The Redesigned BCM MK2 Upper Receiver

@Joe Goins Additional flex under sling or bipod load when compared to the old carry handle upper. Geissele marketing will tell you that their mounts return this stiffness to the upper. The consequences are POI shift at best, or additional locking lug wear and OOB at worst. If you can return some of the original stiffness to the upper without adding weight why wouldn’t you?

@Aono Can you objectively define how much "POI shift," "locking lug wear," and out-of-battery detonations increased since removal of the carrying handle? (I don't see how the upper receiver affects the internal wear-and-tear of the gun.)

@Joe Goins I can’t quantify it, but it’s structurally obvious and well known in terms of POI shift off of a bipod. Picture the upper as an upside down length of roofing gutter, and imagine how that would bend and the sides flex outward under load. Forward of the ejection port is where that happens. There’s a reason everyone has moved to nice long barrel nuts that help to prevent the handguard from flexing off of the nut, or moved to designs that attach the handguard straight to the upper instead. The thing is that the receiver on the other end of that nut is going to have the same problem to deal with and so is still flexier vertically without the carry handle bridge on it than it would be with that extra cross member that was originally designed into it.

Just imagine a front sight base gas block off the rifle. Try to bend it lengthwise and you can’t. Now shave off the sight tower and bend it again. It’s bendier, right? Obviously yes.

Imagine how locking lugs interface with the barrel extension if the barrel extension could gimbal freely inside a receiver channel made of say polymer. This would cause uneven wear, lockup, and with very high round counts can end up affecting headspace under load. Again I can’t quantify it, but there are plenty of anecdotes of say non free float uppers blowing apart at the barrel nut under sling load for exactly this reason. Monolithic uppers and things like integrated receivers all exist because of this. Whether the effect is enough to matter to you depends on what you personally are doing. But the fact is that if you remove material like a carry handle then the part you are left with isn’t going to be as structurally sound. This BCM rearranges the meat to ameliorate the areas that flex the most (roofing gutter). It’s an elegant fix considering that previously the solution was simply to make the whole thing meatier, like a Vltor MUR.