ThompsonMachine.net

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My Gun Projects

Here are a few pics of an HK 91 build .

$119 parts set as received from FAC/Century

$129 set from FAC

The first step is to remove the bbl/trunion assembly from the rec scrap. There are 4 plug welds that need to be ground away . A dremel tool with a reinforced cut off wheel does a great job, just be carefull not to grind to deeply into the trunion. The trunion may hit the coking tube , if so once the trunion has been cut loose the two plug welds on the cocking tube will need to be ground away, they lie on either side of the rec aprox 1/2-3/4 inch from the front edge of the rec. In addition to the two plug welds you will need to cut the weld that runs around the joint where the rear of the tube meets the rec. In the pic above the cocking tube has been cut off flush with the front of the rec as it was bent and I just needed to get it out of the way so I could slide the trunion forward.

Trunion pulled from rec scrap, this is realy all that required to remove the demil rec from the parts set. Evrything else is pretty straight forward .

Here is the area where the gas tube will interfer with removal of the trunion from the rec scrap. Some folks choose to split the bottom of the rec with a cutoff wheel and remove the trunion with the cocking tuve still in place and then remove it from the rec scrap.

The FA grip housing will need the front two ears were the push pin fits cut off flush with the front of the housing so it will fit in place on the Semi rec.

Trigger housing from the parts set on left, semi housing on the right.

The next step is to remove the fcg components from the trigger housing as dispose of the auto sear and trip lever as they are NFA restricted parts and not needed for this build.

The trigger housing will have to have a block perm welded in place to prevent the insertion of the auto sear. The first thing to do is cut away the entire area at the bottom front were the pivot pin for the fa sear passes.

Now that the front of the trigger housing is machined away you will need a shelf of sorts for the front of the housing to engage the protrusion on the semi rec. This block also sits right were the FA sear was located perm blocking insertion of the FA sear. This block also keeps the front of the trig housing assembly in place.

Here is the block welded in place.

The last step of the trig housing mod is to drill a .194 diam pin hole to hold the sear spring in place.

This is what the spring you will need to make should look like, it presses on the underside of the sear holding it firmly agaisnt the bottom of the hammer.

The original FA hammer on bottom , US made semi hammer on the top. If you decide to reuse the original hammer you will need to grind away the FA sear notch at the front.

Here is the finished semi trigger group.

The next important thing is the removal of the trip area on the bottom of the bolt carrier that trips the auto sear. A dremil tool with a couple of cutoff disk will do a great job of removing this area. Do not under any circumstances skip this step or the mod to the trigger housing. I should note some folks simply choose to weld a pin in the FA sear hole and not grind away the trip on the carrier, Use your own discretion here. The method I have detailed is the way HK was required to do it for the duration of their importing of this type firearm.

Here is a pic of the trip area ground away.

A few holes need to be drilled in the rec to weld the trunion in place, I like to drill a 1/4" hole at 2, 6 and 10 positions as viewed from the front, and two small holes in the top area to weld the cocking tube in place. The original cocking tube is also welded were it butts up agaisnt the front of the rec.

Here is the grip housing, I bead blasted, parkerized then powdercoated.

Same with the rec.

Here is everything ready for assembly.




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