Ok, want to build a 9mm upper for use on my M16 lower. I see there are several bottom load mag blocks, but looks like they are held in place with some kind of tension screws. Would the Hahn top loader work well? Not wanting the dedicated version because I want the ease of swapping uppers. Just seems that if I'm going to be taking the upper off anyway, it would be just as easy to remove the top block while I'm at it. Anyone use one? Just as reliable in function as a bottom loader?

If you want to easily swap calibers, go bottom loading block.
I ran a RRA bottom loading block on my Sendra ETA spelled SENDRA wrong. Duh.M16.

Reason being, I didnt have to remove the BHO to make the swap.
It ran like a thief in the night

I use the bottom loader because I want the BHO to work and don't want to have to pull the roll pin on the bolt stop every time I want to remove it.

The tension screw (which actually actuates a roller, doesn't dig into your lower at all) works well in my experience (primarily using it on an SBR'ed BCM lower) and most of the time the block is held in with the magazine catch, so it's not going to just fall out of the gun if the tension screw loses tightness.

I've also had no problems with positioning and feeding -- works great every time.

ETA: This is with a Hahn Precision block. Quite happy.

I'm in the same boat OP.
Just bought a colt m16a1, and plan to shoot 9mm out of it.  

What you describe is my plan as well.  Top drop in Hahn adapter.  As far as I know you do not need to remove the Bolt catch to drop it in for the non dedicated mag block(it's simply drop in).  And I don't want a tension screw pressing on my $25k lower receivers magwell.  If your removing the upper receiver to swap uppers/calibers, removing the magblock is a simple part of the process.
Based on what I've read....DO NOT use the last round bolt hold open block with your m16 lower.  It'll cause unnecessary damage to your bolt catch ears on the lower itself.


Not that I've heard of this being a problem, but in theory, there should be even less problems with a top drop in, since it can't slide down at all if the tension isn't tight enough.  Like I said, I haven't really heard of this issue, but with the top drop in, it can't happen whatsoever.

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