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Posted: 9/21/2008 5:07:31 PM EDT
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I have a rifle that the FIRST shot every time is dead center through the "X". This is the "cold barrel" shot. The next 4-5 shots will print 5/8" or less... BUT, they will be 3"-4" low and about 2" left! Any ideas? Bedding issue? I have NO clue. This is an 8MM Mauser if that matters. |
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It is quite normal for a barrel to have a different point of impact from when it is cold and clean than when it is warm and fouled. It is an unusual barrel that does not have this point of impact shift. ETA: The primary reason for the shift is that the fouled barrel will have a different pressure curve due to frictional changes. Becasuse of the different pressure curve, the barrel will be vibrating at a different point when the bullet leaves the muzzle. The velocities may be the same, but the forces that start the barrel to vibrate are not the same. Thus the barrel vibrates differently. |
OK but... today I took 5 shots, 1st through the X, next 4 low to the left. Put the gun away, shot my AR & 1911 awhile. Got the 8mm out again, it's plenty cool... 45 minutes later. 1st shot (on same target) touching the 1st shot from last time ( sideways figure 8). Next shots off again low to the left. |
Try shooting some groups of 10 to 20 shots. At one of your range trips, shoot one shot and let it sit for 5 to 10 minutes. Then shoot a second shot, let sit, etc. Then see what your overall group looks like. Also, try shooting about 20 shots with ~1 minute between shots. This 1/minute shooting should represent a moderately warm barrel without it getting too hot, and the temperature of the barrel will be relatively consistent. You may find that where the rifle seems to be printing the first shot when cold, is not really an aberation at all. The rifle's real grouping potential should be revealed with the larger number of shots. |
This. Ask any bench rest shooter about fouling shots. Makes much more sense than cryo/stress relief. Every barrel will do it, some more than others. |
A 4" group at 500 yards is great. At 100 yards, it sucks. No, I don't work for Ruger. And my Ruger M77 is pretty damn good once the barrel is fouled. First shot out of the cold, clean barrel is about 2" high. The group is nice and centered, under 5/8" on good days. But every first shot out of a clean barrel is 2" high at 100 yards. The residue from the previous shot leaves the barrel with a thin layer of carbon "goo", this reduces friction until the fouling builds up to the point of reducing bore diameter where it increases bullet resistance. How long does that take? Usually over 200 rounds in that rifle. Even my Shilen barreled .308 does it. Somewhat to a lesser extent but it is there. Thankfully, with modern propellents and non-corrosive priming, cleaning the bore isn't needed after every shooting session. So why does a Mini 14 shoot so bad? Well, it is because James Sullivan just shrunk the M14 to "design" the Mini 14. In doing so, he didn't understand barrel stiffness DOES NOT SCALE IN A LINEAR MANNER. When the bullet passes over the port, it causes a severe barrel whip from the masses set into motion. This whip is several orders of magnitude greater than the normal barrel whip. Because the whip is a forced oscillation, any slight difference in bore time will detract from accuracy. Plug the port on a Mini 14 and prove this. I did many years ago. |
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POI shift of 4" at 100 yards is not acceptable and is a sign of bad metallurgy. Consider those ultralight Kimber rifles with their pencil thin barrels. No POI shift. Why? The Mini-14 is not a good analogy because it has all kinds of issues other than its thin barrel. |
Please explain HOW the metallurgy changes the point of impact. PLEASE. Show the MECHANICS, not just spouting words you don't know the definition of, hoping to baffle with BS. |
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Calm down. A barrel that has not been properly stress relieved will warp (bend in one direction) when subjected to heat (round being fired/ bullet going down the bore at a rapid pace), causing a POI shift. A barrel that has been properly stress relieved will not warp. |
The original poster is using an 8mm Mauser. Not the most accurate rifle in the world. Plus, likely being a war-time production the tolerances are not going to be that good either. A four minute of angle group for a military rifle of that vintage is not uncommon, nor to be bitched about either. Back in those days it was thought that the land diameter was more important than the groove diameter. As such, many rifles of that age will have land diameters that are consistent but the grooves will vary. Barrel makers now know that the groove diameter must be very close to land diameters for good accuracy. I for one would think that a 4-5 minute 20 shot group from a military 8mm Mauser would be acceptable. For a modern rifle that would be horible accuracy---but a challenge to find out why, and try to improve it without rebarreling. |
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I've had that before. Glass-bedding and free-floating have always cured it for me. Stress relieving is often done by barrel makers, and for hammer-forged barrels in particular. Some are done via heat, and other makers (Kreiger comes to mind) use Cryo treatments. It would only be my opinion, but I would think bedding and floating are more likely culprits in this case, and that floating the barrel would be the most likely fix (since the subsequent shots with the warm barrel grouped well.) |
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