Explanations Welcome!

Did you go back to some of the benchmark loads that shot well in the PRIV brass...and repeat the greatness?  If not, you should.  Make sure nothing else has happened to the scope, rings, mounts or other variables in the rifle set up.  Just a thought....as a layman, that's what I would do.

That being the case it could be that your cases need to be annealed if you stick with PRVI brass. As it work hardens you are going to have issues with fliers. I've seen it with brass I've tried to use too long.

Danno,

The brass, all of it in many makes, was once fired factory ammo.  Got a buddy that's been building up brass bought over the counter.  The PRIV stuff is one tough animal and I think my back teeth loosened after finally getting the handle down on the press and then breaking her free on the way up.  I ALMOST decided to stop sizing these but half his brass is this stuff.

I just finished a 1,500 rd ..38 spcl project for him.  Now it's .243 and .308.  We have a barter system.  I reload anything he wants...he buys the components...and he gives me cases and cases of canned goods that him and his wife make.  Tomato sauce, beans, peas.  It's a great deal for me....and for him.  

I have not made much rifle ammo for him yet.  Have 100's of sized, trimmed and primed cases ready to go for each.  I have been loading 5 rd batches using components I have on hand.  He will shoot them and we will tweak if needed.  He is a short range (<200 yd) hunter so, extreme precision/tuning is not needed.  Once we settle on something good for each gun, we'll load everything until there are no more cases.  He'll be set for a long time and I will have food in the pantry.

Not sure what your problem is but a couple comments on the brass you are using.

1) I actually am somewhat fond of Privi brass. I think it's a sleeper brand. It reminds me of PMC brass from years past, I really liked that stuff.

2) Lapua brass prior to firing was tougher than woodpecker lips when I ran the necks through a die and tried to seat bullets. Perhaps it is the lot I have? First time I ever bought Lapua brass was last year for a 22-250AI. Trying to initially neck size and seat bullets was some of the hardest working brass I have used. I was actually ringing the bullets seating them. After firing it seems much, much, much better.

No help in your problem but an observation. May be neck tension changes? I doubt it would cause 2" groups or stringing though. I would think neck tension would just cause the groups to open or close up a wee, not string or blow up to 2"

Everyone does things a little different. When I headspace, I set headspace to new brass that I intend on using in that particular gun after checking the brass dimensionally. I set it up tight, with slight bolt resistance when locking into battery. From that point forward, I neck size until a full resize is needed due to a dramatic change in loading resistance. After the load is tuned to my satisfaction, everything stays the same. If brass is changed, the load must be retuned due to possible volumetric differences.

Neck tension is a big deal IMO. I devised a method to measure neck tension. I size the case and insert the projectile to normal seating depth, Install an RCBS collet type bullet puller, Attach a digital fish scale to the handle of my press and pull the press handle by pulling on the fish scale. The fish scale can be set to display peak pressure so that it reads the peak force required to pull the bullet. This measures the force needed to extract the projectile from the case, which is the same motion that is performed when the projectile is discharged. This needs to be performed on samples of the brass when new and prepped as well as after sizing. I have found neck sizing dies to produce less neck tension than full form dies.