Thought I'd share my experiences since apparently the 450 isn't immune to this malady. I recently switched out a 450BM barrel because of a rough chamber. I couldn't seem to dial in a load the new barrel liked. Factory ammo wasn't any better. Nothing better than 3 moa was a disappointment compared to the 1 moa my "bad" barrel had delivered. Checked all the usual suspects with nothing standing out.
I recalled a 6.5 Grendel build that I had to torque the barrel tighter than I'd liked to index the gas tube. It wouldn't group either. A friend with much more experience than I suggested re-torquing the barrel. I ditched the conventional barrel nut and used a clamp on handguard so I could use whatever torque I wanted, (I normally use 55 lbs) and suddenly it shot great, and still does !
I thought I had torqued the new Bushmaster barrel to 55 lbs, but I tore it down and re-torqued the barrel anyway. Magic ! Back to one ragged hole today. My friend told me he's done that with several rifles that started to lose consistency over time as well.
Anyway, it's a simple thing to try if a new build isn't up to snuff, or if a usually accurate rifle loses it's edge. I didn't touch the gas system or break the barrel loose from the receiver. Just loosened the nut & re-torqued it. Maybe it's the change in torque, or maybe it's just re-seating itself, I don't know. I just know it's made me much happier twice now !