Howa 300WM sudden loss of accuracy

[devinhal]

Hello All,
I have had a Howa 1500 chambered in 300 Win Mag for about 5 years now and have been very impressed with the accuracy with both factory ammo and loads I've developed(.75" at 100 yards) considering the cost of the rifle. I haven't made any changes to the rifle and have been using it mainly as a hunting rifle. Recently I started working on a new load based on the components that I had access to. I also saw this as a good time to get a new lot of brass to last me awhile(Hornady). Since then, I have tried loads with 2 different powders(H1000, H4831SC), 2 different bullets(180 Accubond, 180 Sierra Gameking), and 3 different primers(CCI Large Rifle Magnum, CCI Large Rifle, Federal GM215M) and the groups have been less than impressive(2.5" or larger at 100 yards). I shot about 80 rounds chasing my tail with the new loads.

Just out of curriosity, I put 5 rounds of my previous load down range and had about a 2" group(Federal Brass, Federal Large Rifle Primer, 180 Barnes TTS, 69.4gr IMR4831). I figured the barrel just needed a cleaning and spent some time scrubbing it good. Shot 30 rounds to foul it back up, but got a large group when I tried my proven load again. The groups were good last year when I put the gun away(about .75" at 100 yards), and I can't believe the groups would open that much in such a short time. Has anyone else ran into a similar issue or have suggestions on what to check next?

Thanks for taking the time to read through this.

 

[shoot4fun]

Could be a number of things. Maybe something is loose? Have you gone thru and checked things like action screws. scope mount and rings?
It could also be that you have picked up a bad habit that you aren't noticing.
Lastly, the current groups tend to be large and round or maybe most together except with a flyer (or two)?

 

[Sieg]

How many rounds through the barrel?

Checked for throat wear/distance to lands?

 

[devinhal]

I have about 600 rounds through it total. I set the new loads 0.030 off the lands like my last load. I will check when I get home to see what the Ojive distance is between the two.

 

[762 ULTRAMAGA]

I have about 600 rounds through it total. I set the new loads 0.030 off the lands like my last load. I will check when I get home to see what the Ojive distance is between the two.

One more possibility is a carbon ring, I had the same thing happen recently and after working the area in front of the throat with Iosso bore paste it was back to .5 moa

 

[devinhal]

One more possibility is a carbon ring, I had the same thing happen recently and after working the area in front of the throat with Iosso bore paste it was back to .5 moa

Good to know. I'll look into that option. Thanks for the info.