Confirmed the butter trick today. Works fantastic, way better than the rubbing alcohol I've been using for this. Especially good for removing stickers on painted parts because the alcohol will take off some of the paint along with the sticky. Thanks for sharing!
When you say mill work, do you mean you have a machine that sands your blasters? Or are you hand sanding all of them? Either way they look good.
We can't really help you too much without internal pics of your blaster. The internal trigger spring reconfiguration is very important and consider gluing the slip clutch. Whenever I spin a mav cylinder or otherwise rotate the barrels I press the turret release button first anyways so you don't lose much functionality. Spring in the front piece of the turret makes a difference as well (between grey thing and turret) as it will push the turret back farther. I did just those mods with a removable turret (I find that the drop clip mod makes the blaster feel flimsy and is not effective for reloading) and was getting ranges decent enough to compete in more casual pistol rounds. Also, I'm guessing you know this already, but if you straight up removed the entire AR it will have no seal at all in the turret which could explain your ranges.
In regards to fixing your brass barrels: Use a dremel sanding cone or some other large sanding bit that fits inside your barrel and lightly sand the edges of the lip (try not to go on the inside of the barrel, not too big of a deal if you do... after all this is a tek3) Once you get that down to mostly flush, go in with your needle nose pliers. Stick the tip down the barrel and quickly start twisting and it will pop the brass out on the edges. Sanding is done before this so that you don't smoosh the brass lip to the inside of the barrel. Post pics too if you want to get more help.
Please don't take anything on youtube nerf seriously; at least not before getting multiple opinions on it. Youtube nerf videos are often free from any sort of useful discussion or standards due to the fanboy mentality of the viewers (note how you felt the need to name drop instead of just saying a youtube tutorial when you weren't even posting a link to it.) If you want to have mods that are reviewed by people who know what they are doing and are actively looking for ways it could be improved you should stick around and check out some of the write-ups here.
I've opened a 5K tank before. It is by far the most difficult tank I have ever dealt with although most of it was probably due to how the previous owner cut it. I assume you have opened up your blaster and have the tank. When you shake it, does it rattle? The seal is a bathtub plug looking thing stuck on a hook instead of a rubber washer on the end of the metal shaft like the AT#K series. Usually the plastic "U" part of the seal the metal hook goes through snaps so nothing happens when you pull the trigger. Also the pump check valve can go bad so make sure you try a known working pump. Anyways, I know that on first glance it might look like a good idea to cut right along the seam but if you do then you will have no way to reinforce the joint when you try to glue it back together again. Cut above or below and leave enough room for electrical tape. When you superglue it back together, quickly and tightly wrap the electrical tape around the joint applying superglue as you go. The force of the tape will help center the halves and when dried it will be stronger than the original plastic itself.