I replaced the stock trigger on my 9-mil serial number registered M16a1 a couple of years ago and I have found that I can now assemble my uppers and lower even when the gun is on safe auto. Is this a design feature of the Geiselle or did I bend/break something in the auto sear? I remember very clearly that previously I was only able to connect upper and lower receiver when the safety wasn't in the auto position, but the safety can be in any position now for easy assembly.

Still shouldn't be able to assemble it in the auto position.

Yep meant auto, not safe.

Damn must have bent my auto sear, good thing I have a spare.

With the selector set to auto, if the hammer is not engaged on the autosear, you will be able to close the upper on the lower. Not so if the hammer is engaged on the autosear, that’s when the top of the autosear is rotated rearward to where it hits the bottom of the carrier when you try to close the upper.

I just pulled one of my SSF equipped M16 lowers and confirmed this.

Thanks for confirming! It's been a couple of weeks but from I remember I had a dead trigger when testing out an under gassed 7.62x39 upper so in all likelihood the hammer wasn't resetting.

I'll test this weekend and provide an update. I don't remember manhandling this thing to the extent that I would have bent the sear enough to close the upper, but it's not beyond the realm of possibility either.