Just wondering how far off the lands are your bullets when they are fired?
Also what is the diameter of the loaded round at the neck before and after firing?
Too much "jump" before the bullet finds the rifling can do bad things to your dispersion.
For kicks duplicate same load and seat your bullets .020" further out and see what happens.
Case in point just last night I was talking to friend in Mississippi and he told me about a test he ran a number of years ago with a 1917 Enfield. He loaded up ammo to the stated OAL length in loading manuals and then he started increasing the OAL about .010" at a time till he got to the point the bullet was barely held in the case mouth.
He said it was amazing in that the stated OAL in manuals had groups his hand would not cover the pattern and they got progressively smaller as the OAL got longer and the last group was under an inch. Everything was the same except OAL.
He had no idea how many rounds were on the barrel and no borescope to look at things inside and no erosion gage so there are several unanswered questions but obviously the longer the OAL the closer he was to rifling and the smaller the groups got the closer he got.
You might try some Sierra 52 gr. BR bullets and see how things go.
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