I realize this thread is over a year old but i'll add to it anyway. I bought a Marlin 512 Slugmaster the spring of the vary first year they became available to the public. Sent mine to Basner Gun Smithing to have the muzzle crowned, a trigger job and the action glass bedded. Cost with UPS shipping was $225 which brought the total cost of the 512 to $475.
Shaved about at least a full 1.5" off the 100 yard group and made it an honest 1.25" 100 yard slug gun. Yes I got plenty of 1" 100 yard groups but day in day out average was 1.25" at 100.
I forget how long after I bought my 512 that Browning came out with the A-Bolt slug gun but it was $800 so I passed on it. That is until I cam across one LNIB for sale at Gunbroker for $550 and bought it.
What always gave me fits about both guns was how unbelievably picky they were about which sabot slug ammo shot well from them. The Marlin absolutely LOVED first run Winchester Partition Gold sabot slugs and would shoot them into 1"-1.5" groups at 100-150 yards and about 2.2-2.5" groups at 200, so very accurate for even a fully rifled sabot slug gun. Used the 512 and 12ga WPG's to kill a fair number of deer at ranges between 150 and 210 yards. Never once did I ever have to shoot a deer 2x with a WPG always bang-flop DRT. But if I tried other sabot ammo from Remington or Federal and results were rather disappointing. When I heard Winchester was re configuring their WPG slugs and discontinuing the ones I loved I promptly went to my LGS and ordered a case of matching lot#'s.
A second slug the 512 shot with even better accuracy then the WPG's was Lightfield's Commander IDS sabot slugs, but although these slugs hit like the 16.6" guns of an Iowa class battle ship, weighing in at 600 grains VS the WPG's 375 so they dropped like bricks after about 160 yards.
Unfortunately I don't hunt in slug gun only areas anymore as my home state now allows the use of certain bottle necked cartridges on private land and any CF rifle or hand handgun .cartridge .357 and above and 1.8" case length and under on public.
Still and all my Marlin 512 was every bit as accurate as my A-Bolt or NEF 920 tracker and killed me several train car loads of deer in the years I did use it and I made my longest ever shots with a slug gun using it including a LRF confirmed 170 yard+ shots on two coyotes I caught out in a picked bean field and a 212 yard shot on a IL WT
buck.
Before I retired my slug guns in favor of CF rifles in my home state I made my best/longest off hand shot ever on a deer using my A-bolt on a absolutely beautiful tall tined 8 point buck who was also the heaviest deer I ever shot. An absolute beast of a buck weighing in at almost 300lbs gutted.