DISCLAIMER: The following post is a general statement directed at no one in particular, intended for the purpose of clearing confusion/misconceptions in regards to short-barreled firearms, particularly that of the Mossberg Shockwave/Remington Tac-14.
I think the problem with the Shockwave/Tac-14 has less to do with the firearms themselves and more to do with general misconceptions in regards to the application of short-barreled firearms.
If you try to use a firearm with a short-barrel and a pistol grip the same way/under the same conditions as you would a full-length firearm with a shoulder stock, then obviously it's not going to work as well.
You'd think that goes without saying, but apparently it doesn't, especially when even trained professionals make the same mistake.
What's more, just because a short-barrel firearm with a pistol grip doesn't function/perform as well as a full-length firearm with a shoulder stock when used in a similar manner does not make it "impractical" or a "novelty" or a "gimmick" and most of all "useless" like certain folks often write them off as, it merely means that they have a different manual of operation.
Firearms such as the Shockwave and Tac-14 work best as close-range self-defense tools, specifically indoors. They have served Law Enforcement very well in that niche for several decades, but their specific manual of operation is practically unknown to civilians because up until very recently such firearms were rather costly and tedious to obtain.
The fact of the matter is, such firearms weren't designed nor intended for long-range use, but at extremely close indoor ranges that a S.W.A.T. team would find themselves in, hence the short barrels. The barrels are short for the sake of maneuverability in cramped quarters, to be fired at ranges in which aiming is unnecessary as it is impractical, yet still offer the same devastating power of a 12 Gauge. Which brings me to my next point...
Contrary to popular belief, short barrels do not adversely affect low pressure cartridges such as shotgun shells in terms of velocity to any severe degree. As a matter of fact, when it comes to the 12 Gauge, anything past 12" begets diminishing returns in terms of velocity gain, so much so that out of your standard 18" barrel the amount of velocity you gain over a 12" barrel is in the double-digits, but hey, don't take my word for it...
Energy by the Inch The Great Shotgun Chop of 2007