With all the talk about a possible BCM Dissipator AR-15 upper, some discussion has centered on weight. A number of people have stated that the dissipator will be quite a bit heavier than other configurations. Without having one to weigh, I can’t say for sure. I can, however, break it down into individual components, and give a “best guess.”
First, let’s assume that we’re going to use the same barrel for both. There may be minute differences depending on the size of the gas block shelf at both locations (for the Dissipator), but it is impossible for me to know exactly how the upper will be machined. The prototype photos, posted by Stickman, show a government profile barrel turned down to .625 forward of the traditional gas block shelf. That would make it lighter than a comparable government profile upper, but I’m going to leave the barrel out of the equation for now.
So, the major difference between a standard 16″ midlength upper and a Dissipator is that the latter has a low profile gas block at the midlength location and a pinned front sight base at the rifle location. This allows the use of 12″ handguards while maintaining the full rifle sight radius delivered by a FSB attached to the barrel. Those who oppose the concept say that an upper with a 12″ rail such as the DD Lite and a rail mounted front sight would be lighter than the dissipator with standard handguards. This is based on the idea that the DD Lite is lighter than standard handguards. While this is true – barely – with the 7″ model, the longer versions are significantly heavier than their plastic counterparts. With mounting hardware, rifle length handguards are 10.8 ounces; the DD 12.0 Lite is (according to DD) 14 ounces.
“But the FSB!” you say. “That FSB is heavy!”
Yes, the FSB is approximately 5.2 ounces, and its removal would seem to make all the difference in the world. However, if one wants a front sight, one must be attached to the rail, and the lightest metal front sights available are around 1.5 ounces. A low profile gas block must also be used, but this weight is a wash between the two uppers, because the dissipator needs one, too.
So we have the addition of 3.2 ounces of rail and 1.5 ounces of front sight countered by the missing 5.2 ounces of FSB; that leaves a roughly .5oz “penalty” for the plastic handguard equipped Dissipator over the midlength with the 12″ rail. Again, this is assuming that the barrel weights are the same. The one pictured in the link above is likely to be slightly heavier than a BCM lightweight and slightly lighter than a BCM government profile barrel.
Now, some will point out that the standard handguards don’t come equipped with a sling mount. This is true – but it is also true with the DD Lite. Figure another ounce or so on the Lite for a QD socket and another ounce for the QD swivel. Although I’d find it to be a somewhat undesirable location for a two point sling, attaching a side swivel to the Dissipator at the FSB would add somewhat less than two ounces.
For those who want to play around with exact configurations, I’ve added (preliminary) dissipator gas block setups to the weight calculator.