Bullet stabilization issues? .300 Norma Mag

[FrenchPewPiou]

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:

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:

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?

 

[rustyinbend]

Wow ... something is very wrong here. Step-1 for me would be to shoot some match-grade factory ammo and see if it's still key-holing the target. That would at least rule in or out something in your reloading components or processes.

 

[FrenchPewPiou]

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 ?

 

[rustyinbend]

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 ?

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.).

 

[Long Range 338]

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 ?

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.

 

[theLBC]

Bullets Spinning Fast Enough? Use Twist Rate Stability Calculator « Daily Bulletin

Berger Twist-Rate Stability Calculator On the Berger Bullets website you'll find a handy Twist-Rate Stabil...

bulletin.accurateshooter.com

 

[918v]

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.).

My guess is that a 10” twist will tumble 250gr bullets at 100 yards.

 

[spife7980]

 

[918v]

OP,

Stick to 215-220gr bullets in that barrel. They will shoot great in all conditions.