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Posted: 3/11/2010 9:03:13 AM EDT
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A buddy of mine is having a problem with his build. The first round is always low. Sometimes only about an inch and sometimes as much as four inches. Denny's recon (iirc) barrel, upper & bcg from Denny also, Laure rail & gas block.
What we've tried 1. Charging from closed bolt 2. Charging from open bolt 3. Different # of rounds in mag (to feed from other side) 4. Different mags 5. Different ammo 6. Different shooters 7. Different optics and mounts. 8. Different lowers To the naked eye there are no apparent burrs on the feed ramps. The rifle shoots pretty well (sub moa) other than the first shot. What the hell is going on? |
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Quoted:
A buddy of mine is having a problem with his build. The first round is always low. Sometimes only about an inch and sometimes as much as four inches. Denny's recon (iirc) barrel, upper & bcg from Denny also, Laure rail & gas block. What we've tried 1. Charging from closed bolt 2. Charging from open bolt 3. Different # of rounds in mag (to feed from other side) 4. Different mags 5. Different ammo 6. Different shooters 7. Different optics and mounts. 8. Different lowers To the naked eye there are no apparent burrs on the feed ramps. The rifle shoots pretty well (sub moa) other than the first shot. What the hell is going on? 1. Is it (A) the first shot after cleaning, or (B) the first shot after letting the barrel cool down? If A, its called a fouling shot. If B, its called a cold bore shot. Earthier you master the fouling/cold bore shot, or live with it. |
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Upper receiver in one hand, FSB in the other, now twist against each other.
If you get any rotation of the two away from each other stationary points, the barrel nut was not tightened correctly, and will need to be corrected. Also, if scoped with a cheap unit/mounts, could just be that the scope is not holding a zero as well. Regarding the front hand guard rail, if not free floating, could be that he is flexing the barrel on the first shot and causing the shift in impact as well. |
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Upper receiver in one hand, FSB in the other, now twist against each other. If you get any rotation of the two away from each other stationary points, the barrel nut was not tightened correctly, and will need to be corrected. Also, if scoped with a cheap unit/mounts, could just be that the scope is not holding a zero as well. Regarding the front hand guard rail, if not free floating, could be that he is flexing the barrel on the first shot and causing the shift in impact as well. no fsb, bbl nut was tightned to 65 ft/lb, laure rail. two different nikon scopes in a larue mount |
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Need a little more specifics on the first wild shot? Is this the first round out of the mag, and if pulls a mag about half empty, cycles the live round out of the chamber, then loads a fresh mag/fresh round, does the wild shot happen again? Yes, its the first round out of the mag. We havent tried the drill described above. Like I said above, we tried different mags, and different load sequences to load from different sides of the mag.....both fully loaded and with only 5 or 6. |
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Are you shooting at 50 yards, 100 yards, farther ? Did you notice any small dings on the barrel crown ? 100 yd, benched on bags. Crown looked good, Smith Vortex installed. I would think a bad crown would affect the whole group, not just the first round. |
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How many shots are in your group? Are you shooting at least 10 shot groups or only five shot groups?
A larger number of shots in a group will tell you whether there is really a first shot difference or if it is just part of the normal dispersion. BTW, a sub MOA 10 shot group is very, very good. The other thing you can do is to overlay the targets with the aiming point in the same place. I'll often use tracing paper to plot all the holes on multiple targets making sure that the targets are all lined up the same and that the center of the aiming point is also the same. If after doing all this and you still determine that there is indeed a difference, then one will have to just learn that the first shot is low and by how much on average. One other thing to test if it is indeed low on the first shot: Shoot a 10 shot first shot group by closing the bolt the same way. Then shoot another closing the bolt another i.e., slingshot and bolt release. This will tell you which method or the other produces larger groups or which is displaces the shot further from the main group. |
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I had a similar problem when I scoped my Colt Match Rifle years ago. I could fire a great group, then fire another group at a different point of impact a few inches from the first group at 100 yards. I discovered I had inconsistent stock weld and installing a high cheek piece corrected my stock weld.
Different holds on the rifle off sand bags will change the vertical point of impact. I noticed when I teach a class that rifles zeroed off sand bags shot lower when the sand bags were no longer used. It could just be that the barrel deflects upward when warmed up and you'll have to live with it. |
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