Notchy, thanks for sharing your research on this topic, you put your knowledge and skills to work on this reply, thank you. BuckI can't find the post, but somebody a little while back asked what the old timers said about blowing down a barrel. I found a few quotes that may be of interest.
The first is from A Treatise on English Shooting, by George Edie. If I'm reading the Roman numerals on the frontispiece correctly, this was published in 1783. Here is a quote from George: "Observe after a fire [shot] never to blow through the barrel, but charge again immediately while the barrel is hot and dry; by this method of immediate charging, a gun seldom hangs fire, and carries much smarter and better..." (p.22)
Edie's chief concern appeared to be keeping the powder charge dry. I'm not certain, but I believe gunpowder in his day was probably unglazed and more prone to absorbing atmospheric moisture than glazed powders. Edie did provide very clear and readable instructions for "washing" the gun after a session of shooting, and he also wrote briefly about safety concerns and prevention of accidents. However, I didn't find anything about embers or accidental discharges.
Here is the famous Audubon quote, circa 1810, which described hunting with a backwoodsman. Note the first line in his description of the loading procedure:
"He blows through his rifle to ascertain that it is clear, examines his flint, and thrusts a feather into the touch-hole. To a leathern bag swung at his side is attached a powder-horn; his sheath-knife is there also; below hangs a narrow strip of homespun linen. He takes from his bag a bullet, pulls with his teeth the wooden stopper from his powder-horn, lays the ball in one hand, and with the other pours the powder upon it until it is just overtopped. Raising the horn to his mouth, he again closes it with the stopper, and restores it to its place. He introduces the powder into the tube; springs the box of his gun, greases the "patch" over with some melted tallow, or damps it; then places it on the honey-combed muzzle of his piece. The bullet is placed on the patch over the bore, and pressed with the handle of the knife, which now trims the edge of the linen. The elastic hickory rod, held with both hands, smoothly pushes the ball to its bed; once, twice, thrice has it rebounded. The rifle leaps as it were into the hunter’s arms, the feather is drawn from the touch-hole, the powder fills the pan, which is closed. “Now I’m ready,” cries the woodsman…."
From Audubon's Journals, Vol. 2, (1972 reprint), page 492.
I dang sure didn't read everything, but in the books I consulted I didn't find anything pertinent from the mid or late 19th century. However, there are several publications from the early to mid 20th century that mention blowing through the barrel.
An author identified as T.B. Tryon wrote a series of articles about muzzleloaders for The American Rifleman in the 1930's. I don't have the original citations, but these articles were collected and published in a paperback book entitled The Complete Rehabilitation of the Flintlock Rifle & Other Works by Pioneer Press in 1987. I bought a copy from Straight Shooter Books, but I'm pretty sure Dixie has it, too. In his article on "The Percussion Plains Rifle," Tryon wrote,
"There are a few points to be borne in mind when loading a percussion-lock rifle. Before charging for the first time, explode a cap on the tube, as the force of the blast will clear the dust and oil from the tube and antechamber. Then blow through the bore to make sure the passage is clear, before introducing the powder charge. In fact the only way to keep the antechamber open on the range or in the field is to blow through the bore, and this should be done each time before reloading..." (p. 30)
Major Ned Roberts also mentioned blowing through the barrel. In loading a hunting rifle, he wrote, "...the bore should first be wiped of all oil and grease. Then with the hammer at half-cock, blow through the barrel to ascertain if the nipple is open..." (p. 99). This was in The Muzzle-Loading Cap Lock Rifle, first published in 1940. A few pages later, Roberts said, "After firing, place the hammer at half-cock, remove any part of the cap that may stick to the nipple and blow through the bore to help soften the powder residue and clear the vent in the nipple before loading again" (p. 105). Roberts indicated that wiping the bore after shooting was not necessary for a hunting rifle, but for best accuracy in target shooting, he recommended "...wiping the bore with a slightly wet cloth followed by a dry one after each shot" (p. 105)
I was pretty sure I would find something about blowing through the bore in Walter Cline's The Muzzle-Loading Rifle - Then & Now, which was published posthumously in 1942. Cline was a fanatic about accuracy, and he believed in wiping after every shot. He wrote a long and detailed paragraph about the correct way to wipe a bore. He mentioned popping caps to clear the flash channel, but I found nothing in his book about blowing through the bore. Incidentally, Walter Cline died from an accidental discharge that was believed to have occurred while he was loading a rifle which had already been capped.
When I was quite young, my dad gave me two little muzzleloading books which I still enjoy re-reading. These were For Beginners Only, by B.M. Baxter (published by the NMLRA in 1949) and The Muzzle Loading Shotgun, by V.M. Starr (self published, no date but likely in the late forties or early fifties). Baxter mentioned blowing through the bore a couple of times. In preparing to load, he recommended that the shooter "Blow through the barrel with the hammer cocked. If free, snap two or three caps" (p. 42). He then went through the loading and shooting procedure. "If after a few shots the ball loads harder and the bore seems to have become rough, the powder fouling is caking. Caking can be prevented, sometimes, by blowing down the barrel after a shot. The powder residue which forms caking is of the nature chemists call 'deliquescent,' which merely means that it absorbs moisture. Blowing down the barrel provides moisture from the breath to keep the fouling soft and moist" (p. 45)
The only comment on this topic I could find in Starr's book was on page 8, where he described getting the gun ready to load: "It is not a bad idea to snap a couple of caps on each nipple and then blow through the barrels and watch the smoke come out the nipples. That gives you a good chance to see how well the passage is cleaned out."
To recap, George Edie (1783) recommended never blowing through the barrel after a shot. In context, this appeared due to the risk of one's breath introducing moisture into the bore, which he believed would contaminate the next powder charge. Interestingly, several of the later writers who mentioned blowing acknowledged the moisture in a person's breath, but indicated this was a good thing in that it would keep fouling soft. My hypothesis is that this may reflect a change in powders. Glazing (indicated by the little "g" in the powder granulation, as in FFg) reduces dust but it is my understanding that it may also give the powder a little protection from moisture. Chemists in the crowd are welcome to comment. I believe most of our powders now are glazed, but I read somewhere that Jack's Battle Powder may be unglazed. I've never had the opportunity to try Jack's, but would be interested in hearing how it stacks up against other powders in terms of ignition, energy, and fouling.
All of the authors I found who mentioned blowing through the barrel (except Edie) indicated it was a good way to check for a clear vent. In my limited review, I didn't find anything about embers or accidental discharges associated with blowing through the bore. I'm not saying there isn't anything. I just don't recall reading about it and didn't find anything in this cursory review.
As for myself, I learned to shoot from my dad, and his techniques and recommendations pretty much concurred with Tryon, Roberts, Baxter, and Starr... pop a cap on the unloaded gun and if it sounds clear, blow through the barrel to make sure. After a shot, and you know the gun is unloaded, blow through the bore to keep fouling soft and make sure the flash channel is clear. My only reason not to continue the practice now would be prohibitive range rules.
As an historical foot note, I would like to add that when breechloaders came into common use in the south, both double and single-shot, break-open shotguns were extremely popular general-purpose hunting guns. In muzzleloading days, hunters would call their hounds by blowing a horn, but if you had a breech-loading smoothbore, you could open the breech and eject the shell, then purse your lips and blow through the muzzle as you would blow a trumpet. This makes a clear, resonant musical tone that carries a long way, especially with a 12 gauge. This was mentioned in the song, "Tate's Hell," written by my old friend, Will McLean:
He blowed through his gun barrel;
The dogs did not hear.
The panther had killed them,
and now Tate felt fear.
The sun was not shining,
the mist it was thick.
"Oh, Lordy," Tate holler'd,
I'm lost up the crick.