Bullet stability question

[Bulseyetom]

I just bought a very old single shot rifle in 219 Zipper Improved. This rifle was built in the 50's as a benchrest rifle and has god knows how many rounds down the tube. I have been shooting it using the prior owners pet load that was developed for this rifle many years ago as well using IMR 4895 powder and 52 grain fb match bullets. I have been able to consistently shoot nice round 5 shot groups at 1 moa or less at 100 yards. Today my first group after several foulers was one large ragged hole of about 1/2 moa. I shot several more groups and then my last group I noticed I had 4 shots in about 3/4" with a fifth shot (not sure which number in the string it was) within 1/2" but the hole was almost a perfect side profile. When I looked at the first group of the day a little closer I see that one of the bullets was sideways but not readily noticeable as the hole was inside the group with just part of the nose outside the other 4 shots. What would cause maybe one bullet in 20 to tumble that many rounds apart? Now mind you this is a 60 year old barrel but the rifling looks pretty good and was not copper fouled bad at all when I cleaned it. The other part that is very puzzling is how close to the group the tumbling bullets hit. The bullets are jumping quite a bit to hit the rifling so the only thing I can guess is that the ones that tumbled maybe slipped in the rifling but again why only 2 shots out of 27 that I shot? I am only seated 1 caliber but I still have jump although I never measured it as I have no option in seating longer, just going to a longer bullet . I have a few old 60 gr Sierra I will try next time and see what they do. Any thoughts???????? Tom

 

[pacificman]

If it were my rifle I would want to determine the twist rate to see if the bullets are compatible. Sounds like you might be just on the edge of bullet and twist rate compatibility. The only other thing that I could think of would be to try different powder and/or primer Just my thoughts.

 

[Bulseyetom]

That load has been and still is a remarkable load 90% of the time. I am just not sure why a proven load for 40 years would go south with the rifling still in good shape. I think I better get the JB Bore Paste out and give it 30 minutes of my time. Tom

 

[Old Navy]

Try a lighter bullet just for fun. Later! Frank

 

[FeMan]

If it were my rifle I would want to determine the twist rate to see if the bullets are compatible. Sounds like you might be just on the edge of bullet and twist rate compatibility. The only other thing that I could think of would be to try different powder and/or primer Just my thoughts.

+1

Did you change bullets or shooting altitude? That could put you out of stability if you were close.