Alright, checking with Edto, the math checks out for the VDR-1; it's just a lot higher velocity, so ignore my previous statement.
For this article though, I would suggest changing the case dimensions, since its current form is deceptively the same as a .50 BMG. Personally, I would go with something that is around 120mm long and maybe 24mm wide at the base; that's roughly a 20% increase to the case dimensions. The projectile would still be 12.7mm in diameter, and the 24mm bit is an optional level of detail, so it'd be ok if you only had it as 12.7x120mm. With this, you can go with the previous DR ratings of Tier 4 for the standard, and 5 for the enhanced round.
Besides this, I suggest you change the bullet construction up however, with the standard round having a ferrium, tungsten, DU or nerimium core, since at these velocities, a normal copper jacketed lead round would just shatter before penetrating. Meanwhile, for the enhanced round, a discarding sabot with a projectile consisting of the previously mentioned stuff would be best.