When the first carbine variants of the AR-15 were developed by Colt (the Model 607 initially, and later the XM-177E1 and XM-177E2), one of the complaints was that they were painfully loud. This should not be surprising, given the blast and concussion created by 10.5″ and 11.5″ 5.56mm barrels. Colt’s solution was to design a muzzle device that acted as a very limited-capacity suppressor, which reduced the blast and noise of a carbine to the same level as a regular (unsuppressed) M16A1 rifle. The actual sound reduction was only a few decibels, and they did the intended job.
It is worth noting that the other rationales sometimes attributed to these moderators (improving reliability and making the carbine sounds like an AK) may have been byproducts of their use, but were not part of the design intent. These were created to take the edge of the muzzle report of an AR carbine, and nothing more.
Today I have one of the Liemohn Manufacturing reproduction moderators. It is extremely well made, and does the job just as Colt originally intended. With SBR and suppressor tax stamps now reduced to $0, it is much more viable to make proper XM-177 clones, and these moderators are ideal for that.
Disclosure: The moderator and cutaway in this video were provided to me free of charge by Liemohn.
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