IDENTIFYING TRUNNION ASSEMBLIES FOR AIR COOLED
GROUND CALIBER .30 BROWNING MACHINE GUNS
In this installment of parts identification for the M1919A4 and its relatives we are going to start by taking a look at the air cooled, ground Cal.30 BMG’s family tree.
All the members of this family descend from the M1917 water cooled “Heavy”, at least from the front of the casing assembly back.
Development of this weapon required that the trunnion block used on the parent weapon, the M1917 water cooled, be redesigned.
The M1917’s water jacket screwed onto male threads cut on the trunnion.
This threaded area was about 4.25 inches in diameter. Far too large for the air cooled barrel jacket that was necessary to support the front barrel bearing of the M1919.
The trunnion was redesigned for a barrel jacket of about 1.78 inches in diameter.
One feature of the M1917 trunnion block carried over in the redesign of the M1919 tank gun was a provision for a “Cartridge Bunter Plate”, a hardened steel rectangle inserted into to the trunnion block at the forward end of the feed way. Apparently the purpose of this plate was to prevent wear to the trunnion by the tips of the projectiles as they passed by during firing.
In 1930 when the Cavalry decided that they needed an automatic weapon better suited to their needs than the M1922 Browning Machine Rifle which was a modified M1918 BAR.
They obtained M1919 Tank BMG’s added different sights, and improved the emergency tripod (“Tripod, Dismounted”) that was supplied with M1919 tank guns for use when the weapon was removed from the tank.
The Cavalry tripod improvement was mainly attaching the tripod to the pintle mounting hole rather than a band around the barrel jacket.
This weapon was presented to the Ordnance as a sample of what they wanted eventually becoming an adopted weapon, Major Item 51-77 AKA the very scarce M1919A2.
The Infantry became interested, and between the Infantry, Cavalry and the Ordnance Branches they developed the M1919A3 and the M1919A2E3 which was the 24 inch barreled version of the M1919A2.
While doing this research, the subject of bronze trunnions came up.
Here’s what we found.
The M1919 Tank machine gun had a trunion fabricated from “Grade C steel” shown on Class and Division drawings 51-18-4A from Revision 2 dated July 25, 1919 to the last available Revision 6 dated June 1, 1931.
Revision 6 converted the Class and Division trunion drawing to D28108 on June 1, 1931.
From what we can determine from the available Ordnance drawings the M1919A2 was assigned the Major Item Number of 51-77 and the Class and Division drawing 51-77-1A and 51-77-3 dated June 26, 1936 listed the trunion and its drawing as D28108.
Drawing 51-77-1A also shows, in the upper right a “List of Specifications” shown in Figure A, shown below, which details materials specifications used in the production of the M1919A2.
Line 19 refers to bronze castings, however, as of now there is no evidence of any bronze parts being used on either the M1919A2 or the M1919A4.
This drawing 51-18-4, Revision 6 (June 1, 1931) is the last Class and Division drawing available for the tank gun trunnion This drawing notes that it was superseded by drawing D28108 and shows the necessary cut for the rectangular cartridge bunter plate along with a few other parts.
One of the drawbacks to the Class and Division drawings is illustrated here. This is a very busy drawing with multiple parts on the same sheet and the sheets were all the same size.
The letter prefix system assigned the only one part to a sheet and the sheet was sized to the part.
The smallest letter prefix drawing was A, and it measured 8 ½ X 14 inches which is today’s legal size paper. The largest letter prefix commonly encountered in small arms is the E size which was 40 inches wide and as long as you needed it to be.
Most likely the E size drawings were stored rolled because they were random lengths and the smaller sizes, A through D, were stored flat in drawer cabinets which we have seen at the Rock Island Arsenal Museum.
This drawing shows the actual bunter plate itself. Note Revision 2 to the “Drawing Pertains To” block which designates the weapons this part is installed on.
Revision 2, 9-30-36 added Major Item 51-78. It is the 22 Caliber training machine gun. Why a weapon shooting lead projectiles would need to have the feed way protected against wear is beyond me, but here it is. Possibly it was felt that the adaptors for the caliber .22 would cause trunion wear and it was more cost effective to produce all the parts the same and that outweighed the actual need for the functionality of the part.
Prior to the conversion from Class and Division to letter prefix drawings mandated in 1922 there were no “Assembly Drawings” just drawings of individual parts often with more than one part to a sheet.
There were, however, “Finding Diagrams” which served the same purpose. This drawing dated June 1, 1931 has one revision, Revision 1, dated 1-31-36 which adds the M1919A2 and its casing assembly drawing C9827. The correct spoken title for this drawing would be “Trunnion Block Assembly”.
Note that the original drawing emanated from the Springfield Armory, which in 1931 was the Ordnance facility responsible for all thing connected with the BMG’s. This drawing was approved by then Major Julian S. Hatcher.
Promotions in the post WWI peace time Army were slow even for an obviously talented officer such as Major Hatcher who was promoted to Major during WWI. After the war those officers desiring to remain in the Army were usually demoted at least 1 grade. He had only regained the rank of Major by 1931.
This drawing D28108 Revision 1, dated 1-31-36 added B131297, the previous drawing, to the “Drawing Pertains To” block. This trunnion block and bunter plate assembly drawing B131297 were the M1919A2 trunion assembly.
SNL A6, Gun, Machine, Cal.30, Browning, M1919A2 and M1919A4 Fixed and Flexible and Ground Mounts dated May 28, 1941 lists this combination of parts as applicable to the M1919A2 only.
This SNL also lists, without an assembly drawing number, “BLOCK, trunnion, assembly (M1919A4) (used on guns of early manufacture) (composed of: one A20582 cartridge bunter PLATE (1) one D35234 trunnion BLOCK (1))”.
This is a sort of transitional assembly of parts superseded by assembly drawing B147093 “Block, trunnion assembly (M1919A4) (used on guns of new manufacture) (composed of one A160461 cartridge bunter PLATE one D35234 trunion BLOCK (1)).
The two previous descriptions of the early and the then present trunnion assemblies were reproduced exactly as printed in the SNL. I do not know why it is presented in this manner.
When the M1919A4 production drawings were approved on September 30, 1936 the D28108 drawing for the A2 and the M1919 tank gun was redrawn as D35234 with only minimal changes.
This is the original D35234 drawing dated September
30, 1936. Most of the
dimensions and tolerances from drawing D28108, the A2 trunnion block,
remain the same. However,
this drawing requires a piece mark, D35234 in 1/16 inch high characters
to be placed on the outside front of the trunnion just below the
threaded extension for the barrel jacket attachment. The
piece mark imprinting requirement was deleted by Revision 13 dated 6-15-42.
The piece mark imprinting requirement was deleted by Revision 13 dated
While the previous D28108 drawing noted that all of the holes in the trunnion used for attaching the side plates and top cover be “Reamed Assembly” the D35234 drawings carried the note by the rivet and top cover bolt hole dimensions “At Assembly with C64005 and C9826“.
C64005 and C9826 are the right and left side plates, as of this writing we have not discovered any sort of documentation that describes the actual manufacturing process so we don’t know it these holes were drilled through the side plates and trunnion at the same time, or these operations were done separately.
Drilling the holes in all the parts at the same time while the parts were held in some sort of jig or fixture would eliminate problems encountered with manufacturing tolerances when riveting the casing, however, we do not know if this was the case.
However, the note must have been placed on the drawing for some reason.
D35234 Revision 1 (8-10-37) changed trunnion design to allow the installation of a new cartridge bunter plate A160641 (Nov. 11, 1936).
The change is noted on this drawing Revision 20 by the 1 in a circle with the arrow pointing to the round hole for the bunter plate.
This “plate” wasn’t a plate at all, it was a hardened steel plug inserted into a .3140 hole drilled into the vertical portion of the feed way directly in line with the center of the rear barrel bearing hole in the trunnion which is also the vertical centerline of the chamber and bore. We are still conducting research into Ordnance Committee Meeting minutes to try to discover the official reasons behind this change.
Our present theory for the change goes like this: Ordnance discovered that very little trunion wear occurred left of the centerline of the barrel bearing hole, there was no wear to the right because the cartridges with projectiles never passed the centerline as they were positioned by the front and rear cartridge stops.
The only appreciable wear was in the center. The cylindrical bunter plate would use less material, require less machining on both the plate and the trunion and provide the necessary wear protection.
The two versions of A160461 pictured above are the first (Fig 7) and last (Fig 8) letter prefix identified drawings of the final version of the cartridge bunter plate.
The upper left area of the drawing depicted as Figure 7 has some handy information on a method of hardening steel to Rockwell C58 to C61.
The Figure 8 drawing, Revisions 2 and 5, are good illustrations of how Revisions to drawings are sometime just clerical changes, and do not affect the part design in any way.
These revisions just added Major Items 51-114 (M1919A5) and Major Item 51-125 (M1919A6) to the “Drawing Pertains To” block.
In the approval block, on the lower drawing in the “Submitted” space, the signature looks like our old pal Capt. Walter Gorton, now a Major, the designer of the D35392 bottom plate with the riveting flanges and integral T&E bracket that replaced the reinforcing stirrup and separate bracket.
All of the previous drawings of the trunnion block have two things in common, there are no instructions for drilling and tapping the barrel jacket locking screw hole and the threaded portion for mounting the jacket is .45 inches long.
Again, we do not as yet have access to the detailed manufacturing instructions, but it would appear that the barrel jacket locking screw and its required holes and threads were fabricated when the jacket was installed on the casing. Early barrel jacket drawings show the locking screw hole predrilled, but this was eliminated by Revision 7 (10-10-42) on Drawing C62503, the M1919A4 and A5 barrel jacket)
Detailed instructions for assembling the jacket to the casing, the location of this hole, the locking screw and the thread specifications are shown on drawing 6535358 the post-war casing assembly drawing dated 5-10-48 that replaced the D35358 casing assembly drawing.
Conventional wisdom has it that the threaded portion of the trunion was lengthened to .73 inches in length to compensate for added weight/stress caused by the addition of the bi-pod assembly on the M1919A6.
The Revisions on the drawings and their associated dates do not bear this out.
The threaded portion was lengthened by D35234 Revision 11 (4-28-42). The M1919A6 was added to the “Drawing Pertains To” block by Revision 18 (7-22-43).
This means that the threaded portion of the trunion was lengthened well over a year BEFORE the A6 became a Major Item.
The 7-22-43 date agrees with the dates on most of the M1919A6 specific drawings and the dates for the approval of the A6 as a substitute standard presented by both Goldsmith and Iannamico.
The A6 was first discussed as a set of accessories for the A4 about November of 1942, which is 6 month or so after the threaded mounting for the barrel jacket, was made longer.
Obviously there was some reason for the nearly 50% increase in length.
My guess was that the trunnion block was originally designed for the18 inch tank gun and M1919A2 barrel and jacket and that either there were complaints from the field regarding problems, or Ordnance engineers decided on the change as a preventative measure. In any event, what we did find was a Draftsman's Work Order that gives the reason as "to remedy the loosening of barrel jacket from the trunnion block".
There are also a couple of O.O. letters cited that may have more to say about this subject but as of yet we haven't been able to locate them.
Since the longer threaded trunnion shared the same drawing number with shorter threaded one this indicates that these parts were considered interchangeable.
If they were not interchangeable, a new drawing/part number would have been assigned.
There is no mention in TB ORD 366, the small arms rebuilding manual published in 1949, about replacing short thread trunnion.
In this picture just in front of the right front cartridge stop you can see the A160461 round bunter plate installed in the trunion block. Also note the absence of wear or scratching on the front surface of the feed way except for the center of the bunter plate.
When this picture was taken this semi-auto had fired about 6000 rounds of M2 Ball since being assembled and Parkerized.
This A6 has the modified Israeli top cover, note the dog leg belt feed pawl, the matching cuts in the belt feed slide and the cover itself and the partial squared off “U” IDF property mark just visible to the left of the top cover bolt.
Forged trunnions were originally fabricated from “WD 1055” steel until D35235 Revision 20, 9-1944 when the steel type was changed to “WD 1050”.
From the drawings we have examined it appears that D35234 Revision 17, 5-8-43, established the use of “malleable iron, special-class B castings” for use as an alternate trunion material.
At first we believed this to be ArmaSteel, the material developed by the Saginaw Malleable Iron Division of Saginaw Steering Gear. However, further research revealed that there were at least two types of malleable iron materials used in the production of M1919’s.
On most drawings which we believe refer to ArmaSteel the term “perlitic malleable iron castings” sometimes with the words “Class A” is used.
The bill of materials listed on drawing 51-125-1A, the Class and Division drawings for the M1919A6, dated 11-30-45 has two listings for malleable iron castings, “Iron, malleable; castings” AND “Iron, malleable; perlitic; castings” with different specification numbers. This leads us to believe that different alloys were used for different purposes.
With the passage of time all of the malleable iron castings have become known as ArmaSteel whether or not ArmaSteel was a specific alloy used, the perlitic malleable iron Class A, or a general term or trade name applied to all malleable iron castings.
The trunion material is specifically called out in the manufacturing note added to the drawing by Revision 17.
This note also advised that on trunnions made from the “special-class B” material the installation of the A160461 bunter plate “may be omitted” instead, they were to have the “front surface of the feed way flame hardened to Rockwell C-39”.
This might explain why some of the cast trunnions examined were equipped with the bunter plate. The instructions didn’t say that the plate HAD to be eliminated they say “may” be omitted.
Some might consider this hair splitting, but I discovered a long time ago that it is best NOT to extrapolate things from the text of drawing notes.
This is another of the many conflicts encountered in this research.
The more you dig around looking for answers, the more you know what you don’t know.
We have not, as of yet, discovered the drawing for the cast trunion which might shed further light on this issue.
This view is the bottom of the trunion shown above. The grainy, pebbled un-machined surface of the trunion and what appears to be a mold seam indicates that this part was a malleable iron casting. This trunion has no piece mark; however it has some sort of identifying mark struck into bottom surface. The meaning of this mark, which seems fairly common, is unknown at this time.
So far, this mark has only been observed on cast trunion blocks. Since Saginaw Steering Gear’s Saginaw Malleable Iron Division was the only known WWII producer of cast trunnions, this mark may have been applied by them or whoever finished machined this trunnion.
Piece mark imprinting of the trunion was never required by drawing B169913 which was supposed to be the master list of M1919A4 parts to be marked and the assigned manufacturer’s marks.
Various marks in different places have been observed on trunion blocks including letters, numbers and symbols.
Photo courtesy of Rollin Lofdahl
This mark stamped on the bottom of a forged trunion block is similar to markings applied by sub-contractors producing parts for Buffalo Arms.
While Saginaw Steering Gear produced almost all of its component parts, Buffalo Arms relied on parts produced by outside sources for approximately 153 of the 189 component parts.
From what we know now, it appears that Buffalo’s suppliers marked some of the parts with a number, most likely furnished by Buffalo, to identify the producer.
Photo courtesy of Rollin Lofdahl
These marks on the top of a trunion block consist of large “P” and two circles with a “C” inside. Generally, on U.S. military arms the “P” symbol is a mark indicating that the weapon has been proof tested using special high pressure cartridges, however, trunion blocks were never required to be individually proofed for the M1919 series of weapons. The meaning of these marks is unknown.
Photo courtesy of Rollin Lofdahl
This milled trunion block has the letter R stamped on the top of the trunion block. It could indicate that this block was produced by Remington, however it is not the prescribed marking.
It very likely that, given the passage of more than 65 years, the meaning of many of these marks will remain a mystery
Photo courtesy of Rollin Lofdahl
This photo shows a cast trunion block note the unfinished areas with the pitted appearance, the sure sign of a casting.
Saginaw Steering Gear’s Saginaw Malleable Iron subsidiary developed ArmaSteel, a trademarked name for a perlitic malleable iron casting alloy prior to WWII. They used this casting technology to produce difficult to machine interchangeable parts for the automotive industry.
The use of malleable iron castings greatly increased production because the part was cast to near finished dimensions and only a small amount of material had to machined away to produce the finished product.
Not only did this practice save machine time, but it also reduced scrap because only relatively small amounts of metal had to be removed.
Extensive testing by Ordnance concluded that the cast parts were just as satisfactory as conventionally produced parts.
Besides trunion blocks, castings were used to make the one piece back plate with integral pistol grip, front barrel bearings/boosters, top covers, bottom plates, top plates and cover latches and various other parts including a cast receiver for the M1918A2 Browning Automatic Rifle.
Innovative production methods and materials allowed Saginaw to lower the unit cost of the M1919A4 from the $ 657 original contract price in 1940, to less than $60 at the end of 1944.
Photo courtesy of Rollin Lofdahl
This is a milled trunion block, probably a Saginaw, as they had a propensity to stamp the Flaming Bomb on nearly everything. Note the smooth finished appearance of this part.
As usual, I have had a lot of help with the research; however I am solely responsible for the content, if there are errors or misstatements I bear the responsibility.
Rock Island Arsenal Museum, Jodie Creen Wesemann who supplied all of the ordnance drawings and publications featured or quoted in this article.
Jo is one of those rare persons with an instinct for the important and that instinct seems to operate in overdrive.
Without Jo, and the Museum’s cooperation none of the things displayed here would not be possible.
Thanks to my good friend and an important contributor Rollin Lofdahl who is always there to help in deciphering all of this information.
Thanks to all those members of the 1919a4.com and beltfedshooters.com forums who have helped with pictures and comments, and for sharing the really odd finds that crop up now and again.
And a special thanks to Jon Moran for his help, assistance and encouragement.
Photos are individually credited; those lacking a credit are the author’s.
The Browning Machine Gun Vol. 1, Dolf L. Goldsmith
Collector Grade Publications, INC
Moose Lake Publishing, LLC
U.S. Infantry Weapons of WWII, Bruce N. Canfield
Andrew Mowbray Publishers