Not a fluke.
What caliber. You shot in a condition that allowed the true accuracy of your 788 to be exposed on 43. Load had a small part to do with it, all the other powder loads would have shot like 43 if they had been shot in the 43 conditions. l looked at your other groups a powder load is not going to help those groups.you don't know how to read conditions, simple but true.
The BR guys, I am one, for the most part have never owned a 788, I have 2 of them. My 788 with Hart barrel and canjar trigger has shot .25 agg at 100 yd in competition, moving backers. The caliber is 6x47 remington. My other 788 is a .223.
Keep at it put out a couple wind flags and watch them, that will close up those other groups and make them look like more 43.Later.
Stephen Perry
Angeles BR