I'm interested in a .40 carbine for some reason.
Why? Most will be around the price of a AR-15 with the exception of Kel-Tec or Hi-Point. Just get yourself an AR-15 in 223/5.56.
With that said, don't diss the Hi-Point. It is very accurate, yes a bit big, but being a blow-back system with a spring recoil stock is very easy to shoot and has hardly any recoil. It is fun to shoot, easy to operate, uses inexpensive 40 S&W ammo (cheap for me I reload) and except for a gritty trigger (it will work itself out) is a good piece of machery. Price is half of the other options so you have extra cash for ammo. Customer service is great from this company and mags are only $18.00 apiece (so you can stock up on them without breaking the bank).
But it is your choice.
They will be hard to find, so when one comes along, jump on it.