Hi,
Weihrauch dont produce a larger mag for the .17hmr, this is mainly to do with the rounds dimensions and problems with the feed if the mag was any longer. they do an 8 shot for the .22lr though.
If you try and force 5 rounds in too many times it will weaken the mags spring.
You can only fit 4 rounds in the mag on the .17hmr, the only way to get 5 shots is to replace the empty mag with a full one when the last round is chambered, this is the only way to get 5 shot capacity.
Spare mags are available, but you'll be paying around £45.
Hope this was some use to you.
out of inrest what mo