I'll try to deal with this detailed reply in order:
"For food?" Well now, let's look at that. It appears from what I read on here that a lot of people don't shoot for food, but for money, or for conservation or crop protection reasons.
I kill animals for a variety of reasons including pest control, conservation and because I enjoy it. I eat most of them, but it's not always the primary consideration.
We can ignore the advice of the Food Standards agency if we want, but the current advice is to discard all meat within 10cm of the bullet path; that doesn't leave much of either bird to eat. Everyone's dining choices are different, but I don't think I'd choose to dine off a species with a low population and which isn't thriving. I fully understand that there are plenty of people who want to eat the last bluefin tuna, or caviar from the last sturgeon on earth. I am not persuaded by the argument that such practices are ethical.
I know people shoot ptarmigan frequently in Norway and other parts of Scandinavia, but the populations there are orders of magnitude larger and the human population an order of magnitude lower.
In short, I'm aware of the legality, my point was that there is a distinction between legality and wisdom.
The most important thing when killing an animal is whether you're damaging the prospects of the species. Respect for the animal makes not a jot of difference to the animal- it's more of a figleaf for the hunter. It's entirely obvious if you compare a hunter who shoots a rabbit with total disregard for the animal or concern for its suffering, with a hunter who respectfully shoots a northern white rhino. I don't think many would consider the latter definitely more ethical than the former. Being sporting is not a case of stroking one's ego, rather the reverse; but unnecessarily using animals as target practice may be.
It may well be possible to ethically shoot a ptarmigan with a rifle, but I'd contend not in the UK and would point out that your comment entirely ignores any consideration of conservation.