Building a Hybrid Yugoslav M70B1 from a Stub with Screws

Do the feed and function testing with the dust cover removed so you can clearly see the results of testing.

After initial assembly, I load four action testing dummy rounds into a magazine and install it to check for feed problems. A number of firms make these dummy rounds and mine are from A-Zoom. These present a far safer alternative than live ammo.

The action should be cycled and when the carrier moves forward, a round should be stripped from the magazine and go up into the chamber. When the carrier is pulled back, then the first round should be pulled out by the extractor, hit the ejector and be discarded from the rifle.

I do this a number of times fast and slow to make sure everything is working okay. Common problems at this stage are dirty bolts that need cleaning so the extractor will work correctly and bent receiver bottoms that are causing the magazine to ride too high. For folks who make their own receivers, this is also the time to do the final trimming of the ejector tip to make sure it engages the cartridge base and ejects properly.

Function-Testing

The fire control group (FCG) needs to be function-tested to verify it is operating correctly. With the dust cover off so you can see what is happening, start with an uncocked hammer and make sure the rifle is unloaded. Do not hold the trigger; pull the bolt carrier back and you should see the hammer get pressed back and when the bolt carrier returns, the hammer should be caught by the hooks of the trigger assembly. Let the carrier return home and pull the trigger. The hammer should be released.

Second, pull the trigger and while holding it back, cycle the bolt carrier. You should see the disconnector grab and hold the hammer. Next, release the trigger. The disconnector should release the hammer and the hook(s) on the trigger should then grab the hammer.

Releasing the trigger should not allow the hammer to follow the carrier back. If the disconnector doesn't grab the hammer, make sure the disconnector spring is installed and that the hammer isnít installed backwards. Also check that the axis pins are fully seated.

If everything looks okay, then there may be a problem with the receiver, so contact the manufacturer. If you built the receiver from a flat or a blank, then either the hole locations are wrong or something bent. Do not change the geometry of any FCG component without first understanding what is going on. Most of the time, a FCG drops in and works no problem.

Third, pull the trigger and make sure the hooks release the hammer and it snaps up. If there is a problem, then make sure the trigger is facing the correct way so the tabs are down by the receiver and the flat striking surface in the middle will hit the firing pin. Also, make sure the hammer spring is under pressure and the bent ends are sitting on the ìlegsî of the FCG.

Last, cock the rifle and turn the selector lever to the safe position. The blocking tab on the selector lever assembly should prevent the trigger from being pulled. If it doesn't, check to see if something is bent or broken.

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