@ 03:09 am (GMT)
David HartleyToday's load development yielded a very satifying outcome. The red bullseye is 40mm diameter. This thread hasn't generated a lot of interest but it's maybe worth sharing the outcome as several suggestions from elsewhere and here said it was the bullet weight and 1 in 10" twist rate, a common concern with .243. But I've shot 100gr Sierras for years in a .243 T3 with the same twist and had ok groups.
On another forum someone suggested load density (fill ratio) as a starting point, so I switched to a slower burning powder (ADI AR2209) and upped the weight.
Load: 40gr ADI AR2209, C.O.A.L. 2.645", Sierra Gameking 100gr BT Spitzer. This C.O.A.L. is a soft touch on the lands, no neck crimp.
I was pissed off to the max with shot #5, which I pulled left with poor exhale / squeeze timing.
Here's an interesting photo of what happens with slightly different values for the powder weight and the C.O.A.L.:
Load: 39gr ADI AR2209, C.O.A.L. 2.625", Sierra Gameking 100gr BT Spitzer. This C.O.A.L. is 0.020" back from the lands.
I started at 38gr and was a bit over it after the 39.5 gr group, thinking it was just gonna be a case of poor stabilisation after all. But then at 40gr, suddenly the group tightened up, repeated it after the barrel cooled, and job done. So there's a sweet spot for sure, and that's where I'll stick. For this rifle, it's a 90% fill ratio and just touching the lands. Anything else is a non-starter.