Illegally borrowed .22LR? You're a very naughty boy. Yes, night sights are illegal for hunting. All hunting is illegal after dark.
A 20-joule air rifle is, theoretically, more than enough to drop a coypu at those ranges. This would however be illegal, as you say.
If you get an air rifle, get a good one. If you prefer a spring rifle, then a Weihrauch HW80 can handle those power levels. They're expensive in France, much cheaper in Holland -- see here.
Diana is another good brand, just avoid Gamo or Cometa, which are widely available in France, but crap. Whether you buy in France (retail or Internet) or outside, you will not be asked what the rifle is for, they'll just take your money and give or send you the rifle,
If you prefer a PCP, which are easier to shoot well, but need recharging from a scuba tank or with a pump, then get a good brand like Air Arms.
For target practice at those ranges, a 3-9X40 scope with Adjustable Objective (AO) will suffice. A reasonable brand like Hawke or Nikko Strirling won't break the bank, but will get the job done.
The other option is trapping. A live trap for fox/coypu costs around 65 Euros from your local agricultural supplier. Dangle a carrot over the rocker plate in the middle of the trap, and you'll catch a coypu every night! However, since you're not even allowed to despatch them in a trap with an air rifle, then use a big stick with some nails in the end -- MUCH more humane!
Yup, the French hunting laws make no sense.
PM me if you need further information.
Edited by Coypu Hunter, 13 August 2015 - 06:04 pm.