Horizontal stringing

[PigButtons]

After reading about the rifle on Bergara's site, probably not the stock with that chassis. I see this kind of stringing all the time with sporter rifles, even though I don't shoot big bore stuff. I would guess it is barrel harmonics; most believe it is only vertical but it is really more of a spiral because of the torque of the bullet being spun up by the twist of the lands. A tuner can help, but most often a little more or less charge will reduce erratic muzzle twitch, for lack of a better term. Think of it like a small shudder that is inconsistent rather than a consistent flex.

 

[daniel brothers]

Harmonics or barrel flex is EVERYTHING. Buying 6 boxes of factory ammo for testing or even reloading is all for the purpose of testing which ammo is the most stabilized within that particular barrel. Pressure or combustion is the test. The fluctuation of those pressures are what cause erratic impacts. No matter what the weather conditions or shooter's skills are..... those impacts will NOT be consistent if the pressures are not consistent.
If your vehicle tires are not first balance properly, then the weather or driver's skills mean nothing... it's going to shake wobble and bounce. After the tires or cartridge pressures are in balance.... then you will have a better driving or shooting experience.
It doesn't matter how beautiful a vehicle can look, if the tires are out of balance, then it makes for a horrible ride.
Getting consistent pressures has to be the first priority with every weapon's ability to shoot accurately and FORGIVING. A load with built in forgiveness is found in the center of the node... but that node cannot be found unless the pressures are stable. The more stable...the wider the node.
A good chronograph will help you find that node quicker than a lot of inconsistent bullet impacts. A good chrony should be everyone's #1 investment... it saves on components and barrel life.
Shooting without first having consistent chrony numbers is like working the vehicle's accelerator pedal up and down while driving. You don't do that while driving, so while settle for that while shooting.
You can shoot those bullets into the night sky as long as your looking for consistent chrony numbers. Get low numbers first by adjusting powder charges before you shoot a target. Get the tires balanced first before testing on the road. If the tires are still out of balance..then go back to the shop and work on it some more. Heck... you can shoot into the ground while looking for consistent numbers.

 

[Ned Ludd]

Shooting a .300 WM that only weighs in at about 10 lb off a bench with a bipod, I am actually surprised the groups are as tight as they are. That's a fair amount of recoil and gun handling challenges to be dealing with when you're trying to do load development. I'd suggest either trying your bipod/rear bag setup from the prone position, or using some type of heavier/more stable front rest, if you have to shoot off the bench.

 

[PigButtons]

Another thing that I learned here is that the rear bag can cause problems with POI as well. Too hard and the rifle recoils inconsistently, or rather jumps, too soft and it doesn't give consistent support. Each rifle has a bag flex that will produce good results. I have four rear bags, one with rice, one with air soft BBs, one that I bought pre-filled with I don't know what but it feels like sand, and one with #8 shot. I have a log book that along with case, powder, primer, and bullet, has the bag ( if the rifle is bag sensitive.)

Of course a consistent placement on both front and rear rest is a given, but just to be thorough.

 

[EOD Rob]

I recently bought a Bergara B14 HMR in 300WM and I am reloading for it. I am using Gunwerks virgin brass, 195 Hornady ELD Match, IMR 4955 powder, Winchester LRM primers and all loads are .020 off the lands l. I tried to do an OCW for it off a bench with a bipod and a rear bag and for some reason the last two charge weights that I shot had horizontal stringing.
Everything was still torqued tight so I don't think that anything was moving, so would it be a load issue or could it be something to do with the nut behind the butt?

Thumper,
I have had a similar issue in the past. In my situation, the fix ended up being the primers I was using. For some reason the rifle just didnt like them. Not saying that this is your issue, but if you try all the other advice and nothing fixes the issue, maybe give a different primer a try.

Rob