows.

That said, lets simplify things.

In long range shooting, generally the bullets used are long, heavy for caliber bullets driven through fast twist barrels. As such, it takes a certain amount of time for the bullet to actually shed the effects of the rifling as it leaves the muzzle and fly true to its own center of cravity. The longer, the heavier, the faster the velocity and the higher the rotational velocity, the farther this distance is before the bullet "goes to sleep".

As such, it is not uncommon to see a long range rifle shoot in the 3/4" range at 100 yards only to shoot the same group size at 200 and at times 300 yards. This is why for long range shooting, if you are not testing your loads at 200 or 300 yards minimum, there is a good likelyhood that you are passing over some very good long range combinations.

On the other side of the coin, a load can cut very tight groups at close range but fall apart at longer ranges. This can be caused by several things. Could be that the bullet is only marginally stabilized. Could be that the bullet itself is not quality and the imperfections on the bullet or the load itself are magnified as the range increases.

This is generally seen more with conventional weight or light weight bullets. Again, its always best to test your loads at longer ranges to confirm if they are actually bad or good loads.

Kirby Allen(50)
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  #3  
Old 11-08-2005, 10:36 AM
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Join Date: Sep 2004
Location: on the rifle range in Utah
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Re: Grouping at different ranges

Fiftydriver hits the nail on the head exactly.

The longer the bullet, the longer it takes to stabilize properly. This is why 6mmppc benchrest shooters have tinkered with different shaped, flat based bullets with short ogives for the last 25 years. They have tried to find the best short shape combination to stabilize as soon as possible. The current bullets in use in that game are stabilizing 20-35 feet in front of the muzzle. This makes them go through the paper at 100 yards already being "asleep" for the majority of their intended flight.


A really cool way to check this on a smaller (slower) scale is to take a longbow and shoot long arrows and short arrows from it and video tape the result and play it back in slow motion. YOu will see the long arrow yaws almost like it is going to go off in a different direction at first but then after a few yards comes right back in line. THe short arrow yaws some still, but it is much less drastic and comes back in line much quicker too.
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  #4  
Old 11-09-2005, 08:53 PM
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Re: Grouping at different ranges

Parallax can cause enough optical distortion to make short range groups larger then further out.

Stringing is quite a common problem at extended distances. This is simply a load not being consistent for the barrel and bullet. At short range, the bullet has not had enough time to diverge so groups are quite small. However, at distance, the bullets fly further apart rapidly making for large groups.

Just a couple more thoughts to add to the list.
Jerry
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