You will often hear the dimensionality argument made in favor of the MUR, though the functional tolerances we're talking about here are really of more interest to a machinist or tooling fabricator than to a trigger puller. For most, the MUR is a beautifully-rendered solution to a problem which does not truly exist.
On the other hand, if you have a billet lower, and/or are looking to build something decidedly non-standard, there is no reason (price aside) not to consider the MUR. This is especially true if, for whatever reason, you prefer an upper with no forward assist, since the MUR offers that option where others generally do not.
I considered the MUR and went with Rainier Arms' new billet upper on my current 6.8 build, though we're still talking variations on a theme. In person, I do prefer the RA, but there are so few in circulation (at least, to this point), that the MUR remains what I would consider the "baseline" product.
Neither are necessary, but when tuning for optimal performance, who can really say if a thousandth here or there might not add up to a measureable difference at some point, after all. While the MUR is costly, it isn't prohibitive, so this is one mistake that you can probably afford to make, if for whatever reason, you're drawn to the idea of owning one.
AC
Stand your ground; don't fire unless fired upon, but if they mean to have a war, let it begin here. -- Captain John Parker, Lexington, 1775.
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