Slamfire;
I believe you’re seeing two different dynamics at play. Not necessarily a barrel twist issue.
The Fusion bullets are plated and a slight difference in plating thickness will cause the degeneration of accuracy you’re seeing. I’ve never been a fan of plated bullets. Coating either.
Took a set of .277 150 grain bullets out to CMP.
First started off at 200 yards with 150 gr Sierra flatbase at 200, then 300, than 600 yards.
Sierra's were stable at 300 yards
all over the place at 600 yards
Blew my mind, did not know what was going on.
Next test load, with Federal Fusions shot well at 300 yards
but looked like the Sierra's on the 600 yard target. I did not know what was going on, checked the scope and rings for looseness, same for the action, I figured the scope had broke. Did not take a picture because I was disgusted and thought there was nothing to be learned. But, still had a load that was a half grain more powder, and decided to shoot them up.
at 300 yards, good group
at 600 yards, good group
when I saw that, it came to me what I had seen with the Sierra's and the half grain lesser charge, was bullet tumbling. If I had a target further out than 600 yards, at some distance beyond those 150 grain Federal Fusions with a half grain more powder would have tumbled.
The 190gr Sierra’s are a different dynamic. Could be run-out causing a degeneration of stability, or, simply a bad batch of bullets due to imperfection in core seating in jacket.
I disagree. I also took 190 Hornady's out, and they shot well at 300 yards, and similiarly tumbled at 600 yards with the same loads.
I had talked to Sierra about the velocity needed to keep those 190's super sonic at 600 yards, and they told me to stay around 2515 fps to be around 1200 fps at 600 yards. Well, the bullets tumbled.
I did load up 190 SMK's in 30-06 cases, went to CMP Talledaga for a Bullseye Pistol Match, but the 600 yard range had an all day rifle match going and could not shoot my load at distance. I am curious to see if an extra 100 fps or so will keep the 190's stable out to 600 yards.
This phenomena of bullet tumbling is real, is not discussed in the popular press because the inprint guys shoot at most, 100 yards. Then they make claims about bullet stability all the way out to 1500 yards. I am going to state, unless you have actually shot your bullets at distance, you really don't know how they will act at distance.