i've been playing around with the 20 round thermelt and 30 round thermelt mags i got from SMG a while back.
the 20s seem to be OK in 5.56 with up to 20 rounds. but a little better feeling when they only have 17 rounds or so.
the 20 round thermelts seem to work perfectly well with 12-17 rounds of 5.45x39 depending on the mag. this is enough to make the 5.54x39 ar a practical all around firearm. as cheap as you can get these thermelts for --like $3.65 a mag -- even loaded to 10 rounds you can sure have a lot of loaded mags for cheap.
and another oddity is the 30 round thermelt mag that i had gave up on using in 5.56 --seems to work very well (so far)with 5.45x39 up to about 27 rounds.
i figure it will be as good as a c-mag 5.45 --maybe even better with 24-25 rounds of 5.45
this thermelt 30 has not worked worth a hoot with 5.56/223 that it is designed for--so go figure?--LOL
this thermelt does have a green USGI follower i put in there in an attempt to get it to run 5.56 rounds-- don't know if the USGI green follower helps or hurts the thermelt with 5.45 it does seem to run OK with 5.45 under 30 rounds.
if it continues to run of with 5.45 with no hiccups i'll report the results.