Bullet stabilization issues? .300 Norma Mag

Hi,

After solving my "super weird issues" which were due to bad primers, I have a new problem with my .300 Norma Mag reloads... my groups are very bad (between 1.7 and 2.7 MOA at 109 yards/100m) and it seems that literally all bullets land sideways on the paper target:
487500748_9528591027220202_6024409072926055085_n.jpg


Here's my reloading recipe:
  • Neck-turned Lapua Brass, trim to length 2.482in, neck OD 0.3355in for a loaded round at OD 0.3375in
  • Hornady A-Tip 250 grs, tried bullet jumps from 0.020in to 0.065in
  • RWS LRM primers
  • Vihtavuori N570 powder (77.9 grains) -- it seems low compared to some guys here on the forum but I'm already above Vihtavuori official max which is at 76.1 grains

I shot twice 15 bullets (plus like 3-5 fouler shots at the beginning):
  • String one (15 shots):
    • Average: 2703 fps
    • Std dev: 9.8 fps
    • Extreme Spread: 34.8 fps
  • String two (15 shots):
    • Average: 2703 fps (same as string one)
    • Std dev: 8.2 fps
    • Extreme Spread: 30.2 fps
I'm shooting with a silencer, but concentricity looks good:
487842999_996522142450608_1349494398747456024_n.jpg


And my barrel is a 26" 1:10 twist.

While my velocities aren't the best, they aren't terrible either, they're pretty stable... I really don't understand what's causing this.

I tried factory 230 grainers from Norma ("match grade" ammo) and it gave better groups (all holes touching at 109 yards), although I couldn't go to the target to check how they landed.

In your experience, what could cause this?
 
I just noticed Hornady recommends a minimum twist of 1:8.5" for their 250 grains A-Tip... my barrel is a 1:10". Isn't that the issue :confused:?
My guess would be "No". If a 1-10 is tumbling at 100 yards, then a 1-8.5 will be tumbling at 100 yards. I think you've got something physically wrong with the rifle (canted chamber, defective crown, etc.).
 
I just noticed Hornady recommends a minimum twist of 1:8.5" for their 250 grains A-Tip... my barrel is a 1:10". Isn't that the issue :confused:?
Yes the twist is incorrect. Most guys are running at least 1:9 and often 1:8 twist. Depending on your density altitude and with enough velocity you might get away with it at high altitude but I assume you are near sea level. It sounds like the 230 grain bullet is as heavy as you are able to stabilize.