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Glider,
Remember there's a huge difference between shooting black powder firearms and smokeless powder ones.
For target shooting the barrel length is critical when matched to the cartridge. I used to be a County Coach for .22 target shooting and we used to go to the Eley Factory to get ammunition matched to the teams guns. Eley ammunition is of a high standard straight out of the box but matching the powder burn to the barrel was a major factor for extreme accuracy.The barrel has to be much too short for it to impact accuracy the way you describe it. The kick of a rifle also has to do with its weight vs the power of the round.
On the other side if a barrel is too long MV drops, and if it is much too long the bullet can lodge itself, not an ideal situation.
The Lee Enfield did have the ideal length for the cartridge but it was more by accident than design. You are correct in the sense that this was almost a dark art around the turn of the century.As for the SMLE's barrel being by accident the lenght it was, I doubt it, the same with other rifles from other nations. The right barrel lenght is a very tricky thing and only holds true if you fire ONE type of ammunition. It's a matter of using the right powder bullet combination, be it slower or faster expanding. The V-patr. for example was packed with faster expanding powder than the regular rounds.
The reason the Pennsylvania and later the Kentucky long rifle and the Baker rifle in use by British skirmishers were much more accurate and with longer range than the standard infantry musket was that the rifles had rifled barrels. The Infantry musket of those days were smooth bores. The rifled barrels imparted spin to the ball which made it's trajectory much more predictable. In addition the rifleman used a ball that more closely matched the bore of the rifle so that the lands of the rifling gripped the ball and that made the powder charge more efficient. Leather or cloth patches were used also to make fit tighter. Less gas from the explosion leaked out around the projectile. With a smoothbore the patch was used only to make the fit where the ball would not roll out of the barrel if the musket was tilted down. The smooth bore musket was not accurate even at forty yards. A soldier would be lucky to hit a man size target at forty yards. A good rifleman could hit a man standing still, in the open at several hundred yards. The advantage of the smooth bore musket was that it could be loaded much faster than the rifle, perhaps as fast as 3-4 rounds per minute. A rifleman trying to be as accurate as possible might take a minute or more to load one round. Also, the rifle, with it's tight fitting bore would be fouled by the black powder residue after just a few rounds and would lose it's accuracy or even be impossible to load. The smooth bore could be fired perhaps 40-50 times before becoming impossibly fouled. The average infantryman of that day was not trained to be a marksman but rather to stand shoulder to shoulder in line of battle and deliver rapid volley fire at ranges of 100 yards or less. Even a hit at 100 yards might not be disabling because of the low remaining velocity of the ball. Of course the Minie Ball and the rifled musket changed everything but many generals did not "get it" for a few years, just like the impact of the machine gun was not recognised for a while. Having said all this, I still don't believe the long barrels of the rifles or smoothbores were necessary except for perhaps sight radius on the rifle. Maybe they were just the "style" of the day.
I had the opportunity to chat with an Israeli Sniper at least he was in 48 and 56 and he says they used the Ross RifleHowever, in my opinion as-well as many expert's the Mauser is the best bolt action rifle in history, and for good reasons as nearly every military commercial bolt action rifle today utilizes its design. (The Isrealies actually used K98k's as Sniper rifles up till the mid 90's)