I tried a bunch of them and found I hate them.
1. Made the shotgun very uncomfortable when slung.
2. Added quite a bit of weight which slows down how fast you can swing the barrel from target to target.
3. Requires the use of the cherry picking method for loading one round at a time vs. belt mounted speed loaders where you can grab and stuff 2,3,4 shells at a time depending on skills and hand size.
4. Can limit other accessories. Such as if you’re stuck using their top rail replacement. I do have concerns about mounting pounds of accessories which are attached by five tiny top rail screws. Any impacts to the the mount is going to be transferred to those screws.
The only one I kind of liked was the Mesa one for the buttstock. It only works with the pistol grip and Standard stocks. It is a real PITA to install for the average user since it replace the sling plate. Having the weight at the rear of the shotgun was less of an issue.
If I had to have shells on the receiver, I’d use one of the Velcro type.