Something not right with the action bar lock. It should positively lock the action so that the action remain locked despite the forend being pumped back. It could be dangerous as the action could be opened too soon after the gun is fired when the pressure is still high. True that if the bolt is short of fully in battery, the firing block may cause misfire. You may want to have the gun looked at.
The Mag cap alone shouldn't be keeping the barrel in the receiver. The pounding from firing the gun will tear it in pieces. It is probably not the cause of your problem.
If the stock fit and recoil bothers you, you may consider shortening it and while installing a recoil pad.
-TL