I originally posted this on a different forum, but it got emailed around enough that someone actually emailed it back to me! Good reading, I will go through and clean it up later -- tacos wait right now!
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In order to understand this, there are a few things
about the functioning of the AR that have to be
defined, I am away from most of my notes and stuff, so
most of the figures given are from memory... but
should be pretty close. For this description, the
standard rifle gas system is with the gas port located
at 13.0" and having a 20" barrel... the standard
carbine system is with the port at 7.5" and a barrel
of 14.5"
The pressures at the gas ports are: 13.5K for the
rifle and 26K for the carbine -- or twice as much.
The dwell time (the time that the gas system is
charged with high pressure) is determined by the
amount of barrel after the gas port. These are nearly
identical between the rifle and the carbine.
Pressure from the port is regulated only by the size
of the gas port and the diameter of the barrel.
These two factors determine the internal bolt
pressure, the maximum pressure that is obtained in the
bolt carrier/piston combination -- for the rifle this
pressure is about 1000psi and for the carbine it is
over 1500psi, half again as much.
When the rifle is fired, primer shot sets the bullet
forward until it contacts the rifling, at this point
the powder charge ignites and sets the shell case
fully back, binds the action and start to propel the
bullet. The bullet jumps slightly again and is etched
by the rifling... it stops again very briefly as the
pressures build to a point for the bullet to overcome
the mechanical advantage of the rifling twist and the
! bullet starts to spin, at this point the chamber
pressure is at max, 50K plus (there are some that
believe there is another, third stop the bullet makes
and some testing suggest this may be true).
As the chamber pressures start to climb, the brass
case expands and becomes plastic, this is essential to
seal the case in the chamber -- the correct term for
this is Obturation, when the case is obturated and
sealed, it is stuck in the chamber, practically welded
in really.
The Lock Time, or the time that the action remains
locked with no attempt to start unlocking is very
important... on the rifle, the lock time is about 550
microseconds, the lock time for the carbine is about
375 microseconds -- this may not seem like much, but
it is much shorter of a time, also keep mind that the
chamber pressures are twice as high in the carbine
when the unlocking starts.
What does all of this mean? When the carbine is fired,
the system attempts to unlock earlier than intended
and while the case is still fully obtucated... this
results in the action bind delaying the unlocking and
stressing the system. As the 5.56N is not drastically
tapered, "squirting" is not a big problem in most
guns. When the internal bolt pressures finally unlock
the bolt, the velocity of the reward movement in the
carbine is much higher than what the rifle was
designed for, it also must start extraction of the
obtucated case... as you know, the AR does not have
any sort of initial extraction, perhaps the single
biggest shortcoming of the design. This has been known
to cause ripped case heads...
At this point, as the bolt start to unlock, it is
rotated to unlock... due to much higher velocities
with the carbine, the rotation of the bolt creates
some centrifical force and helps to "float" the extractor...
the extractor on the AR is not balanced and the
forward part of it weighs more -- some argue that the
pressure of the extracted cartridge case keeps the
case head against the bolt face and test have shown
that the extractor does not generate enough force to
actually life from the case, but the fact is that the
extractor does float enough to negate some of the
spring pressure, and the contact with the case
rim becomes "soft". For this reason, it is much more
likely that the extractor will simply pop off, rather
than actually rip the case.
Balanced extractors and different designs have been
developed (LMT), but the best solution to date has
been stronger extractor springs and spring buffers.
That about covers the FTE issues...
Back to bolt velocity. The high speed of the bolt has
a couple of other detrimental effects, one of the most
common is that the bolt is cycled so fast that as it
returns to battery, it actually has enough force to
"bounce" off of the barrel extension when closing and
locking... this bounce back is very small, but can be
enough to cause the weapon not to fire... this "bolt
bounce" is pretty well known.
One other problem is that the bolt can cycle so fast
the magazine spring can not keep up with it and the
round stack is not properly aligned and forced back
into place before the bolt returns to batter --
therefore there is no new cartridge picked up and the
bolt closes on an empty chamber, this is what some
call "ghost loading", or bolt-over-base jams... this
is far worse in full auto fire as the bolt does
actually move faster in full auto than semi auto; this
is due to the fact that the top cartridge in the
magazine does not apply force to the bottom to the
bolt causing drag.
The common solution to this issue is to use a stronger
recoil spring and a heavier buffer... this works, but
is treating the symptom, not the problem.
PigTail and expansion chamber gas tubes attempt to
fool the rifle into thinking that the gas port is,
located further away that it really is, but they are
not as good of a solution as actually moving the gas
port out...
I guess that about covers it for a quick rundown, of
course all of this is not nearly as simple as it
sounds.
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Originally Posted by K.L. Davis
Re: Detail description of the cycle
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