Tuning Calling FX gurus, help please

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I cant figure out why my Crown MKII keeps doing this, see attached pics. Bone stock rifle, shooting NSA 23 grain slugs. Been shooting the same ammo since I bought the rifle. Just started this tonight. I have taken the barrel off a couple times, check and made sure the transfer port was tightened all the way, yadda yadda yadda. Idk what is wrong. It happens at random, juat shoves the slug against the end of the barrel and blocks it. Thanks in advance. 
 
Only thing I can think of is that the liner is not being pushed far enough back in the barrel inlet. Possibly a piece of debris or o-ring is stopping it from going into its home position.

I modified the port, enlarged and oval shaped it on my impact inlet, and with the pin probe installed it did that also. With the stock probe there was no problem.
 
How much of the threads are exposed on the liner retention nut. Just wondering if the liner for some reason isn't seating all the way in the SS piece and up against the TP. Mine has 9 threads exposed. The liner should be butting up right against the shoulder in the TP with no gap available to grab the slug. As Airgunman said, maybe the problem is actually starting before it gets to that spot. Does it do that with pellets?

Jking
 
You didn’t mention what cal. If it’s a small slug ( short) my guess is the mag is not feeding consistently. I’ve had occasions when certain slugs and even some pellets just would not feed. Too small or more often too big. The other suggestions are good because clearly there is too much space allowing the slug to turn some place. Magazines are problematic and for the most part I avoid them. 
 
My bet is on the fact that it's not completely seated into the brass port block. Normally when removing liners from Crowns, one marks where the brass nut is on the liner before disassembly. This at least gives something to go by for depth. As stated in previous posts, it is somehow separating from the seated position in the brass port piece. The O-Rings/brass nut should be tight enough to compress the O-Rings to stop any movement. It's not supposed to be lightly tight, firm is more like it. Gorilla tight is too much. You also may try using Viton O-Rings as they are somewhat stiffer, though I have not had issue with the factory O-RIngs.

mike