Yep. TM mags do tend to piss gas everywhere when filling.
Pop this over the inlet valve and push down with the gas filler nozzle, and hey presto, silent filling.
If it's leaking from under the filler valve (which if new it shouldn't do, but could) they are simple enough to fix. You can even add a drop of silicon oil over the valve and that should feed down any gaps and expand the o-rings.