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Given
time and some of that on CAD I could
show some aspects graphically. But I
will attempt (operative word) to do
this thru description. Even better would
be a Flash animation - not easy to produce
but maybe one day!
The ''cup'' (depression - center right
in elipses) in the frame is actually
receiving the barrel lug base protrusion
at fully unlocked - at which point it
actually could prevent feed ramp from
being any further back than level, with
the mag' well. It is tho not designed
to take that much of an impact!
Along with that at unlock is the actual
and theoretically main limiting factor
- the pin, in the barrel lug slot. That
should prevent the barrel setting back
any more on it's own so - the ''cup''
is actually a clearance device shall
we say.
Now, also at unlock the barrel and lug
is at its lowest point, having dropped,
thus the contact with the wear areas
we have seen by the exterior of chamber
profile. I say contact - in fact from
a design POV I expect with the incredibly
tight tolerances in this gun, we have
a theoretical ''barest kiss''.
However, if we consider the extreme
violence of the event (firing) during
a full cycle - it seems very likely
to me that a two or three thou over
travel downwards, and maybe a thou or
so to rear (heck, this is a demanding
gun to design!) - and we can see the
slight attrition to the surfaces and
thus a very small disturbance of the
aluminum there.
I am still personally content that this
is self limiting and I guess if tolerances
were sloppy as heck, it might not even
happen. Put this another way - it is
a process that will occur during early
useage of the gun, after which time
it cannot happen any further.
Let me try and describe in words - the
cycle of events -
At the instant of the slide beginning
rearward travel, as round fires and
pressures begin to back off from peak,
we have the barrel moving rearward by
approx 1/4", as recoil springs are being
compressed by slide - (exerting as they
do said rearward force against barrel
lug) - at which point the lug slot allows
it to drop on the pin, and there it
stays while slide continues all the
way to the rear.
The slide progresses then forward, stripping
a new round - which is fed into the
lowered barrel and chambered, assisted
by the feed ramp. The completion of
the auto load is aquisition of full
battery with the slide having moved
fully forward, the recoil assembly is
now relatively relaxed, and we see the
bolt face area impinged on the round's
base to complete the action.
The last 1/4" or so as around goes into
battery and the slide closes, is when
the lug slot then rides over the pin
to lift the chamber area (barrel rear
if you like) once more and go into full
lock-up. This incidentally is why that
lug/pin area needs lubrication.
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