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Posted: 3/20/2009 10:00:59 PM EDT
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Why do different bullets shoot left or right out of the same gun? I can see up or down, but not left or right. I tested two different bullets our of my 1x9 AR the other day and this is the result:Targets Click on a pic to make it bigger, then click on Original at the top to really blow it up. The two targets are at the bottom. I made no attempt to adjust the scope. I didn't even try my "plinker" loads that day as I was testing loads in another gun also. "Scatter" groups out of it but most were in the same area. Just some bigger than others. So what's the scoop on this? Makes it hard to figure out how to adjust the scope.
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Nice set up. Looks like you have your own private range. You might want to hang up some wind ribbons. A slight cross-wind could explain the difference. The heavier bullet may be less sensitive to the crosswind. The B.C. for the 69 gn is going to be higher than the 52 gn, so even though it leaves the bbl a little slower, the velocity will decay less.
You can run some parametric variations using an Excel spread sheet from Dr. Pejsa to see how your parameter variables affect the POI.[www.pejsa.com] |
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Yeah, I live on a 92 acre farm. Wind was not a factor that day. Very calm. If there is much wind at all, it blows my box targets over. Or the targets off the box. Just trying to figure out why different bullets shoot at different points of aim on this gun. My 7.62x39 AR shoots scatter groups, but they are generally around the same area. You can see that here:7.62x39 Not the best scope on that gun for target work, but at least I can adjust it to where all rounds hit in a general area.
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The barrel vibrates in 2 axes, forming a cone of deflection. Because the bullet's inertia also has a reaction in the barrel, the position of the muzzle at the time of the bullet's exit can be anywhere.
This is why uniform internal ballistics with respect to the barrel time are highly influential on accuracy. |
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Quoted:
The barrel vibrates in 2 axes, forming a cone of deflection. Because the bullet's inertia also has a reaction in the barrel, the position of the muzzle at the time of the bullet's exit can be anywhere. This is why uniform internal ballistics with respect to the barrel time are highly influential on accuracy. This sounds like baloney, but it's not. There is something different about the construction and distribution of mass between the two bullets that cause the barrel to respond slightly different so each bullet leaves the barrel when it's pointed in a slightly different direction. At 100 yards, just a fraction of a thousandth of an inch ay the muzzle is magnified into a large distance, easily seen. Don't think about the barrel vibrating only up and down parallel to gravity, think about the muzzle of the barrel describing an elliptical shape as it vibrates. Two bullets of the same weight that strike the target in different locations can probably be brought to the same point of aim by adjust the powder charge and maybe type for each bullet. |
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I also think the barrel resonates differently based on where it is supported so if you were to keep shifting the rifle on your rest the POI may slightly change.
Might not be as much of an issue since you have a FF but still something to think about. I like testing loads in the way I will be shooting it which is usualy from a bipod with or without a rear rest. Also you could have some wind closer to your shooting position which will have a big affect on your bullet but none at the target so the box wouldn't blow away. Based on your pics though I am not sure it is either of those issues. |
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As far as second order effects, yes, barrel whipping can come into play. You want your mode shapes to look like the third or fourth down in the diagram so the bullet exits parallel to the static barrel centerline. Also, as Keith_J points out, the vibration mode would probably be a swirl rather than a 2-D shape.
Another second order effect: If your rifling is clockwise from the shooter's view, then the gyroscopic precession as the bullet arcs down will steer it to the left. |
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Starting to think this may be a 69grn "problem" with this 1x9 AR. I shot some rounds today and brought out the 1x12 AR. You can see the results here: Targets The 1x12 prints in the same general area. The 1x9 prints the 69ers a lot further to the right. No attempt was made to adjust the scopes on either gun. Once again, the 1x9 gun printed tighter as the barrel heated up. First three shots are what spreads the groups out. Didn't make a difference in the 1x12 gun. I'll admit, I'm not the best shot, but still not sure what direction to go on sighting the 1x9 gun in.
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Are you using a carry handle scope? If so the high head position could be exaggerating the parallax in the scope. Are you loading up a mag at a time and shooting each group at all at once? That would explain why the groups print in different places, but still stay in a group.
ETA just looked at pics in first post. Answered my own question. Still think it could parallax in the scope. Also might be a heavier recoil of the 69gr load. If you don't have it now you might want to try filling the buttstock with weight to reduce the recoil effects. I shoot and coach Highpower and have never had a horizintal shift in impacts when switching bullet weights and that is after wearing out two barrels. I have used bullets weighing from 52 gr up to 80 gr and my no wind zero has been the same with all of the bullet weights. all of the barrels have been 1/8 or faster. I have seen that while working rifle sight in and it was usually from the shooter taking up a different head position, inconsistent shoulder pressure. Good luck with it Dave |
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Thanks for all the answers. Another question. Why does the 1x9 AR shoot tighter groups as the barrel heats up? I always thought the opposite was true. Barrel heat doesn't seem to affect the 1x12 gun. All barrels act differently, I don't believe it the twist of the barrel causing this. It's the molecular structure of the steel of the barrel making the difference 
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Quoted:
Thanks for all the answers. Another question. Why does the 1x9 AR shoot tighter groups as the barrel heats up? I always thought the opposite was true. Barrel heat doesn't seem to affect the 1x12 gun. We don't have enough information about your rifles to draw conclusions about details like this. If both rifles have free floated forearms, troubleshooting stringing during barrel heat up is difficult or impossible depending on the amount of money you'd want to spend. On the other hand, the first thing to suspect for non floated barrels is bearup against the stock, and this applies to AR's, TC Contenders, bolt guns in one piece stocks, or any configuration where the barrel bears against the stock. The point of bearup causes the response frequencies to shift (higher), whether stock is just in contact with zero or non-zero force. As the barrel heats up from shooting, the magnitude of that contact force changes, and whether the force increases or decreases is irrelevent as it causes the response frequency to shift, the shape of the barrel to change while it vibrates, and the place the bullet strikes to shift. This points up the desirability for floated barrels on rifles that will be shot in rapid succession. Rifles bedded with contact, such as factory Remington rifles, intended for hunting where one or two shots will be fired without significant heating of the barrel and chamber have always seemed to shoot fine, because they did in that application, but somewhere around the third to fifth shot, don't be surprised when bullets start stringing. A heavy pre-load at the contact helps mitigate this problem, and going into details about the subtleties will take a couple more paragraphs that I don't want to type right now. Free floating does not eliminate the need to know the difference in point of impact for the cold bore, clean bore shot, cold bore, fouled bore shot, or hot bore, fouled bore shot for a particular rifle. Whether cryogenic stress relief eliminates that issue I can't argue from first hand experience; I've seen all the andecdotal advertising about the benefit, and I'm still skeptical, partly because I want to know the rest of the history of the barrel, too. Rifling twist rate would be about the last place I would look at to understand whether it was the cause for the difference in point of impact between two rifles (except for precession, which is affected by wind more than gravity), except in one instance; if one barrel is very heavy, and the other an extremely light, minimum gage contour, the torsion "impact" load applied to the barrel by the bullet may be causing the dinky barrel to wind up in torsion, hence vibrating with a torsion mode, that superimposed on top of the conventional bending modes causes a real mess to sort through, espcecially if they couple. I would rate this as a very low order effect that is important in only extreme cases of light barrel contours, heavy bullets with long bearing length and big powder charges, and probably one or two other factors that don't jump out right now. The best approach to solving this "problem" is to find rifles and loads that work by trial and error experimentation tempered by existing experience about what works. A science project with instrumentation and reams of arcane analysis and insufficiently funded experiments might be fun and interesting, but in the end, both methods will yield pretty much the same rifle and accuracy. If it works, it works, and we know how to build accurate rifles and the limits of the possibilities. |
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The 1x9 AR has a 16", stainless bull barrel. Got that complete upper from Model 1. The 1x12 gun has a 20" heavy barrel from Quality Parts. Before they became Bushmaster. It has an upper from CMND and a M-16 style bolt in it. Both barrels have target crowns. Both barrels are free floated. Both uppers go on one lower. It is a Double Star lower with the RRA trigger in it. Standard A-2 stock and buffer assembly. I've tested different loads with these bullets and am getting near the max on them. On the next load, I'll back them back down as I didn't see an appreciable difference. I had only shot the 69gr bullets through the 1x9. Then I decided to try the lighter weight bullets through it. That's when I saw the shift in POI. I still think the "settling" in as the barrel heats up is me. The 1x12 gun has a much better scope on it. I think I'll just sight the 1x9 gun in with the 69gr bullets. Leave it at that. It's obvious the 1x12 gun doesn't like them. Use that one for the rest.
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What distance were those targets fired from?
How much difference in center of POI are you seeing? A 3 mph breeze (or a 3 mph change in the breeze), is just barely perceptible on your face but can move a high BC bullet 3" at just 200 yards. If the breeze "fishtails" without changing velocity, you can move six inches. |
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