CMC 3.5 LB. Drop-In Trigger – Curved – For Small Pin .154″ Diameter Only.


SKU: CMC-91501 Categories: Brand:


CMC curved-bow drop-in fire control group for AR15-pattern rifles.

  • Self-contained drop-in assembly for easy installation
  • High-quality match grade trigger with 3.5lb pull weight
  • Contained in 410 stainless steel house
  • Includes oversized hammer and trigger pins and clip retainers
  • For small-pin (.154″) lowers only
  • Not recommended for use with 5.45×39 rifles
  • Also available with straight trigger bow.

Additional information

Weight 0.25 lbs
Dimensions 8 x 4 x 1 in

Detailed Information

Combat Reliability & Service Life

The newest
generation of CMC drop-in AR15 trigger modules incorporate the following
features: Same Reliability and Service Life as Traditional AR15 Triggers – The
most important features of the CMC trigger group is the fact that the primary
design objectives and resulting functions are made to be enhancements to the
original design of the traditional AR15 trigger group, and specifically not to
be compromises. The module box that contains the CMC trigger group is designed
to keep side to side wall thickness as close to standard as possible. This means
that if build ups of grease, grime, sand, powder residue, etc. occur the
internal parts still have the clearances to continue to function as the factory
standard designs would function under these conditions. Had the internal
geometry been closed in around the moving parts to center and/or contain the
internal parts it wouldn’t take much to contaminate the parts, lose clearances
and result in malfunctions. The stainless steel sheet metal module is a very
high tech, close tolerance, and critical component with +/- .001” tolerances in
areas critical to function. A very high quality component that is not easy or
cheap to make.

1.  There
are no screws in this design

 No matter
what methods are used to secure them, screws can and will come loose and we have
done everything possible to avoid Murphy’s laws. And we all know that when
Murphy’s laws strike, they will normally occur at the worst possible times. So
not only would the use of screws for attaching the trigger module to the
receiver or to lock some part in place offer no benefit, they would be very
counterproductive to the design objectives. Nor would screws secured with
adhesives add to the effectiveness of the mechanism. The CMC design purposely
does not rely on any screws or adhesives that could in any way could allow some
part to loosen and cause a failure.  

2. There
is no adjustability in this design

If some
well-meaning, unqualified human can tamper with a perfectly functioning
mechanism, they will. We feel that we can’t allow that for people who put a
premium on reliability and the huge majority who understand the disadvantages
wouldn’t tolerate it anyway. Adjustability means an additional part or parts
would have to move in addition to the components that absolutely must have
movement. Clearly adjustable features would offer no benefit to reliability, but
could definitely compromise reliability. Furthermore, do to the nature of the
design adjustability is not necessary. 

Why is
adjustability not necessary? The only valid reason we can see that adjustability
would serve any purpose toward reliability in an AR15 rifles is to ease and
reduce the time during the installation of the old style trigger groups, where
you begin with a bunch of loose parts. You see, the fact is, every different
brand of AR15 rifle receiver is more than likely built to a different set of
nominal dimensions and tolerances, even when based on military specifications.
To add to the problem, many AR15 rifles are reverse engineered without any
reference to the military prints. To make a precision set of individual parts to
one recipe (the same geometry for every respective part) that would function
reliably in every different brand of AR15 is clearly and obviously unlikely.
This is surely why we don’t see a complete set of individual parts that will
drop-in, without any fitting required, being offered for sale. 

So if you
ignore the reliability factor and Murphy’s laws, adjustability in a in a loose
set of trigger group parts might be useful. But obviously, to maximize
reliability its best to have a set of non-adjustable trigger group parts
installed by a qualified, experienced gunsmith who can fit, file, hone, check
for safe and proper function, and then keep that group of trigger parts
dedicated to the rifle they were fit to, exclusively.

3. Consistency
Is Critical To Reliability 

Who is a
qualified AR15 trigger group gunsmith And who isn’t? How do you determine who is
and who isn’t? Leaving the answers to those questions up to the so called
“experts,” with the CMC drop-in AR15 trigger you are buying consistency that
narrows the only remaining proficient gunsmithing questions down to what AR15
gunsmith is qualified to insert two pins and proficiently determine the results
have yielded safe and proper function. If you want to take any given trigger
module out of one rifle and move it into another, and then again to another
rifle, and so on, the same level of AR15 gunsmithing proficiency apply; who can
proficiently insert two pins and determine the results have yielded safe and
proper function. The lower receiver has to be pretty far outside of the military
specification geometry for the CMC drop-in AR15 trigger module not to fit and/or
yield safe and proper function.  

Tactical Grade Quality & Trigger Pull 

We believe
the necessary elements to match precise trigger control with a fire control
system that will dependably function to up to the highest standards are
experience, knowledge, state-of-the-art design, testing, very close tolerances,
precision manufacturing and quality control. After years of producing and
improving processes, and geometry; the newest generation of CMC drop-in AR15
trigger modules have brought every element of what yields a Tactical Grade
quality up to the most demanding levels. 

Trigger Pull Dynamics 

A clean,
crisp trigger release and 3.5 pound nominal pull weight. Key elements to any
trigger group that allows the shooter’s performance to meet the potential of the
firearms. CMC’s drop-in AR15 trigger modules are factory set to a nominal
trigger pull weight of 3.5 pounds. To most people, a truly clean, crisp breaking
trigger pull set at 3.5 lbs. seems much lighter. If you know anything about the
outstanding marksmanship demonstrated at the Camp Perry Championships by
shooters using a minimum 4.5 lb. trigger then you will not have any doubts about
what is possible with a 3.5lb trigger pull.  

Over-Travel & Take-up 

elements are critical to the overall performance of a world class shooter, but
are also important to any shooter who wants to demonstrate precision shooting.
Unlike traditional AR15 trigger groups one of the most spectacular elements of
the CMC AR15 drop-in trigger modules is that match grade over-travel and take-up
can be accomplished with geometry and without the need for screws or other
moving parts. Sure, some people have different preferences, but due to the many
years CMC’s trigger modules have been in production and the many thousands sold,
customer feedback has made us very confident that the design of these products
is such that take-up and over-travel is right in the sweet spot for both. And it
wont change because it’s done with geometry on not moving parts that can come
loose (usually at the worst time).

 1.  Every
component of the CMC drop-in trigger modules maintains tolerances of +/- .001”
(one thousandths of an inch) or less wherever two critical surfaces engage or
interface. The same tolerances apply to each and every hole location.  

2.  Surface
finish is critical for obtaining that magic “feel” that is absolutely necessary
with a Match Grade Quality trigger group. It’s not easy and takes of a lot of
time to develop an effective, precision tooling set up for producing mirror
finishes on the components with critical surfaces as are incorporated in the CMC
trigger modules.


important is reliability to you? CMC’s drop-in AR15 trigger modules get their
outstanding combat reliability and service life, and match grade quality &
trigger pull without compromises like reduced sear/hammer engagement or using
reduced power springs. The cheap, easy ways to reduce trigger pull weights are
by minimal sear/hammer engagement and reducing spring rates on the trigger
return spring, and the hammer spring. But CMC’s way of thinking is that the
cheap, easy methods are very counterproductive to the objectives for this
design. For one, reduced spring forces increase lock time (the time from when
the trigger releases the hammer until the hammer strikes the firing pin) which
reduces accuracy potential. Second, reduced spring forces can cause trigger
groups to fail much easier in grimy, dusty and/or fouled conditions. Third,
reduced spring force on the hammer causes light strikes of the firing pin on the
cartridge primer. The potential for light strikes has two problems that are
especially concerning in AR15s; failure to discharge the round is the obvious
problem, but also, if the rifle is fired a lot without cleaning (especially in
nasty environments) fouling can cause light strikes to occur long before they
would have with full power springs. Also, for those of us that like to go to the
range and practice or shoot a lot its nice to be able to reliably use
inexpensive military surplus ammunition; which can be done with the CMC trigger
groups. However, for those of us that also like to do serious precision shooting
with high quality ammunition its nice to know that you have a great match grade
trigger pull that facilitates precision with very fast lock time. What are the
benefits of settling for less? 

– The amount of engage of the sear tip to the hammer cock notch is extremely
critical for reliable function under varying conditions. The cheap and easy way
to obtain reduced trigger pull weights is by reducing engagement to bare
minimums. Maybe that’s OK for bench rest rifles, match only rifles or rifles
that are always kept clean and carefully handled, and reliability is not of
grave concerns. However, the CMC drop-in AR15 trigger modules are designed to be
well suited to all aforementioned usages without any compromises to reliability
under all conditions of usage.

Springs –
Another one of the cheap and easy ways to obtain reduced trigger pull weights is
with reduced power springs. The springs used in CMC’s drop-in AR15 trigger
modules are custom made using the finest music wire spring materials available
and are spec’d out to replicate the forces of the springs used in military
weapons. This means reliable discharge with inexpensive military surplus ammo,
faster hammer lock time to maximize accuracy, and longer lasting reliability in
various environments.

Finishing – This was discussed previously, but the subject also belongs in this
area. Reducing sear/hammer engagement and reducing spring forces to allow for
lighter trigger pull weights both reduce the requirements for superior surface
finishing on engagement surfaces. With the full force springs like CMC uses;
lighter, crisp, clean trigger pull is unobtainable without extremely smooth,
consistent, highly polished surface finishes on all engagement surfaces. It’s
not the cheap, easy way, but it’s the best way from CMC’s point of view.

Materials –
CMC uses only the highest grade materials and manufacturing processes available.
Each one, the newest materials we are incorporating, the heat treating
specifications and some unique and additional processes not used before in these
applications, are the results of intensive studies by a team of highly qualified
engineers. No, we won’t say exactly what makes all these elements proprietary or
others would surely copy us as they have in the past. But we will tell you this:

Extreme Testing – To validate the engineering and design of the newest
generation of CMC trigger modules, we implemented an extreme testing protocol.
We commissioned an engineering firm to build a machine that replicates the cycle
of an AR action. Using this machine, the carefully documented tests include
25,000+ round cycle testing of multiple groups, analyzing each component part
and its critical dimensions several times throughout the course of the test.
Then the same parts used in the 25,000 round testing, were live fire tested for
500 more cycles and checked again for failures. No cracks, fractures or failures
of any kind were found. Therefore, every characteristic of the triggers groups
undergo hundreds of thousands of rounds of cycle testing to ensure reliability
before we ever release a design update. 


Which AR15
Rifles Are Compatible – The CMC drop-in AR15 trigger modules are designed to fit
any lower receiver manufactured to be within the standard military
specifications for nominal dimensions and tolerances, except Colts ® with sear
blocks. CMC does offer models of the drop-in AR15 trigger modules that will fit
Colt ® brand rifles with larger (.171” diameter) than standard retaining pin
diameters. It’s always recommended to have an experienced and qualified AR15
gunsmith to be the one to qualify safe and proper function of the trigger