Grooves...
Differences exist in the manufacturing and components of plastic shotshells.
The most famous hull ever - smooth.
Do a search sometime on Mike Jordan with Winchester.
Find an older shotshell reloading book showing the various hulls, and what recipes work with what powders, wads, loadings. This will show some histories and developments over the years.
What one IDs and learns is a Hull, may be of a certain construction and used across various lines in a shotshell mfgr line - And - sometimes used for private branding. For instance in the old days a shotshell might have the Sears, Montgomery Ward, or a Farm Supply store name on the box/ shell.
Knowing how to ID allowed one to make informed choices on buying new shells ( hard shot, soft soft, worth reloading, or not...similar/same as factory loading that had patterned/ not patterned well in their gun...etc)
When the plastic hull came about different qualities of plastic were used. So different types of equipment used to make the Hull were avail.
Winchester Literally changed the plastic shotshell industry with the One Piece Compression Formed Hull 40 years ago. This hull was used in target loads to many of the better hunting loadings did many things. This Compression formed hull literally RULED for 40 years. Everyything was and still is compared to this hull. [ see note to follow**]
Tidbit - at first it failed. Too thick, so tweaks were make in composition of plastic,, thickness, skiving the mouth of hull...etc, then Win hit upon the famous result.
This Compression formed hull allowed many folks to shoot more because they could reload - for many reloading had been reserved for only the elite, or trap, skeet shooters. This allowed regular folks to shoot clay games more, and allowed regular folks to reload to save monies for hunting birds small game,ducks and waterfowl ( before non- toxic shot) a learned benefit was allowing one to get a load that patterned best in THEIR gun for Task...remember guns back then had fixed chokes.
**The equipment that made this Famous Compression formed hull finally wore out. Winchester weighed the cost of replacing this equipment, new dies, and everything. Decison was made to NOT spend the monies, The new AA hull is a two piece hull nowadays.
--Activ shotshells had NO metal - except for the primer. All one saw was Plastic. This was a great shotshell, yes it was reloadable. I reloaded many of these.
The height of the brass has no effect on velocity - great mental/ psychological deal for selling tho" . Many of us have reloaded buckshot loads in AA target load hulls ( low brass) as well as reloading the Heavier Magnum loads [ 1 1/2 oz , grex buffered, #4 hard shot] for waterfowl. IN the same note, I have / others have reloaded the 1 oz target loadings in high brass hulls...makes no difference.
Grooves - actually more of a result of mfg choice of plastic, the equipment, how the hull, brass basewad...components were put together.
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Most common failure seen on a shotgun - no matter what platform. Dirty Chamber.
Wisps of 0000 [ 4 aught] steel wool or the finest grit of Scotchbrite pad wrapped around a brush, battery drill will take out the plasticized crud from chambers.
Even by hand will work.
We did NOT have this problem with the old paper hulls btw.
I still likes and shoots the paper hulls...
HTH
Steve