Open Bolt Trigger Mechanism

.300M

.GS7" PIVOT PIN HOlE SHOULD BE DRILLED THROUGH BOL! AND FXTRACTOR SIMULTANEOUSLY.

pari should he made slightly oversize and dressed to exact fit during assembly.

If i lie closed-boll configuration is adhered to, commercial firing mechanism components, which include a hammer, trigger, disconnector, safety, and necessary spring, arc available from commercial and surplus parts «sources. These are available at attractive enough prices to make it more sensible to buy them than to build them. Unfortunately, several of the surplus parts suppliers got the idea in their heads that they could rip off consumers by doubling, or sometimes tripling, the price of these small parts when the assault weapons ban went into effect. This, of course, didn't work out since there were many more such paits available than there was demand for them. As a result, prices have again returned to normal and should remain so for years to come. An example of this is that all required firing mechanism parts can be obtained from dependable parts suppliers such as Quality Parts Co. in Windham, Maine, for around S40. These arc new parts, and Quality Parts ships promptly, without any B.S. or excuses.

Diagram tf37

the open-bolt version will require manu* facture of a sear, trigger, anil disconnector. I hese are designed to interchange with the closed-hull parís, using ihe existing pivot pins and pin holes, with no chango in the lower receiver. This system uses a combination trigger bar and disconnector, which is hinged to the sear. The lower end of this part engages a projection protruding from the forward end of the trigger. Consequently, when the trigger is pulled to the rear the scar is pulled downward, out of engagement with the boll, allowing the bolt to travel forward, pushing a cartridge from the magazine into the chamber. At the forward end of the bolt stroke, the firing pin crushes the primer, causing the round to fire. At this point, a combination of recoil and «as pressure causes the bolt to move to the rear, ejecting the fired ease in the process, where it is again caught by the sear and held in the rearward, or cocked, pusition until another pull of the trigger causes the cycle to be repeated. I he ' lump/' or projection, at the top of the trigger b<ir is depressed by the forward-moving bolt, which moves the lower, booked end of the bar out of engagement with the trigger. This requires allowing the trigger to move forward again before a subsequent shot can be fired.

156" DIA.

.400"

.175"

- .125"

-11

.125" T +

j

— .250

fc>

J -450

.600"

*

M16 Auto Sear Placement

D ¿gram 438 TRIGGER (OPEN BOLT)

1.600"

.187"

?ocr

A 50"

riz:

1.15C"

SEAR (OPEN BOLT) Diagram *39

THIS PART IS MADE FROM .125-INCH-THICKNESS, 4EAT-TRCATADLE STEEL. T SHOULD BE MADE SLIGHTLY OVERSIZE AND FITTtD DURING ASSEMBLY.

PIVOT PIN HOI F IS .087" IN DIAMETER.

Trigger Mechanism Plans Wood Homemade
.250"

TRIGGER DRAWBAR AND DISCONNECTOR

Diagran

Full automatic lire is achieved by pulling the Lrigger farther to the rear, against the pressure exerted by the li-shaped wire spring located at the rear of the trigger. This causes the forward end of the trigger to contact a projection located on the lower end of the scar, which, ¡1 the trigger is held back, holds the sear out of engagement with ihe holt, allowing it to move forward and back unimpeded I his causcs full automatic fire.

The sear is cut from 5/8 y.625)-inch-thick steel plate. This must be of a type that is shock-resistant and capablc of hc^t treatment. As usual, 4140 or 4340 will serve admirably for this. Note that ihe disconnector spring hears against the pivot pin. proiiding friction to hold it in place. The disconnector spring is .VI6 inch in diameter, wound from .018-inch wire, and the uncompressed length is between .300 and .400 inch, ilic scar spring, located irt the lowrr fronL side, can he a section cut from the Ml Carbine recoil spring used in the closed-bolt version.

I'hc trigger is made from the same material with <\ projection at the forward end on the right side tu engage the trigger bai•disconnector. Iwo small 1/16-inch-diamctcr holes are drilled in the location shown to accept the U-shaped wire spring, which is formed from .065 inch music wire. lliis spring bears against the lower side of

Ar15 Trigger And Bolt

RELATIONSHIP AND PLACEMENT OF OPEN-BOLT TRIGGER PARTS

the safety shaft, providing resistance to rearward movement of the trigger after the first, or semiautomatic, stag«'.

I he trigger bai should he made from similar material, exccpt that it is only I/8 ineh thick Ruth the hook at the bottom and the lump at the top should be left slightly oversize to allow fitting during assembly.

For use in the riosed-bolt version, an original firing pin can hardly be improved on. Here again, one can be purchased for les* than the coat to make one. A drawing is included, however, in the event that manufacture should be required. Drill rod will suffice as material for this. Stems from automobile engine valves or shock absorber shafts ran also be used. The experienced lathe operator will know to use a sharp cutting tool, set exactly on center, taking light cuts. Those who have never done this before may gain some valuable, although unpleasant, experience when making a pait such as this.

3.P85"

065r DIA.

.100"

FIRING PIN

+2 0

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