On my rain covers, they're only stitched back a very few inches from the muzzle...if I'm sitting on a deer stand and see a deer coming I want to shoot, I just slide the rifle back 3-4 inches and the end slides off to the side...meanwhile, underneath the covering I set the trigger and slip off the hammer staff...then at the appointed time, I just raise the rifle up to my shoulder and the material just lays there across my lap.
For me its a comforting feeling to see raindrops dribbling onto the rain cover and running off, knowing the rifle is dry underneath...I keep it folded inside the large rear game pocket of my hunting vest (that I use as an equipment vest)
There is a massive difference in setting in stand or blind and out walking in the snow or rain or even snow covered pine trees, falling down perhaps etc etc. The wind can turn a clear day into a heavy snow storm and it usually does.
If its well below freezing snow is not a horrid treat once the gun is cold. Warm gun and snow is as bad as anything except pouring rain maybe. If snow or water gets in the muzzle and runs water back down the bore it WILL get around the patch in most if not all cases to some greater or lessor extent.
I can lean the gun in its cover against a tree and the wind can blow any amount of snow down on it an it will be dry.
The full cover also prevents snow buildup on the barrel or in/on the sights making an aimed shot impossible until cleared.
There are many reasons I use a full length or near full length cover.
The Indian Trade rifles imported by the English in the 1780s came with a "case".
Go to the Mountains for 20-30 days, live in a wall tent with no fire (the clients burn wood at an alarming rate and its cut with a cross cut saw, by you and another guide before the season). Rain, snow, sunshine, horseback or foot, I use a full length cover. It works. Anything else is just a leaky bandaid.
Unfortunately, so far as I know, for the Colonial Period there is little of no documentation for guncovers aside from the cover with RCA 48 and the mention of "cases" with the trade rifles.
Dan