The U.S. Caliber .30 Carbines - Trigger Housing Group

The U.S. Carbine Caliber .30

Trigger Housing Group






Trigger Housing

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    Trigger Housing
    Retaining Pin

Trigger Housings


The first trigger housing used with an M1 Carbine was a modified Winchester M1907 trigger housing used in Winchester's first prototype submission to the Ordnance "light rifle" trials in August 1941. The first design was further modified for Winchester's second prototype, adopted September 30, 1941 as the Carbine Caliber .30, M1.

During the months following the adoption of the Winchester design, both Inland and Winchester produced a number of variations of the trigger housing and other parts as prototypes for testing. Since these housings were prototypes and not used during production they are not covered in detail here.


Example of one of a number of prototype Trigger Housings that preceded the type I housing.

Manufacturing Variations
Throughout Production

At the time contracts were issued to each contractor and subcontractor for manufacturing trigger housings, each of these companies had a variety of different machinery available with which they accomplished this task. As a result, machining variations exist that were specific to a particular contractor or subcontractor. These variations did not affect the design and/or function of the trigger housing specifications as set forth by U.S. Army Ordnance. They were simply different machining methods to produce the same end product.

Several contractors/subcontractors had more than one method of machining the trigger housings, resulting in slight variations from the same manufacturer.

Machining Variations


 Straight CutCurved Cut 

Mag Well Rear Wall
-Front View-


The vertical grooves on the rear wall inside the magazine well need not travel the full height
of the magazine well. This was simply out of necessity due to available machinery.

Mag Well Rear Wall
-Top View-

While the wide vs. narrow rear wall varied throughout production, the opening on the right side of the wall of the
M2 trigger housing was standard for all housings used with a select fire mechanism to accommodate the disconnect lever.

Saginaw Steering Gear, Grand Rapids

The machines used by Irwin-Pedersen to hold the trigger housing in place for machining used holes or partial holes on either side
of the trigger housing. The most obvious location was between the trigger pin and rear of the housing. Sometimes a partial hole of
a quarter circle or less is on top of the magazine well side walls, directly behind the housing's front retaining lugs (black arrow).
Less common is a partial hole on either side of the lug at the rear of the trigger housing (blue arrow).

Saginaw Steering Gear inherited these machines when they took over production at the Irwin-Pedersen Grand Rapids facility.
Trigger housings manufactured by Saginaw Gear at the Grand Rapids facility may or may not have the partial holes in the
sides (blue, white and black arrows).

Location of Manufacturer Markings

The vast majority of trigger housings were marked with the letters assigned to the prime contractor or their subcontractor who manufactured the housing.
Late Winchester trigger housings tend to be stamped lightly. The W may be hard to see or appear absent.

Manufacturer markings are most commonly located on the right side of the
housing below the rear lug. It's not uncommon to find a letter or numbers
elsewhere that had meaning only to the manufacturer. Winchester often
added a letter or number to the bottom of the housing between the safety
and mag catch, the meaning of which was only known to Winchester.
The orientation and size of the manufacturers mark sometimes
varied overtime, most notably on housings manufactured by

The National Postal Meter manufacturers mark is often on the left side
of the housing below the rear lug. The letter was usually accompanied
by a number. The numbers ran sequentially throughout production with
the lower numbers appearing on their earlier trigger housings.
Underwood consistently placed their manufacturers mark centered at
the top of the rear wall of the magazine well. A small ordnance bomb
was usually stamped on the bottom of their trigger guards.

The Trigger Housing "Types"

For the purpose of consistency with the current commonly used terminology the trigger housings shown here are grouped into six different "types". Although these types are numbered sequentially I - VI, several of these types were manufactured and used concurrent to one another.

Five of the six types of trigger housings were machined from a solid steel billet and varied only slightly from the first to the last. Types III, V, and VI were the same milled trigger housing. The difference was one small hole added late in production (type V) and/or the cuts necessary for a select fire mechanism (type VI). These differences are discussed in detail below. The type IV housing was made of stamped steel panels brazed together, making it unique and easy to identify from the others.

All six variations could be used on any M1 and/or M1A1 Carbine, regardless of who manufactured it and/or when. The M2 Carbine and other models converted to select fire required the type VI housing or an earlier housing converted for use with the select fire mechanism.

Not one of the prime contractors and/or their subcontractors manufactured and/or used all six different types of trigger housings. Underwood was the only company to manufacturer and use types I-IV. Saginaw Steering Gear's Saginaw facility manufactured and used the type III housing only. IBM used only the type IV trigger housing. The type V and VI housings were only manufactured by Winchester, Inland and Springfield Armory as all other companies had ceased carbine production by the time the type V housing was introduced.

As all companies served one master, U.S. Army Ordnance, when one contractor's inventory ran short of a particular part it was common for another company to provide parts from their inventory to keep production running. This was referred to as "integration" and fairly common with one part or another. Integration was often documented but not always. War Baby Volume I by Larry Ruth documents many of these occasions.

Usually when a prime contractor received trigger housings from another prime contractor, the original markings were not changed or altered to indicate which contractor actually used the housing.

Type I

Winchester, Inland, Underwood,
Rock-Ola & National Postal Meter

Incorporated December 19, 1941. The rear wall inside the magazine well on type I trigger housings was the narrow style.

The hole in the rear of the trigger housing that accommodated the hammer spring and it's plunger was counter bored/recessed
to facilitate assembly and re-assembly. Inland produced several thousand early type I trigger housings absent this feature
before learning the hard way. Many, but not all, of these early Inland type I housings were later counter bored as shown above.

Type II

Winchester, Inland, Underwood, Rock-Ola, Quality Hardware,
Irwin-Pedersen, Saginaw at Grand Rapids & National Postal Meter

Same design as the type I except for the machining of the metal between the uprights at the front of the trigger housing.

Type IType II

Type III

Winchester, Inland, Underwood, Quality Hardware,
Saginaw at both facilities & Standard Products

Same design as the type II, with the elimination of the angle cuts on the rear of the lug at the back of the trigger housing,
leaving the lug square. Introduced during the summer of 1943.

Type I & IIType III, V, & VI

The type III trigger housing was the only housing manufactured by Saginaw Steering Gear at their Saginaw facility.

Type IV

Underwood, Rock-Ola, Quality Hardware,
Standard Products & IBM

All other trigger housings before and after the type IV were machined from a steel billet. In an effort to simplify the manufacture of the trigger
housings, metal plates shaved to the final dimensions of the trigger housing were copper brazed together then heat treated and case hardened. All
of the type IV trigger housings featured the full length vertical cuts in a wide rear magazine wall. This housing was incorporated May 22, 1943.

The plates and copper brazing are easily observed on the outside of the housing.

The hole for the trigger housing pin was recessed so the pin fit flush with the side of the housing.

Type IV trigger housings were the only trigger housing used by IBM.
Inland participated in their research and development but did not use them in production.

While this process eliminated much of the machining process and cut the cost of manufacture significantly, the requirements for heat treatment and
case hardening after brazing timed with cutbacks in carbine production made this version less preferable than the milled trigger housings.

Type V

Winchester, Inland,
& Springfield Armory

In mid 1944 Inland and Winchester began adding a small hole approximately .06" in diameter on the left side of the trigger housing
just behind the magazine catch. This hole provided access to the rear of the magazine catch spring and plunger. All
other manufacturers had ceased production before this addition. The specific purpose of this hole has not been found in any
documentation. It is believed to have been for cleaning, lubrication and/or drainage.

Type I, II, III, IVType V & VI

The small hole was sometimes added to earlier trigger housings during the post war rebuild process.

Type VI
The M2 Trigger Housing

Winchester, Inland,
& Springfield Armory

Recommended for adoption concurrent with the M2 Carbine September 14, 1944. Produced concurrent to the type V housing which was made for use with the M1 Carbines.

The rear wall inside the magazine well was machined on the right side (black arrow) to allow clearance
for the disconnect lever. The left side was notched (white arrows) to hold the selector spring.
The area below the front of the sear was milled deeper to provide additional clearance between the sear
and hammer as the disconnect pushed and held the sear down for full automatic fire.
Types I, II, III, and V on left, Type VI on right.
The outside of the hole for the trigger housing pin was counter bored to provide clearance for a
weld that attached the trigger housing pin to the disconnect lever.

Modified Trigger Housings
for use with
Select Fire Carbines

After the introduction of the M2 Carbine many semi-auto M1 Carbines were converted to select fire without changing the M1 to an M2. Their trigger housings were modified to function with the select-fire Disconnect Lever and Selector Spring but are absent the counter bored trigger pin hole of the M2 trigger housing. Those modified retained their earlier features. The small hole on the left side of the trigger housing was sometimes added to the earlier trigger housings during the post war rebuild process but was not necessary for select fire operation.

The design and construction of the type IV trigger housings (welded & brazed) was inherently unreliable for conversion for use with select fire parts, although a few have been found to have been converted for use with these parts.

Note the absence of the small hole, indicating this was a type III trigger housing later converted for use with a select fire carbine.

Trigger Housing Retaining Pin

Length: .875" - Diameter: .177" - Head Diameter: .2"

The first trigger housing pin utilized a spring to retain the pin within the
right side of the trigger housing when the housing was removed from the receiver.

Receivers designed for use with this pin had a milled recess in the right side
of the trigger housing lug so the pin would clear the lug during disassembly.

The early spring loaded pin was replaced in early 1943 by a simplified trigger housing pin used throughout the majority of production.

Typically the fit of this pin is slightly loose. It is held in place by the stock when the carbine is assembled.

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