I would say have a look at the crown of the barrel and make sure there is nothing awry there. Are you using a silencer?
However, all barrels should give SOME resistance on pushing a pellet in, even those with throats, it's possible yours is a duff over-size barrel. The rifling marks look exceptionally shallow. Is it too late to send it back and have it swapped? The only other thing is to test it with some 5.6mm pellets like Defiants and see if they do any better.
Very frustrating problem.