- Does Exactly What It Says On The Tin
Ten bucks for two quality hopup buckings that you can buy from Shorty's walk in store without having to pay shipping... You can't go wrong, really.
"Upgrading" your hopup bucking is rarely a worthwhile proposition unless the one that came in your gun is REALLY terrible. However, buckings do wear out with time and use, so sooner or later you'll want to replace a clapped-out one.
Or, you could be in the same boat as me and discover that some ignoramus at the factory apparently used petroleum based lubricant to install the factory hopup bucking on an Echo 1 SCAR, and be forced to destroy your bucking in the process of a barrel upgrade.
To the high-rated comment above about hopup buckings not contributing to double-feeding problems, yes they very well do. Unfortunately, most people have only a very dim concept of how an AEG mechanism works, so advice you're likely to find on the internet seems to be based on hazy mis-recollections of what other people said on some forum somewhere, voodoo superstition, and myth. A "problem with the tappet plate" will likely result in BB's not feeding at all, because the tappet plate is spring loaded and will return forward and stay there if disconnected from the sector gear, preventing any BB's from being loaded into the chamber unless it breaks in a VERY specific way (above the spring, or the spring is lost or removed). An air nozzle that is too short or broken can result in double-feeds, but this should be immediately obvious to anyone who looks at it.
Suffice to say, one of the leading causes of double-feeds and related problems is the hopup bucking or its nub not restraining the stack of BB's trying to push into the chamber when the air nozzle is retracted, allowing more than one in at a time. This is supposed to be accomplished by the lip of the hopup bucking and providing enough friction for just one BB in until the air nozzle/tappet plate comes forward and pushes it up to the hopup nub (essentially "chambering" a BB like a gun would chamber a cartridge). The Mad Bull Shark buckings have three square nubs around the lip to provide extra friction and retention, and would probably help with double-feeding issues that are not otherwise mechanical.
(Note that the tappet and air nozzle are always supposed to be forward when the gun is at rest, preventing any BB's from getting into the chamber. This part is UNLIKE a real gun, which has a round in the chamber at all times. When you fire, the tappet plate is pulled back and then pushed forward again before the piston comes forward to launch the BB. This is when double-feeding happens: Too many BB's get pushed in front of the air nozzle by the magazine spring before it comes back forward.)
HOWEVER, if you have hopup bucking related double-feeds it is most likely because your bucking is worn out or stretched out, or improperly installed. The extra retention provided by the Mad Bull buckings is "nice" but probably not actually NECESSARY versus a regular hopup bucking that's also new and not worn out. It may, however, help the Mad Bull buckings last longer. I haven't used mine long enough to find out, so I can't comment on that just yet. To check if your bucking is worn out and not retaining BB's, pop your barrel out but leave the bucking on, and try dropping a BB down into it. A bucking that's still holding will prevent BB's from falling in. They'll rest on the lip and you'll have to push them down in with your finger to get them to go down. A worn out bucking will allow BB's to fall right through, or allow them through with very slight pressure. (How much pressure is "too much" depends on how tightly wound your magazine spring is. If your lowcap mag double feeds when full but stops when it gets towards empty you have a marginal hopup bucking.)
Mad Bull also claims that the Shark Bite buckings can slightly increase velocity by holding the BB in the bucking a hair longer allowing more pressure to build up behind it. This would depend an awful lot on the timing of your gun, basically relying on the piston coming down early, before the tappet/air nozzle push the BB forward and out of the bucking's "Shark Bite" area. I'd say this claim is pretty bogus. I didn't measure any FPS increase between this bucking and my old stock one, and I wouldn't hold your breath that you'd get any real increase, either.
Bottom line: Mad Bull Shark Bite buckings good, same price as regular non-shark buckings so no loss there, extra BB retention can't hurt, chance of small FPS gain can't hurt either but is probably nonexistent.
If you need to replace a worn out bucking, go for it. If you're expecting magical FPS increases, don't.