Wading into unknown waters....

[troutabout]

I may have figured it out but I'm calling tomorrow to get someone else to look at those bore gauges.
The barrel is definitely stamped 16.8. I watched him drop the bigger guage in the barrel and it went in 1/3 of the length of the guage. I'm betting since there were only two brass guages on the ring that each one is graduated (stepped) to represent 3 sizes each. The big one he used probably reads 16-12-10 and the little one 28-24-20.
So either it's 16 gauge or it's been back bored to 12 - which I doubt.

 

[troutabout]

When I called the shop the guy said the two brass gauges were 12 and 20 gauge - tapered to define the degree of choke. "Modified" choke is what fit the barrel end but my vernier calipers measured .671 so that's hard fact to go forward with. I think 16 ga. is correct.

 

[Stumpkiller]

This is a side action lock just like the single gun pictured.

Side hammer, but if this is the image it's a back-action lock as opposed to a sidelock.

 

[troutabout]

That it is. My powder and shot are here so Saturday morning I'll take it to the range and Try it out with a soft load. I was thinking 35 or 40 gr of powder and a half ounce of #7 shot. Then I'll go to 70 gr and an ounce of shot. Targets will be standard computer copy paper. None of my local shops carry the replacement 6.7mm nipple I need so there's another order to place next week. Current nipple looks great but I'd like to have spares.
Wish me luck .....

 

[Stumpkiller]

Good conservative loads. I'm a 16 bore fan myself - 0.662" - and I like one oz shot and 70 gr FFg. +/-.

Kit Ravenshear, a redeemed Brit who was a former Tower of London gunsmith, used to post this poem in his catalogue.

More powder,
Less lead,
Up close,
Wide spread.

Less powder,
More lead,
Shoots far,
Kills dead.

 

[troutabout]

Interesting poem and I'm sure it's based on experience. When I used to live in a rattlesnake infested county in Tennessee I kept a 22 revolver loaded with the old style .22 crimped brass LR#12 rat shot. At 25 feet the pattern was tight and deadly on the snakes. But when I shot the same round through a single shot rifle there was almost no holes in a piece of notebook paper !
I think the explosion mixed with the shot in the barrel of the rifle, whereas the short pistol barrel had no distance (plus side loss from cylinder end) to mix with the lead shot.
His poem makes sense to me.