Check for leaks and blockages

Rubber tube on the end of the gas tube will allow you to check for leaks of the gas block to barrel/block to gas tube, and holding the bolt inwards will also allow you to check the key to top of carrier for leaks through the front of the key.   Normally CLP around the edges of everything that your checking for leaks will show the bubbles when the two are compressed with about 100lbs of air via compressor gun nozzle if you do have leakage, but you can use some soapy water that will bubble up better if desired.

[youtube]https://youtu.be/ZNAF1nM_g1c[/youtube]

As for blockages, pretty easy to see if the gas block passage is not indexed correctly on barrel gas port, since the fouling ring will not be centered on the barrel gas port.

Next on the list, and make sure that the upper has been cleaned of all storage grease, including using a chamber brush with CLP by hand to scrub the chamber.  Also, since the uppers have been pulled apart, check to make sure that the carrier still moves freely in the upper receiver, and the gas tube is tweaked correctly to align with the carrier key.  Hence gas block passage has to be centered on the barrel gas port, so we tweak the gas tube over the middle of the barrel to correctly index it with the carrier key.

And no, not a fan of light weight B/C's.  Hence the math of the bolt unlock timing is gas port size, recoil spring tension, and then weight of B/C and buffer.  
If the bolt unlocks too fast, then this cause short stroking since the residual pressure in the bore is too high at unlock, and the spent case is too pressure bound to the chamber for a clean pull.  So by going to a light weight B/C, you have to go to a heaver buffer to bring the math back in check, and resolved/helped nothing since you still have the same reciprocating weight over all in the end.

Bring me up to speed on your theory for this?

Hence with either a full lenght stock, and a telescoping stock, the B/C still cycles the same distances back, and what changes is lenght of the buffer receiver inner void, and the buffer in play in the void distance.



Math of B/C cycling correctly/bolt unlocking at the correct time, is recoil spring tension, buffer and B/C weight, and then gas port size.

So with full size stock, you have the longer buffer weighting 5.15oz, and the spring at 11.75" long.
On the carbine spring since it can only be 10.5" long to prevent it from coiling binding up at back of stroke, and that the standard buffer is 2.9oz, the spring tension has to be greater to make up the difference for the lighter buffer to keep the bolt unlock timing correct with the same amount of B/C travel within the void of the receiver extension.

Again, front of buffer, to the amount of distances that it's going to retract into receiver extension before the buffer bottoms out on inner void back wall of the tube is the same;  no mater if a full lenght or  shorter carbine receiver extension is in play.

So on the carbine receiver extension since the standard carbine buffer is lighter by 2.25oz's, a stronger tension recoil spring is required to keep the bolt unlock timing correct.
Note: If you need to make sure you have the correct recoil lenght spring in play for the buffer/receiver extension is play, just compress the recoil spring on the buffer, and the top of spring coil should be below the top of buffer bumper before the spring coil binds out against itself. As for checking if you have the correct lenght buffer in play for the inner void of the receiver extension, check the distance of the bolt face to the back of ejection port window edge with a full charging handle pull.  The face of the bolt should stop 1/8"~1/4" in front of the back edge of the ejection port window, and the last half of the CH pull should be the same tension to the end of pull of the handle/

Still not sure that you are getting at?

Here are the gas port pressures on the different lenght of barrels at the gas port, and the gas port is sized correctly to deal with these pressures to get the bolt unlocking at the correct residual bore pressures so the spent case can be pulled cleanly.



Again, gas port size to created the needed bolt timing unlock with the amount of pressure at the gas port as the bullet passes, then you have buffer mass and spring tension as the final parts of the unlock timing math.  So recoil spring lenght is determined by  the depth of the receiver extension void, and the spring tension is dictated by the weight of the buffer in play.

As for why carbine cycle rates are more violent that say a rifle stoke, with the barrel gas port closer to the pike of the dwell burn spike, different ammo's (223 55Kpsi verses Nato 62K psi), you get more ammo pressure variances (wider swings) to the math of gas port pressure to the B/C,  than a gas port much farther down the barrel.  Also, carbine rigs weight less than a rifle rig, so your going to feel more felt recoil as well.

Hence rig set up for 223 ammo, in carbine or rifle will be about the same unlock speeds (felt recoils greater on the carbine due to the lighter weight), but when you switch over to 5.56 nato ammo with greater working pressure, it's going to speed up the carbine rig cycle rate more than rifle cycle rate, since the  gas port is much farther away from the chamber on the rifle  instead.  Again, look at the pressure at the gas port for different barrel gas port locations, and the higher the working pressure at the location, the greater your going to get from variances off the  base line standard pressures down the port.

So we never use a carbine buffer in a full lenght receiver extension, since this will case the back of the key to crash into the lower receiver at back of stroke (the the bolt face retracts back past the back edge of ejection port window), and we never use a full lenght 11.75" spring in a carbine receiver extension, since it will coil bind out before the buffer bumper can make it all the way to the tube back wall to allow a full stroke.

As for carbine 10.5" spring in a full lenght receiver extension (where you should be using a 11.75" spring), bad idea since the spring does not have the needed amount of preload to start with, and can end up with the bolt unlocking too soon isntead.

So simply, break the rifle down to upper and  lower to start with.   Buffer lenght and recoils spring parts are totally dependent on the receiver extension lenght in play, and have nothing to do with the upper receiver at all.

Now in the case of a carbine receiver extension with its needed carbine spring, and back to the upper receiver play on it, we can play with the weight of the buffer to bring the bolt unlock back into check if we are running into over function problems with the bolt unlocking too soon.  Hence we have a rig set up for 223 ammo, but are going to run Nato ammo in with it greater pressure, or even MK262 with it slower burning powder that will increase the amount of gas port pressure.  Here, we can increase the mass of the buffer to slow the unlock back down to normal timing.