After a good bit of tinkering, I think I finally have the .257 AI just about dialed in. I initially started with a ladder test with 100 gr. Barnes TTSXs over AA4350. ESs and SDs were fine, but not much to write home about. I was somewhat disappointed with accuracy, though. Knowing that this is an unbedded, full-stocked rifle prone to heating up rapidly and using a 10x Swaro, I threw out 5-shot groups and went with the average of the two best 3-shot groups from each 5-round string (I know, I know, statistical insignificance, whatever). I also threw out one outlier from each 10-shot group when calculating average velocity and SD. All loads were with Nosler .257 Roberts +P brass fireformed to .257AI and CCI BR-2 primers. Insert your usual disclaimers about verify for your own purposes, no representations and warranties, please don't blow yourself up using my loads, etc. but load data was pulled from the Hodgdon/Accurate Reloading Center website.
| Charge- AA4350 | COAL | Avg. Velocity (9 shots) | Extreme Spread (9 shots) | SD (9 shots) | 3-shot Avg. |
| Load 1 | 2.885" | 2868fps | 90fps | 26.7 | 0.900" |
| Load 2 | 2.885" | 2928fps | 39fps | 15.6 | 1.165" |
| Load 3 | 2.885" | 2977fps | 67fps | 21.8 | 1.081" |
| Load 4 | 2.885" | 3028fps | 40fps | 12.2 | 1.313" |
| Load 5 | 2.885" | 3073fps | 33fps | 20.5 | 1.050" |
I then decided to perform a seating depth testing using the Load 5 charge, based on the lowest ES. Those results were more encouraging:
| Charge- AA4350 | COAL | Avg. Velocity (9 shots) | Extreme Spread (9 shots) | SD (9 shots) | 3-shot Avg. |
| Load 5 | 2.835" | 3049fps* (7 shots due to Labradar gremlins) | 104fps (wtf) | 14.5 | 0.793" |
| Load 5 | 2.810" | 3086fps | 54fps | 18.4 | 0.831" |
| Load 5 | 2.785" | 3042fps (8 shots/ aforementioned Labradar gremlins) | 40fps | 16.7 | 0.943" |
| Load 5 | 2.760" | 3046fps | 25fps | 8.7 | 0.649" |
| Load 5 | 2.740" | Cold killed the Labradar batteries at this point | Null | Null | 0.784" |
One of the 2.760" COAL 3-shot groups measured slightly larger than a gnat's ass at 0.266". That should do for now:
I'm not enamored with velocity, though. Knowing that I am working with a 21" barrel, I still thought I could consistently average 3100fps. Accurate's load data predicts 3293fps from a 24-inch barrel with Load 5. I have a pound of Hodgdon 4831sc on hand and may tinker with it, although I would expect it to give lower velocities based on it being a slower burning powder than the AA4350. Based on the advice of one of the forum's sages, I might also experiment with magnum large rifle primers. I also have a stash of Winchester and CCI standard LRPs. The temps were in the low 30s, so I wonder if that combined with the BR-2s could explain the SDs being a bit all over the place.
What say you, wise men of the interwebs? Another powder or primer you might suggest I experiment with? Or should I leave well enough alone with a 2/3 MOA group and a sub-10 SD?
