HK does make a version with both collapsible stock and hi-cap mags, which is called the
UMP.
Good news is it comes in 9, 40 & 45 versions,
Bad news is it is select fire and ONLY available (through Hk) to LE/Military. Navy version lower has S/1/F while standard version has S/1/2/F.
NFA if you can find one in the US, which would have to be an USC/UMP conversion registered as a SBR, but production of the UMP didn't start until the 1990's, so no original UMP's would be permitted under the pre-1986 law.
:-( So far, no folder rigs or hi-cap mags for lowly "
citizens".
Navy version pictured