The only parts not interchangeable with their GI counterparts were those parts specific to a particular model, the rubber buttplate they used starting about 1966, and the Phillips head recoil plate screw with it's matching escutcheon nut.

All metal non-GI parts, with the exception of the barrels, were manufactured using investment casting, a technology that by the 1960's had progressed to a degree not available when the WWII GI carbine parts were made.

Barrel Group

The threads on the breech end of the barrels used by Plainfield Machine are compatible with the GI receivers. GI barrels may be used as replacements for the barrels used by Plainfield Machine. Plainfield Machine Co. did not use surplus GI barrels. If a Plainfield Machine carbine has a surplus GI barrel it was added after the carbine was sold by Plainfield Machine.

The first barrels used by Plainfield Machine were made using a sleeve that included the barrel threading and gas piston housing, into which was inserted surplus Springfield 1903/1903A3 barrel machined to carbine dimensions (example s/n B311). For further on this practice refer to National Ordnance. While this may sound like a poor method of making barrels, the end result has proven otherwise. Next to surplus GI receivers, barrels were the one part most difficult to obtain in the early years as no one was making them. 1903 barrels had four lands and grooves, 1903A3 barrels had either four or two lands and grooves.

About 1962 Plainfield switched to acquiring their barrels from Small Arms Manufacturing of Bridgeville, PA, predecessor to the current barrel manufacturer E.R. Shaw. The first barrels 1962-1963 were 12 groove, followed by 6 groove barrels 1964-1969, then the standard 4 grooves common to the GI M1 Carbine barrels in the 1970's.

Small Arms Manufacturing .30 carbine barrels were also used by Universal Firearms and National Ordnance. It appears these companies purchased barrel blanks onto which each attached their own gas piston housing and finish reamed the chamber. The gas piston and/or the manner in which it was attached to the barrel was different for each of these companies.

The gas piston housing used by Plainfield Machine throughout production (and Iver Johnson 1978-1982) was slightly inset into the barrel and brazed in place.

The angular cut of the four sides of the barrel between the receiver and forward of the gas piston housing
evolved over time into a smaller angular cut most noticeable just forward of the gas piston housing.

By 1966 the Plainfield logo appeared on the underside of the barrel on the flat area between the receiver and gas piston.
This does not appear to have been done consistently throughout 1966-1978.

Plainfield Machine Front Sight Location

The first couple inches at the front end of the M1 Carbine barrels are milled slightly smaller than the rest of the barrel, for mounting the front sight.
Also consistent throughout Plainfield's production was the location of the front sight along this smaller diameter. The M1 Carbine front sights engage
a key hole and are secured in place by the pin in the front sight. Plainfield consistently placed the key hole cutout at the rear of this smaller diameter
mounting the sight at the rear of this area. Other carbine front sights are normally mounted midway along this smaller diameter.

Normal M1 Carbine front sight location

Plainfield used surplus GI barrel bands having a bayonet lug as long as they could obtain them. About 1966 they switched to using their own barrel band, which had no bayonet lug (depicted above).

Front Sight on Sporterized Models

Slide

Surplus GI slides were used well into the 1960's. Refer to USCarbineCal30.com for further information on surplus GI parts. When supplies dwindled, Plainfield switched to a cast slide. Some of their cast slides have the PMC logo on the bottom of the body of the slide, some were left unmarked.

Stock Group

Plainfield Machine Co. did not use surplus GI stocks and/or handguards. If a Plainfield Machine carbine has a surplus GI stock and/or handguard it was added after the carbine was sold by Plainfield Machine.

In addition to the stocks on their carbines, Plainfield also offered individual stocks for sale.

According to a son of William Haas stocks were initially manufactured by Plainfield using walnut. After the first couple years they were outsourced to a contractor in New Jersey.

The PS-1 (slinghole) and PS-2 (no slinghole) Military model stocks were made using American Walnut until 1966. By 1968 they were made from "hardwood with a military type oil finish". Paratrooper and sporterized models came with stocks made from American walnut. Hardwoods commonly used for manufacturing rifle stocks include birch, beech, cherry, mahogany, and walnut. Most commercial carbine manufacturers used whatever wood was most affordable and available at any given time. Plainfield obviously used walnut and birch, but appears to have also used beech (a lighter wood), and possibly even cherry.

Beech Wood Model PS-1 Stock

Walnut Model PS-1 Stock

Walnut Model PS-2 Stock

Walnut Sporterized Stock

Stock manufacturer S.E. Overton in Michigan supplied Plainfield Machine with the following stocks [War Baby II by Larry Ruth, p. 751]
Date	Quantity	Style	Misc.
July 23, 1968	5000	M2	Birch
Jan 7, 1969	5000	M2	Birch
	200	M1	no sling aperture
Apr 22, 1971	3000	M2
	2500	Standard Military
	500	Sporters	no sling aperture
	500	handguards	no sling aperture

Ventilated metal handguards became standard on the military version by 1965. The number, orientation, and shape of the holes
were started by Plainfield and used by Iver Johnson, IAI, and currently Auto Ordnance and after market manufacturers.

About 1966 Plainfield replaced the GI style recoil plate screw with a Phillips head screw. Threading of
this screw and the escutcheon nut the screw was mated too were not compatible with their GI counterparts.

About 1966 Plainfield replaced the GI style buttplate with a
rubber buttplate held in place by two Phillips head screws.

Rear Sight

Surplus GI rear sights were used throughout most of the 1960's. Refer to USCarbineCal30.com for further information on surplus GI parts.

In the late 1960's Plainfield began using their own rear sight. At some point they added their initials to the left side of the sight. The Plainfield rear sight is interchangeable with it's GI equivalents.

Trigger Housing Group

Plainfield used surplus GI trigger housings and parts as long as they could obtain them. Refer to USCarbineCal30.com for further information on surplus GI parts.

In the late 1960's Plainfield switched to using a cast aluminum trigger housing similar to those used by Universal early in their production. Injection mold marks can be observed on the left side of the trigger housing. It appears they used several different casting molds in the 1970's. These may be replaced by a GI trigger housing. All parts within the Plainfield trigger housing are interchangeable with their GI counterparts with the exception of the hammer pin used with the aluminum trigger housings. If a GI trigger housing is used to replace the aluminum trigger housing the hammer pin will also need to be replaced.

The only part within the trigger housing group that has been found to have a marking
specific to Plainfield Machine were some, but not the majority, of the rotary safeties.

Part I	Part II	Part III	Part IV	Part V
History of Plainfield Machine Co. 1951-1978	Serial Numbers & Dates of Manufacture	Models & Flier/Manual Downloads	Receivers & Markings	Parts & Markings