Buffer should be carbine, 2.9oz and 3.25" long.  
To check recoil spring length, buffer face down on a table, and spring on buffer, and compress it all the way down until the spring coil binds on itself.  Top coil of spring should be below the top of the buffer bumper top end.

As for carbine receiver extension, send a tape measuring down the inside of it, and from the back end wall of the tube, to lip of tube, should be 7".  If it coming up as a void depth of 6-1/2" ish, its a air soft knock off for one of the AR BB guns ,and will not work since it void is too short for a real firearm.

As for B/C, CLP around the base of the key to top of carrier, hold the bolt inwards, then pressurize the front of key to about 100lbs of air.
If you have leaks between the bottom of key, and top of carrier, it a problem and key will need to be pulled to clean up two surfaces, and new key bolts used with then torqued to 37inlbs, and the key metal staked back into the side spines of the bolt heads.

As for other gas leaks or blockages, check the gas block to barrel for leaks or miss index of the gas block passage to barrel gas port.  Hence on some gas blocks, you have to hold them off the barrel shoulder a touch to get the block passage to match up with the barrel gas port.

From here, pull the bolt off the carrier, dry fit the carrier into the upper and check the alignment of the gas tube to the carrier key.  If gas tube needs to be tweaked to align it, do so over the center of the barrel.

On bolt catch, shotgun the upper open, pull the catch up by hand, and let it go to make sure its not binding in the receiver slot.  Next lock a empty mag into the well with the upper still shotgun open, and make sure it lifts the bolt catch up all the way.  If tab of bolt catch is just slightly engaging the back of the mag follower, push the mag forward in the well, follower in the mag forward, and see if the bolt catch tab misses the back of the follower.  If yes, then will need to mod a 9mm bolt catch to work with the lower/mags, for a better engagement of the catch tab to mag follower so it not missing the back of follower as the follower and the mag more forward in the well.


Last check, pull all the way back on the charging handle, and the face of the bolt should stop 1/8"~1/4" in front of back edge of ejection port window, and last inch of pull on the CH, should be the same tension as the center of pull.