zog54, you did the best thing and sent the rifle back to the manufacturer for analysis.
You can describe your issues to us here until you're blue in the face and you will get a lot of supposition only.
Don't take that the wrong way. Some of those suppositions were educated and plausible. All were well meant.
In the end, without some really sharp pics, not taken with a cell phone, or better yet, hands/eyeballs on, we can't and shouldn't give you a definitive answer. Anything we tell you without all of the information could cause some real issues that could end up dangerous to you.
In relation to your issue, a friend of mine has a 340 bolt action Savage 30-30 with similar issues. It will only handle handloads at minimum specs, with 150 grain spire points. It will not shoot off the shelf ammo of any sort, safely. He only uses it for shooting pigs and steers in the head and the rifle hangs on a nail, through the trigger guard, in the barn. The magazine is long gone and he has a piece of wood that he carved and fit snugly into the well to drop a bullet on from the top so it will chamber easily. No, the rifle isn't rusty and the bore is shiny and bright, without fouling.
The thing is, a grain over minimum and smoke comes out of the bolt from blown primers and the bolt is very tough to lift and extract the fired case. Sometimes the neck will be cracked, right to the shoulder.
We did a chamber cast, after pulling the barrel. I love that Savage nut. That chamber must have been cut with a worn out reamer. It was slightly smaller than minimum spec but, would chamber CLEAN full length sized or new brass without issues. Oh, I forgot to mention, it has a firing pin that protrudes about .025 further than it should. This isn't likely to cause over pressures but it is likely to cause blown primers in the extreme.
First thing we did was to hone down the firing pin to median length. Reassembled the rifle and at least is stopped the blown primers. It didn't do anything for the other pressure signs.
The chamber cast showed that the chamber was true to the bore, so the very tight, under spec chamber had to be the issue. That would have made a great match chamber, under the right circumstances. With a rear locking rifle, of which I will admit limited experience, this may have been the cause of the high pressure problems. At this point, I should also mention this 340 had almost no leade. I didn't have another to compare it to. Maybe I could have gotten away with just doing a throating job on it? That requires a bit more time for set up and to be honest, this rifle just didn't need to be that good. It just needed to be able to accept factory loads or hand loads of a similar equivalent, although the action is very strong and capable of handling some very stout loads.
I have an old 30-30 reamer on hand and recut the chamber. I didn't cut it deeper, other than the throat. Just cleaned up what was there to median tolerances. It shoots well now, without pressure issues. Kills pigs and slaughter steers like a hot darn. Definitely accurate to 3 meters max. IMHO, the neck was so tight and the throat was so short that along with the over length firing pin piercing the primers the pressures were very much higher than they should have been under normal circumstances.
Is this the issue your rifle has??? I have no idea. It may be something similar or it may be something else altogether. I hope they settle the issue for you before hunting season. Factories or their support facilities can be painfully slow at best.
