I just got a QC10 Glock small frame lower and the shelf is not as long as a normal mil-spec lower.  It does have a low shelf and the RDIAS seems to be at the correct height, but I am concerned about the shelf not fully supporting the RDIAS.  Should I have a gunsmith make an insert for this?  Or is this just fine the way it is?

Also, I have a Gibbz upper that looks like the takedown lug is too big.   Is it better to remove material from the back first, then time?  Or remove from both the front and back and shim as needed?

As you can see from the images, it is just barely too big.   Really, I would prefer this over it being to small.

That QC10 lower is in fact machined to M16 internal specs, except for the sear holes not being drilled.

Your DIAS might need support at the front if it's not held firmly in place by the takedown pin post, or between the takedown pin post and what's there of the shelf.

I had some uppers on which the DIAS fit so tightly that it came out with the upper when the halves were shotgunned open, and others where it just sat in the lower and could move some unless I shimmed it.  Some folks even add a set-screw to the DIAS body to snug it up to the takedown pin post.

So if it get held in place, I don't need to worry about any stress on the RDIAS?  The shelf is about .3 inches shorter than on my SP1 spec lower.  

Also, should I remove material from the back of the takedown lug?   Doesn't seem like it will take much

Most important thing is that the DIAS is in a correct-enough position to work, and doesn't SHIFT - it's holding back the hammer and needs to catch and release the hammer based on interactions with the carrier, without jarring about.  If it catches the hammer and then jerks forward, that's gonna put stress on the body and the pin holes.

If you can measure the takedown pin post, see if it's thicker, relative to the hole on the front or the back, and work to keep it centered around the takedown pin, which is what is supposed to provide the index for a DIAS. If it's even, then take off evenly from the back and front until it fits, otherwise, take the proud side first and work toward a balanced center.

Sounds good.   I didn't think the RDIAS had any stresses on it, but I wanted to make sure.

This doesn't get them in trouble by doing this?

Are you reading a Life in Limbo?

No, as long as the sear holes weren't drilled.

Interesting. So I guess since nearly all manufactures put the high shelf in that it has more to do with reducing legal liability?

Maybe somewhat - it's also cheaper. Metal not removed is less machine time, so the end product is marginally cheaper to produce.

As an update, I filed down the takedown lug and was able to get a really nice fit on it.

I had to file both the front and rear take down lugs to make my QC10 lower fit any upper I could find.  Not a big deal, but not the quality I expected.