You may find as I did that the vibration and bouncing about resulting from extended carry in a vehicle eventually loosens the wads in Brenneke slugs, offering less than usual accuracy. Those in my circle who continue to use them generally swap theirs out at specified periods, from weekly to monthly, depending on preference and experience.
I've also noted the possibility of tearing or shearing of the relatively thin rims of Brenneke slugs in autoloaders, and at least potentially in a pumpgun with a heavy-handed operator. Even the shell stops of the magazine ding up the rims of the Brenneke rounds, requiring frequent inspection, at least.
They do work swell in double guns and singlebarrelled shotguns, and in those weapons increase the odds of getting the job done with a single, well-placed shot. And for a gun that rests in a less severe opereating enviornment, they're just fine.
But I use other slugs in the shotguns carried aboard my car, truck and motorbike. The triple-ought buck loads you use should do just fine, though I go for something with a bit less flash at night. Different guns have differing preferences so far as shot loads go, and my old 870 tends to prefer 20-pellet #1 buck loads. But either triple-0 or reduced recoil 8-pellet 00 buck loads are available, and are sometimes my choice as circumstances dictate.
That Brenneke slug remains a particular comfort as the first load up when travelling in bear country, though....
Interesting. The S$B Rottweil Brennekes I bought in mass quantities from the Cheaper Than Dirt outfit a few years back have some featuers that may differ from the ones you're describing. First of all, the top of the slug is set back from the rolled-in mouth of the plastic shell casing. This would appear to protect the round and the slug inside it from getting too dinged up in the course of handling and load-and-fire. The wad is somewhat unique in that it is a thick copper (or plastic) platform resting directly on and compressing the poweder load. A short pillar rises from the center of the platform to flare out into a thinner platform upon which rests the botton of the Brenneke slug. The shell casing itself is transparent, so all of this is visible.
They certainly are effective. I've shot two horse-chasing dogs with them and it was a solid put-down each time. Didn't emerge from the other side of the critter, either, which suggests some nice expansion.
I've also got some solid copper Barnes expanders that I haven't yet had occasion to try on varmints of either the four- or two-legged persuasion, but Lurker says that they do a really nice job. Perhaps some of the post election festivities will offer a suitable proving range, but we'll have to see how it plays out.
The Benelli has an 18" barrel, and it seems to like the 000 buck loads I feed it just fine. Nice spread, and it's a good compromise betrween pellet weight and quantity.
BTW, all of these are 2 3/4" - no need for magnums for social work, and the recoil is more manageable.