So i just tried to deadhang in a Chinup position, i cant. I can do over hand or hammer grip but not palms facing me.
A couple things spring to my mind:
-When using the rings, just let your hands rotate in such a way that lets your body feel okay, if that's possible at the moment. When you hang at the bottom, you are maintaining the supinated grip. If letting your hands rotate to a pronated position at the bottom alleviates the issue, I would start there so you can still do your pulling without aggravation.
-Without bombarding you with shoulder mobility stuff, I'll give you a drill I like, and then I'll post a link you can read through if you like.
-The drill: adjust your rings or bar such that your feet can rest on the floor. You are going to do dead hangs with your feet on the ground. If you use the rings, try and rotate your hands as best you can into a supinated position
without feeling pain or discomfort, and lock your elbows (not forcefully, just keep them straight). If you use a bar, find a setup such that you can hang supinated at a comfortable angle of shoulder flexion, one that doesn't aggravate anything (even if it's just past 90 degrees). Do this drill in a way that allows your body to expand and loosen up. If you can only do one "rep" at a time, that's fine.
a) you will do a long exhale until you feel your abs start to engage. Maintain the engagement and take a
silent inhale through the nose. You ought to feel your ribcage expanding. This ab engagement should not be forced; you aren't doing an ab exercise. It's just enough to keep you from arching your back when you inhale.
b) the above-mentioned ab engagement will prevent you from arching your back to compensate for shoulder flexion, and it ought to help you to feel a lat stretch.
c) do this a couple times a day, for something like 3 rounds of 5 breaths. Just let additional ranges of motion come on their own.
That's the simplest drill I have found to help with the chin up position. I know you've posted about back issues, so if anything here aggravates it, obviously stop and reassess.
A quick and general "how and why:"
-the elbow must transmit all the forces to and from your hand to your torso. When you move your shoulder through that 180 degrees of flexion, the humerus will pass through a series of different relative rotations. When we push or pull the elbow sees relative motion between the forearm (radius and ulna) and humerus. If things aren't moving well, the tissue around the elbow gets the brunt of it.
-Additionally, the humerus ought to end up in relative external rotation at its max ROM approaching 180 degrees. If it's not, and you are trying to supinate your hands, you are essentially twisting your hands the opposite direction of your humerus.
This remains one of the best shoulder health articles I know of off-hand:
www.conorharris.com