Any insights are a welcome addition. I have a lot to learn about slug guns. I imagine you have quite a bit of knowledge as you build them and hunt exclusively with them in state.
I've only learned out of frustration. I don't like modern guns that go way down in value. And I don't like buying new guns where the actual gun "cost" was 15% of finished purchased price. (insurance, distribution, marketing, etc). So those are some of my frustrations I had to swallow my pride to accept. The other issues was quality slug availability. Historically, the most accurate slug ever made was a barnes slug (federal loading?) that was discontinued 4 years ago. Today, a box of 5 of those slugs sells for nearly $80. They were that good and everyone had their guns regulated to that extinct load.
On the open market today, in 90%+ of guns, the most accurate slug by several orders of magnitude was the 20 gauge Remington Accutip sabot slug 3". So instead of doing custom load development (which is illegal in many slug states...factory loads only) it began the "so what is the most accurate slug gun that can be made?".
-Best of the West slug gun built on a Savage 110 (MSRP is around $6000?)
-TarHunt built on the Savage 110 (MSRP is around $6000?)
-A New England Arms Ultra Slug Hunter. A $115 total piece of junk that now sells used for $700 and needs a LOT of work to be good.
-A new in the box savage 110 heavily customized to overcome SERIOUS defects. (Gun is $700 and you throw away most of the parts)
Things I learned along the way in my plight where handloads weren't an option (some of the insights may be of value on your journey)
-Glass bedding is CRITICAL CRITICAL. The recoil is so fierce, the wood compresses or the screws back out. Either way, accuracy is due to harmonic consistency. The only way to torque down the screws and have them stay put is to glass bed actions.
-Feed sucks on slug rifles. Glass bedding can help with feed as it makes sure the bottom metal/magazine stays put on the correct angles too.
-The most accurate guns use one-gauge down in barrels. Heavy barrels matter. The ultra slug hunter barrels (Verney Caron I believe makes Hastings for almost ALL shotguns?) were 10 gauge barrels with 12 gauge bores, or 12 gauge barrels with 20 gauge bores. That's how a $115 gun could shoot exceptionally well and its why people will drop $1000 on a new in box old stock one today when Walmart sold them for $115 four years ago.
-Good mounts and rings are essential...stuff moves under the recoil of these "bore rifles".
-3" shells have a 90% chance of shooting better than 2-3/4" shells across the board. Why? Its a freebore issue or a gas pass through issue. Bullets closer to the forcing cone / lands appears really important.
-Extraction can be an issue. Bolt slug rifles are important and useful.
-Cleaning regimens need to be similar to muzzleloaders. The fouling of lead, copper, and plastic is a mess and it builds in layers. A bore snake and done is not a good plan with a slug rifle.
In the end, 3"-5" five shot groups are possible at 300 yards, but I found it cost $2800 or more to get there, and in the end I own a laminate and plastic pile of crap that will be at a garage sale someday. Never have I spent more for less...$2800 can buy a hell of a fine vintage rifle that appreciates in value, whereas I was forced to buy tactic-cool junk due to legal restrictions in the midwest...still sour on that. (as I look at three of these custom rifles staring at me in the case)
For the sake of argument, have you tried the new barnes factory 12 gauge slug OR the Remingon Accutip 3" just for comparisons? I realize you're cutting your bullet weight in half if you do, but your KE and penetration are up. I would think something at 1750-1900fps at 500gr might be more effective than 1000gr at 1000fps for close work?
In the olden-days I think the 12 bore paradox rifle was shooting a 2-1/2" 12 bore load of cordite with a 730gr piece of lead at 1150fps and 4.3tpsi (*Westley Richards) . Another was 2-1/2" case with black powder and 750gr lead at 1000fps for 1.8 to 3.5tpsi (*proof house records). These were with fosberry bullets. <- This tells me that bullets being lighter might be necessary to safely get you the speed you want?