The U.S. Caliber .30 Carbines - Parts

The U.S. Carbine Caliber .30

Bolt Group






Trigger Housing

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The Flat Bolt

The Flat Bolt was used
from the beginning of production.
By mid 1942 a web (black arrow) was added under
extractor to strengthen the right bolt lug.
The web had been absent prior.

Throughout production bolts were given a
punch mark on top of the bolt when the
carbine it was on successfully passed
test firing. This was equivalent to a
proof mark. On the example at right the
punch mark can be observed between the
bolt lugs (white arrow).

The rear of the bolt and face of the hammer were designed to prevent the hammer from contacting
the firing pin (A) until the bolt rotated into the locked position and was ready for firing (B).
The rear of the bolt and face of the hammer were hardened to operate as a matched set.

The rear of the commercial bolts made by many different manufacturers post WWII were/are not hardened to
GI specifications. Use of a commercial hammer or bolt with a GI hammer or bolt is not recommended.
If the two are intermingled these surfaces should be inspected regularly as wear or damage in these
areas can remove this important safety feature.

The Round Bolt

On November 18, 1943 U.S. Army Ordnance standardized a number of design changes to various parts for the U.S. Carbines. These changes included five changes to the bolt design and changes to the slide in relation to how it interacted with the bolt (refer to the type V slide with part number 7160091). The most obvious change to the bolt being the elimination of the lightening cut along the top of the bolt that changed the bolt design to round instead of flat, this bolt has become commonly known as "the round bolt".

Contrary to widely circulated rumors, the round bolt was not specifically
designed for use with the select-fire Model M2 Carbine. It was an evolution
of the bolt for all U.S. Carbines, cal. 30. When the Model M2 Carbine was
later introduced the round bolt had already been in use with production of
the Model M1 Carbine and was simply the standard when the M2 Carbine
production began. Either bolt was acceptable for use in either model
during Ordnance inspection and rebuild operations.

The change in the shape of the bolt being the most obvious has overshadowed the other four changes to the bolt design that accompanied it's change in shape. Several of these changes were very small and difficult to observe, in some cases requiring a micrometer to tell the difference. Collectively these changes were meant to increase the safety and reliability of the carbine bolt. Some of these changes may have preceded the date of the Ordnance announcement at several of the prime contractors.

The Design Changes in their Historical Perspective

While these changes were simply the evolution of the bolt design and the details below are normally not included on this website of carbine basics, when placed within their historical context these changes present an excellent example of what can happen when introducing multiple design changes into mass production lines at ten different facilities at their peak of production. The result being some of these design changes were implemented at different times by different manufacturers with several contractors unwilling or unable to make the changes. With researchers and collectors many years later trying to make sense of what was done, why, when and by whom.

During the month of November 1943 (when these particular bolt changes were standardized) M1 Carbine production peaked with 528,765 M1 Carbines manufactured by nine prime contractors at ten separate facilities. This was an average of 17,625.5 carbines a day. Rock-Ola produced the least with 22,754 (average 758.46 a day). Inland produced the most with 121,851 (average 4,061.7 a day). Add to this each prime contractor and/or their subcontractors were required to produce parts for use as spares for carbines needing a replacement in the future. The lead time required to have the bolts ready for assembly into a carbine varied from one manufacturer to the next.

During the same month Ordnance had been discussing the end of production for several prime contractors in the near future. The contractors were aware of these discussions and that it had yet to be decided which contractors.

The Design Changes

Elimination of the lightening cut to increase the bearing surface between the bolt and receiver.
This also eliminated one step of over fifty required to manufacture the bolt. The overall
length of the bolt was increased by .022" to help prevent out of battery firing. The bolt
body itself accounted for .012" of the change. The remaining .010" was added to the skirt
at the rear of the bolt (see below).

Carbine bolts have a ramp that engages the cartridge in
the top of the magazine as the bolt moves forward (above).

The lead-in angle of this ramp was slightly increased and appears as a slightly larger "hump".

Flat Bolt
Round Bolt

A 1-2 degree angle cut at the rear of the right bolt lug where
it engaged the body, to strengthen the lug.

Flat Bolt
Round Bolt

The addition of .10" to the length of the skirt on the rear of the bolt (upper arrows).
A redesigned cam surface on the rear of the bolt that engaged and cocked the hammer
(lower arrows).

The width of the right lug was decreased slightly to contribute to the increase in dwell time before the locked bolt was engaged by the slide and moved
rearward, extracting and ejecting the spent casing. This change coincided with changes in the slide design that served the same purpose. Ordnance
documents found to date have not described how this decrease in lug width was to be accomplished.
Examination of the bolts has revealed two basic right bolt lug designs:
  • Those having a curved/flat tip on the front of the right bolt lug, shown in the series of photos in the column on the left below.
    The machining of the curve varied slightly from one manufacturer to the next. Some have more of a curve than others. Some have a slight edge at the
    front towards the top before curving down and to the rear. But all have the same basic shape and width. While some of the manufacturers machined or
    left a slight edge towards the top of the curved/flat bolt these are distinctively different than those that follow.

  • Those having a pointed tip on the front center of the right bolt lug, shown in the photos in the column on the right below. These are also
    consistent in shape and size. Post WWII Ordnance drawings of the round bolt indicate this lug design.

Bolt lugs with the pointed tip have a width approximately .05" less than those with the curved/flat tip. This decrease in width is consistent with
the design changes that accompanied the round bolt.

The majority of flat bolts have the rounded/flat tip, though flat bolts with the pointed tip are not uncommon. Time wise these were manufactured
concurrent to one another, including by the same manufacturer. There are several theories as to how this happened but they are only theories.

Curved/Flat Right Bolt LugPointed Right Bolt Lug


Blue arrow indicates the location of the decrease in width by .05" +/-

Note the slight edge towards the top of this curved/flat bolt

Flat bolt with pointed lug

Front edges varied slightly from one manufacturer to the next

Round bolt with pointed lug

The End Result

  • Round bolts were not manufactured by or for Standard Products, National Postal Meter or the Saginaw facility in Grand Rapids, MI.

  • Flat bolts manufactured by National Postal Meter having the pointed lug design appear to have been manufactured in lieu of the round bolt and are absent the other design changes.

  • Round bolts were manufactured by or for Winchester, Inland, Underwood, Rock-Ola, Quality Hardware, IBM and the Saginaw facility in Saginaw, MI

  • Winchester, Inland, & Saginaw in Saginaw, MI: all round bolts have the pointed right bolt lug and all other design changes

  • Underwood: a small percentage of round bolts have the curved right bolt lug with all other design changes. The majority have the pointed right bolt lug and all other design changes

  • Different manufacturers integrated their round bolts into their carbines at different times between December 1943 and March 1944.

  • All prime contractors other than Winchester and Inland had ceased all carbine production by May 1944 (6 months after the announcement of the design changes that accompanied the round bolt).

Ordnance documentation has been found that the presence of an X on top of a round bolt approximately 1/2"
behind the lugs indicates the bolt does not include all of the design changes that were accompanied the
elimination of the lightening cut that produced the round bolt.

Underwood was contracted post WWII to manufacture round bolts as replacement parts. These bolts featured
the pointed right lug, all design changes and added a drainage port under the hole for the firing pin.

What difference does it make?

One of the great things about the .30 caliber Carbines is all parts were interchangeable regardless of who made them or when. Changes in parts designs were made to cut production costs and time, improve reliability and/or function, and/or to increase safety. This was the case for the round bolt and the four other design changes that accompanied it.

While any bolt will work with any slide in any carbine, the best performance, reliability and safety can be expected by using a round bolt with all of the design changes in conjunction with a type V slide marked 7160091 on the bottom or the type VI slide.

This information may also give you insight into just one example of many that you'll be in for should you decide to get seriously involved in collecting these carbines. Consider yourself forewarned, these war babies can be addictive.

Bolt Breakage

While most GI bolts (flat and round) have a very long lifespan, if a bolt is going to crack and/or break
the most likely place it will happen is the right bolt lug under and aft of the extractor.

The most common reason for this type of crack/break is related to the forces exerted on the right bolt lug by the slide.
Any interference with the movement of the left bolt lug, such as dirt, grime and/or burrs in the receiver will place
stress on the right bolt lug. This can also occur with anything that impedes the movement of the right bolt lug

Improper headspace and/or "hot" reloads can be another reason for these cracks/breaks.

Firing Pins

Firing pin design underwent changes during production resulting in three types of firing pins. These changes were to the rear of the firing pin and easiest to observe by viewing the firing pin from the rear.

The type I firing pin was in use from the beginning of production. On October 11, 1943 U.S. Army Ordnance Springfield Armory changed the design of the rear of the type I firing pin to the type III firing pin to help prevent the hammer from hitting the firing pin when the bolt was not rotated and locked. Type I firing pins already in service were to be modified as carbines were inspected by Ordnance personnel by machining a simple angle with compound cut on the left side of the firing pin at the rear, resulting in the type II firing pin.

Modification or replacement of the type 1 firing pin was mandatory during Ordnance rebuild operations post WWII.

The purpose of the tang on the rear of all firing pins is to engage a cut in the rear of the receiver bridge
that held the firing pin back until the bolt was fully rotated and locked. Along with the design of the rear
of the bolt and face of the hammer this was intended to prevent out-of-battery discharges. The changes
producing the type II and type III firing pins removed part of the left rear of the firing pins to
distance them from the impact of the hammer.


Design changes to the Extractor during production resulting in three types of extractors. These changes were to the opening (black arrow)
and lip engaged by the extractor plunger (white arrows). While the type I extractor can be identified by examination of the extractor while
assembled inside the bolt. Identification of the type II versus type III extractors usually requires disassembly of the bolt and removal of the extractor.

Type 1
Type IIType III

The type I extractor was used from the beginning of production. The small notch engaged by the extractor plunger did not reliably retain the extractor within the bolt, occasionally allowing the extractor to be thrown from the bolt by the centrifugal force of bolt rotation. During February 1943 Ordnance issued directions to change the design to the type III extractor with type I extractors already in service to be modified to incorporate the lip and notch design similar to the type III extractors. The latter produced what is referred to as the type II extractors. In March 1943 Ordnance reported these changes were already in effect at Inland, Winchester, and Underwood. The type III extractor remained in use through the end of production and beyond.

Extractor Plungers & Spring

The extractor spring and plunger sit within a hole inside the right bolt lug (arrow).
The ejector sits within the hole in the bolt face (bottom right).

The spring remained unchanged throughout production. The plunger underwent a redesign early in production.

The type I plunger was used from the beginning of production in conjunction with the type I extractor. When the
type I extractor was redesigned the plunger was also redesigned to better engage the lip of the extractor.
The type II plunger was used throughout production. Ordnance documents indicate that the type II plunger
was to have a shorter shank than the type I plunger. Generally the shank on the type I plunger is longer
than that of the type II, however, short shank type I and long shank type II plungers are occasionally

The extractor spring and plunger force the extractor towards the casing while
allowing enough spring compression for the extractor to move over and around the case rim as the bolt moves forward.
Note the orientation of the type II plunger to facilitate proper engagement of the lip inside the extractor.

A Word of Caution!
While it's possible to disassemble and reassemble the bolt group without a bolt tool (see below), depressing the ejector
while inserting the plunger and spring, firing pin, and keeping everything aligned to insert the extractor can present
a frustrating challenge. The plunger can be an eye hazard if allowed to launch while under spring tension, and good luck finding it.

Ejectors & Spring

The ejector spring remained unchanged throughout production. One end of the spring was tapered to fit the protrusion at the rear of the ejector.
There were four variations of the ejector. Each differed in the angles at the front of the ejector in an effort to improve ejection.

Used at the beginning of production. Featured three bevel cuts on it's face. The bevel facing towards the bolt face often varies in size. Used sporadically in 1943 concurrent to the early ejector. Two bevel cuts forming a wedge shape. Flat face with a 45 degree bevel cut around the edge of the face. Flat face with a 360 degree conical shape. Likely made by a few manufacturers in lieu of the 45 degree bevel cut as opposed to a new design.

Bolt Tool

Disassembly and reassembly of the bolt group is faster and much easier when using this tool. Authentic originals
are available but more expensive than commercial reproductions. Either can get the job done.

Turn the screw until it is fully extended outward. The bolt is inserted into the tool from the rear, aligning the pin inside the front
of the tool with the ejector and rotating the solid lever into position between the extractor and extractor plunger.
Do not use the forked lever for disassembly as one of the prongs may break. It will be used during reassembly.
Holding the bolt and tool together with one hand while turning the screw with the other will force the the extractor/spring
assembly down into its recess as the bolt moves forward. The extractor can now be removed, followed by the firing pin.

Before rotating the screw out to release pressure on the plunger, cup one hand around the front of the bolt/tool
to catch the plunger if it pops out while backing the screw out. Once the screw is backed out the plunger and it's spring
along with the ejector and it's spring can be removed. It's a good idea to sit the plunger and it's spring in a location
where they will not roll or be bumped as they are easily lost. A magnet or magnetic tray is very helpful for retaining
small parts.

Reassembly is started by inserting the ejector and spring. The ejector should be rotated
into the proper position that will allow the extractor to be inserted in it's hole without being impeded.

Insert the plunger and its spring into the recess of the bolt with the plunger oriented with the cutout for the extractor lip
towards the bottom of the bolt. Swivel the forked end of the lever to engage the plunger to hold it and it's spring in place.

Turning the screw to force the bolt fully forward within the tool will cause the lever to push the plunger (arrow)
into it's hole. Insert the firing pin, then the extractor. The screw can then be rotated out and the assembled bolt removed.

If the forked end of the double ended lever breaks, replacements are usually available without having to replace the entire tool.

Should you have questions, assistance is available on our Discussion Forum.

The Discussion Forum also serves as a reference desk for the more advanced material that could easily overwhelm a website and is often subject to opinions that may vary
due to a lack of original documentation. A number of researchers and authors are present on the forums, helping others and seeking information for various research projects.