Make sure you purchase a dog with good bird finding and pointing genetics.
Professional dog trainers have to use pigeons because of the economics. Pigeons are cheap and often reusable. I have never used an e-collar, but again understand why the pros do. A good pro will likely have 8 - 10 dogs or more in training. That means your dog will have about 15 - 30 minutes twice a day with the pro. He has to make that time as effective and efficient as possible. In the summer there are about 3 - 4 hours early and maybe 2 - 3 hours late where temps are manageable.
I have trained six Brittanys myself over the course of the past 39 years. Every dog has become an excellent bird finding machine with solid points and good retrieving skills. I hunt my dogs on pheasant, grouse, ducks and geese (over water and in the field). I am fortunate that I own a well experienced dog or two when I am training my new pup. Some times injuries to the older dog(s) put the young dog in service quicker, but usually I am not in a situation of putting the dog in too much too soon. That said most of my dogs have pointed wild birds at 9 months of age or so.
A few of my dogs have seen pigeons when I belonged to a local dog training club, but I found that summer training to be "forced". Again, the Pros have to train in the summer if they want to earn a living. I have stopped that long ago.
Patience. Number one key to developing a good bird dog. Take a slow and systematic approach to exposure to gun fire.
Yard work, walking, socializing is the second key to developing a good bird dog. Establish your role as the boss in a consistent manner. Breed and individual dog characteristics will dictate intensity. Most of my whoa training is simply done on our nightly walks. Whoa steady to the point where I can walk past or around the dog until I release it. No barrel, no table, etc....
I work my young dog (often on a 30 ft check cord) all summer long at a gun club field or two. I do not worry about bird exposure. I want the dog to learn to run through grass, get wet from the dew, chase a mouse, grasshopper and/or song bird. The green grass has a pretty strong ability (chloroplast) at masking bird scent. It helps wild nesting birds and young broods avoid predators.
As late August - September rolls around I start putting out 1, 2, 4 chuckar ... using a 30 ft check cord we work on finding, pointing and steady ... birds flush ... no gun ... they usually fly and we can re do. I have found that exposure to multiple birds once or twice a week is far superior to every night ... same field ... same flagged pigeon launcher..