In case you don't know about them:

Better Mag Adapter

Salem 6 Magazine

At the time of this writing, they do not hold the bolt open on last round and there is nothing to that effect mentioned on their site. The pictures on their site and in some reviews I found appear to show a different follower than the one currently shipped with the magazine.

Vendor was very quick to ship and answer emails and confirmed that their mags do not hold the bolt open on last round... They mention that they will clarify that on their website...

Interesting. Don't see a lot of doublestack rimfire magazines

Interesting

I got some range time today with them... I was shooting CCI minimags on CMMG dedicated upper .... The 4 Salem mags I got converted an otherwise reliable setup in a jammomatic ... Couldn't even get 2 shots in a row without a double feed.... Switched to OEM S&W 15-22 magazines and not issues at all...

While a nice feature, it's not one I get too hung up over. The 10/22 doesn't have a last-round-bolt-hold-open feature, and Ruger still managed to sell hundreds of thousands of them.

Right, but no BHO in an AR15 means a different manual of arms... Not a big deal if it is a range toy but if it is intended as a trainer... Of course, the 4 I got will give me plenty of practice clearing malfunctions if I decide to use them....

Not to mention that the fact that they do not hold the bolt open, not even in a MP-15-22 (which does have that feature) is not stated on their web site ...

Good luck if you decide to use them