From 1895 to 1902
Sporting Life routinely reported the guns and loads used by competitors at major shoots.
These are the 25s at 1901 GAH at Live Birds
https://digital.la84.org/digital/collection/p17103coll2/id/42076 
I recall ONE GAH at Live Birds competitor who was using a 16g; 1899 GAH. E.A. Sturdevant shot a 16-gauge Parker at 26 yards, with 2 3/4 drams Du Pont, and 1 ounce No. 8 and 7 shot. He killed 23.
Even the ladies used 12g; Annie Oakley mostly 1 oz 2 3/4 Dr.Eq at targets and 3 Dr.Eq. at Live Birds.
April 4, 1896
Sporting LifeOne of the pleasantest features of the recent Interstate shoot at Kirkwood Park was the presence of the two lady contestants, Mrs. M. F. Lindsley, “Wanda,” of Cincinnati, wife of Milt F. Lindsley, manufacturer of the King's Smokeless powder, and Annie Oakley, who is the acknowledged champion pigeon shot of her sex in the world. “Wanda” was attended by her husband, who felt well satisfied when his clever wife stood in the big match after a dozen men had dropped out, among them being some of the best pigeon shots of the country.
Her score was equal to that made by such men as Captain A. W. Money, George S. McAlpin, William Wagner, W. S. Edey and W. H. Green. The quality of the birds were a little too fast even for some of the best shots, and “Wanda” can be congratulated for her pluck in entering such a contest.
She used a 12g Smith gun, 7 pounds weight, 3 drams of King's Smokeless powder, 1 1/8 ounces of No. 7 and 6 shot in Peters shells.
Sporting Life, April 2, 1898 GAH report
https://digital.la84.org/digital/collection/p17103coll2/id/45659Mrs. W.P. Shattuck shoots a rather heavy charge. She uses an L.C. Smith gun weighing 7 3/4 pounds, 3 1/4 drams of Du Pont powder 1 1/4 ounces No. 7 1/2 shot, in first barrel, and 3 1/2 drams, 1 1/4 ounces No. 7 shot in second barrel; loaded in a 3-inch Leader shell.
