Anyone have experience with them? There's not nearly as much info on them as the Stern, but they don't require bolt modification to work with PSA. That intrigues me greatly. I've got a coupon to Sylvan Arms and am having a hard time not pulling the trigger. Convince me not to, or convince me to fo.

I have one.  I only have Glock brand mags and the fit is snug, not really drop free with empty mags.  Fits just fine in most of the AR pattern lowers that I've tried it on.  Did not fit into a 2008 vintage DPMS (non-mil spec?) lower.  No issues with operation in semi in all lowers that it fit into and in full auto (in Colt M16A1 lower).

The Sylvan unit is also not compatible with the New Frontier Armory (NFA) LRBHO upper I have.  To wit, the bar on the right side of the top of the Sylvan adapter interferes with the lower right inner edge of the NFA upper.  It does look like one could grind off a little bit of metal on the NFA upper and on the Sylvan adapter to provide a fit while retaining adequate structural integrity at the loss of aesthetics (that are not visible with the adapter installed and the upper mounted on a lower.  That should provide LRBHO, if desired, but I did not want to grind parts and I do not want LRBHO when I shoot 9mm in any AR except with the dedicated AR9 Glock mag lower so I have not tried it.  Also, rather than use the screw provided with the Sylvan unit, I have taken to stretching an o-ring over the standard AR mag release button to prevent inadvertently dropping the unit out of the lower.

Best of luck with whatever you decide to pursue.

MHO, YMMV, etc.

Colt or glock? I've got both, but it took me a while to find a bolt(spinta) that would work with the colt, little picky on the mags too. Glock I had no problems with, other than hating on the weird mag release. Will say some of the best customer service I've ever encountered.

I guess I'm buying a Sylvan Glock magazine adapter.

Thanks y'all.

Works like butter, just have to get used to the mag release in front.

FWIW, I decided that I wanted the flexibility to use the New Frontier Armory (NFA) LRBHO upper mentioned above with the Sylvan Arms adapter.  I filed and finish ground then "blued" with a black permanent marker a section of the upper on the right inner lip below the ejection port to provide clearance for the adapter cross bar.  I did not want to compromise the structural integrity of the adapter, so I left it unmodified.

MHO, YMMV, etc.