Chokes and Chambers
 

While he was refinishing the Damascus barrels, my gunsmith told me my "new" 1891 Parker GH 12 with 30" tubes had 2 1/2 chambers and Full/Full chokes. Sound right?

Yes. Your Parker was intended to shoot the then standard 2 5/8th shell but Parker intentionally cut the chamber short to get a better gas seal when the shell was fired.

So what size shells would you shoot in such a gun? I have read on here where people say to go ahead and shoot standard 2 3/4 shells in such chambers, whereas others have said not to.

http://www.rstshells.com/store/m/2-12-Gauge.aspx

Others say 2 3/4 inch shells are acceptable but if you chose to use 2 3/4 inch shells you should be aware of the chamber pressure that they produce. I really recommend that you not exceed 9500 psi and a shot charge of 1 ounce to keep recoil energy down to protect your 125 year old wood.

Or, was it chambered to shoot brass shells which did come in 2 1/2 inch length? What are the bore diameters?

P.S. 1891 was the year that Union Metallic Cartridge Co. began offering their loaded CLUB paper shells with the extra cost option of being loaded with American Wood or Schultze smokeless powders. While the price of the loaded shells varied from $23.50 per thousand for the lightest 20-gauge shells to $32 per thousand for the heaviest 10-gauge shells, the American Wood was $10 extra per thousand for 20- thru 12-gauge shells and $12.50 extra in 10-gauge. Schultze was $15 extra in 20- thru 12-gauge and $20 extra in 10-gauge.

If you measure a Winchester or Remington shot shell you will find that they are 2 5/8 extended

Parker recommended three loads for your gun
1. 1 oz and 2 3/4 dre This was said to be a nice bird load
2. 1 1/8 oz. and 3 dre. This was the standard 12 ga patterning load for all except the heaviest 12 s. Your gun would have been patterned with this load. It still remains today as the standard trap load.
3. 1 1/8 and 3 1/4 dre.

See patterning hang tags and Parker's The Small Bore Shot Gun as source

All of these loads are suitable for your Parker as made and are commercially available today from many sources . Some people have their own favorite loads but the most informed place to start is with the Parker documents

I think there might have been some confusing information conveyed earlier on in this post.

Chambers should measure 2-5/8" long in your gun. The intended shell length would have been 2-3/4". The 1/8" shorter chamber was to create a better gas seal which was intended to help with shot performance.

To my knowledge, there never was a 2-5/8" shell. True 2.5" long chambers in this time period was an English thing.

The 2 5/8" 12ga was the standard before the 2 3/4" shell was made available. I have seen boxes of old 2 5/8" factory ammo for sale in the used/junk bin at the Kittery Trading Post. If my memory serves me the Winchester M97 was the first gun designed to specifically shoot 2 3/4" 12ga ammo, which had higher pressures than the 2 5/8" shell. Maybe Dave Noreen can fill in some of the details here.

So if ordering RST, would you order 2 3/4 or 2 1/2?

Bruce that is not correct. Just checked a Winchester AA and Remington Nitro 27 Handicap and the case length is exactly 2 and 3/4 inches for each.