I neither have nor use Pistol Grip Only shotguns.
However, at one point in my checkered past, The State Of MD not only paid me to shoot them but made me teach others to do so, like 300 one year.
Since I had to do a demo for each group of rookies and fire to qualify myself, I shot hundreds of rounds from the hip that year. I observed hundreds of others do the same, with skill levels from none to prettydarngood.
Nobody shot as well without a stock. Nobody. People who aced the qualifier with a stocked shotgun had loads of trouble with the folded version.
It's not just me. No major instructor will teach a PGO seminar, none turn up at 3 gun matches and folks who actually go into harm's way have them only for breaching.
It's true that will time and effort a small amount of effectiveness can be reached using a PGO. However, the same amount of time and effort invested in a stocked shotgun will reap major amounts of expertise and effectiveness.
BTW, that Mossberg with a real stock will make a fine HD shotgun. Just add wear marks.
My closing argument, is this. Here and elsewhere for more than a decade I have challenged the PGO fans.
The challenge.....
We'll shoot the course of fire of THEIR choice.
We'll use the ammo of their choice, from birdshot to slugs.
We'll shoot at the distance they choose, from a foot to a football field.
I'll use a fairly stock 870. They can use ANY shotgun, ANY accessories and sights. Belt feed, lazers, no problem.
Score wins, time decides ties, though there will not be any. Heck it won't even be close.
Results posted rat cheer,and over on THR.
In over a decade, no PGO fan has taken up the gauntlet.
None. Talk's cheap......