Any blowback operating system can experience an OOB. Urutau is actually fairly resistant. When built to spec, the gun won't fire if the bolt is more than 2mm out of battery. Normally, 2mm leaves enough of the case supported to avoid case wall blow outs.

A firing pin that's too long or has the collar positioned incorrectly can let the gun fire more than 2mm OOB. If you combine that with an enhanced feed ramp barrel that leaves more of the cartridge unsupported, now things can go badly. Equally problematic is a DIY barrel that isn't properly headspaced.

As for why the bolt stops short of fully going into battery, the list of possibilities is long. Bolt binding on the guide rods, bolt hanging up on the receiver, feeding round hanging up on something, weak recoil springs or springs binding on the guide rods, etc.

The bottom line is that multiple things have to go wrong for an OOB. But on a full DIY design, there are lots of opportunities for things to go wrong.

I've had two OOBDs early on in the build process. Both blew the Urutau in half, with the force going down and left.

The problem is in the original bolt face itself. It can rotate after the first few shots, and will trap the firing pin forward, so that the next round picked up from the magazine is struck by that extended firing pin before it can be pushed into the breech.

My fix was to purchase the metal bolt carrier group from Urutauparts.com, and it does work well.

I have an all-new issue: runaways. There's a problem, possibly with head spacing, and I can't figure out what to change in order to stop the Urutau from firing an entire magazine after the first trigger pull. It will continue to fire without the trigger, with the hammer not moving at all. The lightweight firing pin keeps slamming forward on its own, firing the next round and cycling the action over and over.

I have increased the strength of the firing pin spring, but that didn't help. I can obtain lighter springs for the two guide rods, and reduce the forward force that way. But after that, if the Urutau keeps mag-dumping, I'm going to abandon the whole thing.