I found a few references that may be of interest. This first one is from The Buffalo Harvest, by Frank Mayer (p.44):
"The use of rest sticks is forgotten now, but we runners [i.e. professional buffalo hunters] couldn't have operated without them. You see pictures of buffalo runners prone while shooting their game, but that would have been fatal to your chances. Let me tell you why. A heavy rifle fired so close to the ground reverberates amd causes more sound than one fired higher above it. So if you were prone while firing you would soon frighten your game away. We used rest sticks which put us about thirty inches above the ground: we either sat while we fired or fired from a kneeling position.
The sticks were a simple device; merely two pieces of hard wood, bolted together so as to provide a crotch in which you put the heavy barrel of your rifle. We didn't use sling straps which made you feel as if you were shooting from a straight-jacket, but merely rested the barrel, held the barrel and sticks steady with the left hand, which made shooting almost like using a bench rest."
Another very interesting quote refers to Indians using cross sticks. This is from "Uncle ****" Wootton, as dictated by himself to Howard L. Conrad (p. 429-430):
"...To shoot with any accuracy whatever, the Indian had to have "a rest" for his gun, and he was always slow about "taking a sight," while the mountain men all shot "off hand" and lost no time in "drawing a bead" on an enemy.
At hunting game with the rifle the Indians were no match for the white hunters. They were experts at finding the game, it is true, but anybody could do that in this country [Colorado and New Mexico] twenty years ago [the book was published 1890, and dictated to the writer in the late 1880s].
When the game was discovered, however, such as deer or bear for instance, the hunter had not, as a rule, much time to hunt up a tree or rock to rest his gun on before shooting. The game usually saw the hunter about as soon as the hunter saw the game, and was up and away if he was not quick in his movements. It was the habit of the Indian hunter, when using the rifle, to carry a couple of gun rods in his hand, and when he got ready to shoot at anything, down would go the rods on the ground in the form a the letter X. With one hand, the hunter held the rods together, and with the other he managed his gun, resting it between the two rods. Sometimes he would make a single rod answer the purpose, holding it with his hand in the same manner, but he never had sufficient confidence in himself as a marksman to risk a shot at game without some sort of a rest. That was when the Indians first commenced using rifles. Of late years, they have learned to use them to better advantage."
There are a couple of pretty well known photos dating to the Modoc War, showing a shirtless Warm Springs (probably Wasco) scout crouched behind some rocks, resting his Spencer on a single "gunstick," using that technique as described by Mr. Wootton. If you Google "Modoc War" or Warm Springs Indian Scouts" and look through the images, you'll probably find the ones I mean.
Another reference to Indians using crossed sticks is in Yellowstone Kelly: The Memoirs of Luther S. Kelly, edited by M.M. Quaife (p. 171):
"...They [Sioux Indians] were resting their guns on their crossed gunsticks and when we arrived within fifty yards some forty or fifty of them who had been concealed rose up and delivered their fire as we circled and returned the volley. The indifferent marksmanship of the Indians alone saved us."
I hope this adds to the discussion. It appears that the professional buffalo hunters used cross sticks bolted together, but their predecessors improvised with loose sticks. Either way, it sounds as if the sticks would be held with the off hand.
Thanks for reading,
Crooked River Bob
"Should have kept the old ways just as much as I could, and the tradition that guarded us. Should have rode horses. Kept dogs."
from The Antelope Wife