I have an old Stevens 5100 that I bought used. I turned it into a cowboy-style coachgun by shortening the barrels to 18 1/4". I used a hacksaw and my friend sanded the muzzles with a disk sander. It looks professionally done. Now all it needs is a drilled and tapped bead and some sling swivel studs (or better yet, a leather back scabbard).
My recommendation would be - regardless of make or model - a 12 gauge with a top-mounted cam lever (most of them will have this) instead of the olde worlde-style forearm-mounted cam lever. Sidelocks are preferrable to boxlocks. Rabbit ears (exposed hammers) are a matter of presonal preference and also functionality. If you're going to us it for sport, go for exposed hammers. If you're a cowboy action shooter, go hammerless. Exposed hammers are more durable than the firing pins of a hammerless side-by. But they make operation more complicatified. It's a trade-off.
If you do get a rabbit ear side-by, don't get one with an automatic tang safety. Anything with an exposed hammer(s) that can be decocked doesn't need a safety. And also, with a hammerless side-by (or over/under) with an automatic safety (as most have), you can turn the safety off as you close the gun. After awhile, it becomes instinctual. Also, I recommend double triggers over single-selective. They're less mechanically complex plus you can fire double-shots if you wanted to. I also recommend extractors over automatic ejectors because they're less mechanically complex and also because they allow you to take out a single casing while leaving a fresh round in the other chamber.
I came up with this idea for a gun I'd like to design (which I don't know how to do yet) that's similar to the Ithaca Auto & Burglar Gun. It's a sidelock side-by-side with double triggers and a walnut stockless grip with a shape similar to the Rodda Howda with a rounded knob on the end. It has modified rabbit ears (actually, they look more like antlers) that can be cocked and decocked independently or simultaneously. Instead of a cam lever, it would have a Thompson-Center Contender-style hook mounted on the trigger guard. It can be broken open with the trigger finger and cocked with the thumb at the same time. The barrels are 12" long (it's a Class-III) and it has a walnut spinter forearm.
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