On 9mm build, face of the bolt on a full charging handle pull should end up about 1/4" just behind the bolt catch, and pull tension of the CH the last inch, should be the same as the middle of pull.

So if bolt is not coming back far enough, check down the tube for maybe a limiting spacer in place that is slightly too long.  If last inch of pull is stronger in tension to get it that far back, then check spring to make sure it not too long (just put it on the buffer, compress is all the way down, and make it does not coil bind up before shrinking down lower than the end of the buffer bumper retainer.  If these not the problem, then last ditch is to just shorten the buffer bumper the needed amount.



As for on the bursting, even with excess recoil of the rig not sprung up correctly, should still not happen.  So either the hammer is not being cocked back far enough for the disco to retain the hammer, or the tail blow to the disco (which acts like a hammer buffer as well), is so strong, that the disco can not recover faster enough to catch the hammer.

Or, could just be doubling the rifle yourself instead.  Hence hold the trigger all the way back, use the charging handle to cock the hammer to the disco, then slowly release the trigger to find the disco release point as the trigger it moving back forward.  If the disco is set correctly, then hammer should not be released from the disco until the trigger is dam near all the way home back forward when the trigger comes to it forward rest position.  If the hammer is being released from the disco with the trigger about half way forward, just too easy to trigger dance with your finger to actually bump fire the rifle instead.

As for cutting the tail off the hammer on a blow back rig, not the best of ideas.  Hence tail off the disco has the disco spring working as a buffer, and with the tail gone as the hammer gets slung way over back, has the hammer crashing into the tail hook that will not act like a hammer over cock buffer instead (read end up breaking hammer pins even faster than normal on a blow back action).  So at most, you may want to shorten the tail just enough that its tail to disco still,  and not hammer to top of hook on over cock instead.

Like where you are going there, but lets expand it.

Buffer length has to do with how far the bolt can come back, and in a block back action, you only want the bolt face on a full charging handle pull, to come back around 1/4" past the back of the bolt catch.  If bolt face is allowed to come back as far as the back edge of the ejection port window, then Bolt moving back forward will have too much of a run against a raised bolt catch, and can crack the bolt catch.  If the face of the bolt comes back past the back edge of the ejection port window, then chancing the modified carrier Key crashing into the front face of the receiver extension ring on the lower, and cracking the lower receiver isntead.

Regarding buffers for a non ramped bolt, standard 9mm blow back buffer, using a retractable stock, is 5.47 and uses a standard carbine recoil spring of 10/5".  

As for a ramped 9mm bolt, since the hammer against the back of the carrier will have less tension as the bolt comes back, you need to increase the weight of the buffer, and in some cases, the tension of the recoil spring needs to be increased as well to keep the bolt cycle speed in check.  Hence system is not a locking one, but just a blow back action, and its the weight of the buffer and the spring tension alone that delays the bolt forward as the round is ignited.

To add to this, the above is just a guide line for standard 9mm ammo, and if you increase the strength of the ammo up to 9 major loads (load used in unlimited USPSA pistols), the spring tension/buffer mass has to be increased as well to keep the cycle speed in check.  Hence the down fall to the 9mm ( and why the USGI did not pick up the action for service use), is the action is brutal on hammer pins to the point that it will crack them.  Again, the disco tail/ disco spring acts like a buffer when the hammer (its tail off such parts) is slung back to over cock, but on the blow back actions, the hammer sling bad is very, very brutal that even a standard disco spring can not buffer most of the time.  Fact it, the KNS type trigger pin sets, are used in the 9mm blow back actions, not due to them not breaking trigger pins, but the broken trigger pin retained to allow the rifle to stay in play for a while longer once the hammer pin breaks instead.

Hence trigger pin is retained in the receiver via the side cups to hold it in place when it center cracks at the center annular groove with the KNS set, and the side pieces are held in place by the screws on the trigger pin side.  As for how long the FCG will work, up to the point that the side plate channels/suports fail after the trigger pin has centered cracked.


And again, if cycle rate of the bolt coming back is not kept in check so the hammer is not way over slung shot back via the carrier, the faster you will be cracking hammer pins.

So buffer length is to stop the bolt at the correct place on the back of stroke (checked by a full charging handle pull), and buffer weight and  recoil spring tension (of the correct length) is to keep the carrier from coming back to hard instead.
Note, to check to make sure you have the correct recoil spring length, put the buffer face down on a surface, install the recoil spring, and compress the spring on the buffer.  If the correct spring length, spring will full compress coils to coils with the top of spring below the buffer bumper.  Hence standard rifle spring, on a carbine length buffer, will end up with the spring coil binding against itself on the buffer, before the buffer bumper can reach the back of the receiver extension (top of spring will end up above the buffer bumper when fully compressed isntead).