The chambering looks fine. Nothing I would worry about. Not saying you (OP), but people love to get their bore scopes out and nit pick every microscopic imperfection in the bore and chamber and blame shitty shooting on these things. 9 times out of 10 it's the shooter or ammo. I build rifles for a living and had the fortune of seeing some pretty grotesque chambers and rifling on rifles that come into my shop that still stacked bullets.
I just helped another SH member with a 7 PRC that wasn't getting the accuracy he expected. I went through the whole rifle, which was also built by another reputable smith in southern Idaho. What I found blew my mind and these are things you may be inclined to check if you have the time and tools. First, the crown was extremely rough. There was no sharp edge to it. Whether it came to him like this or something happened during cleaning, I have no idea but that is a red flag for accuracy. Second, I made the call to take the barrel off and re-crown it. When I did so, I was able to remove the barrel from the receiver with a little more than hand pressure... I probably could have loosened the barrel by hand had I tried that first. Literally zero torque. Now, in a lot of cases having a hand tight barrel works, in fact benchrest shooters do this and we only hand tight the barrels on the Terminus Zeus QC actions (albeit they are retained by set screws), but the standard in non-quick change repeater rifles is a barrel torqued on with anywhere from 70-110 or so ft lbs of torque. Between re cutting the crown and torquing the barrel to spec, this rifle went from 1.5-2" to .5" with the same ammo.
In a nutshell... systematically go through everything. After you shoot, a nice little test is to stand the rifle on the butt and loosen the rear action bolt. Index your off hand so it is touching the stock and action so you can feel any movement and loosen the front action bolt and see if the barreled action drops back in the stock to the rear. If that happens, bedding is in your future. Check all action bolts, I use 60 in lbs or torque on those and blue loc tite in some cases. Check scope rings to ensure they are tight up against the front of the picatinny slot and torqued properly. Check scope cap rings and torque to scope manufacturer's spec. Get a magnifying glass or jeweler's loupe and inspect the crown. Make sure you are judging accuracy with a fouled barrel. Barrels have different "personalities" and some like to be a bit dirty.
Ammo... ammo is huge. I've never had the best results with any Hornady bullet. To me they are just decent.... I'd try some Bergers, specifically the 245 EOL. You opted for a twist rate that suites the heaviest 30 caliber bullets so put it to use. Spinning the living daylights out of a lighter so so bullet won't do you any favors in the accuracy department. Run a bullet better suited to your fast twist and I'll bet you have better results. The 245 EOL is incredibly easy to load and tune, more so than the 245 LRHT in my experience, at the cost of only a slightly lower BC.
Shooting... I don't pretend to know your shooting experience, specifically with heavier recoiling rifles but In my 20+ years of doing this and training people, heavier recoiling rifles are harder to shoot small with unless you've done it a lot. Even experienced shooters get a little jumpy on the trigger after a few rounds in anticipation of the recoil. I have a good buddy who shoots a 6 BRA like a house on fire, you put my .25 MOA 300 NMI in his hand and he's a 1.5" shooter at best. Not saying that is you, but something to be aware of in the journey to sort this out.
Barrel.... I have a couple X Caliber barrels on personal rifles... nothing precision based (300 BLK / 375 Raptor). My experience with them is limited but they do appear to be good. Would that be my first choice for an all out, high-end precision build, as others have said, nope. Bartlein, Brux, Proof are where its at with a few other really good manufacturers out there like Ace, Benchmark, Mullerworks, and others. Now, if your barrel the culprit? Probably not. Your chambering appears concentric from the limited pics, and based on what I've seen the X-Caliber blanks are just fine... but then again, any manufacturer can put out a bad barrel from time to time. That would be the final thing I looked at.
Be systematic in your approach to tracking this down. If the rifle checks out, turn to ammo. Some rifles just don't like certain bullets and even powders. Like my good buddy says, don't try and fit a square peg in a round hole. Just move on and try something else.
I hope that helps, please feel to reach out if you need any help.
