The Design Changes in their Historical Perspective

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 Lug	Pointed 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 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.

Extractors

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 II	Type 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
observed.

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.