  Solar flares are explosive releases of magnetic energy. Hard X-ray (HXR)
flare emission originates from both hot (millions of Kelvin) plasma and
nonthermal accelerated particles, giving insight into flare energy release. The
Nuclear Spectroscopic Telescope ARray (NuSTAR) utilizes direct focusing optics
to attain much higher sensitivity in the HXR range than that of previous
indirect imagers. This paper presents eleven NuSTAR microflares from two active
regions (AR 12671 on 2017 August 21, and AR 12712 on 2018 May 29). The
temporal, spatial, and energetic properties of each are discussed in context
with previously published HXR brightenings. They are seen to display several
'large-flare' properties, such as impulsive time profiles and earlier peaktimes
in higher energy HXRs. For two events where active region background could be
removed, microflare emission did not display spatial complexity: differing
NuSTAR energy ranges had equivalent emission centroids. Finally, spectral
fitting showed a high energy excess over a single thermal model in all events.
This excess was consistent with additional higher-temperature plasma volumes in
10/11 microflares, and consistent only with an accelerated particle
distribution in the last. Previous NuSTAR studies focused on one or a few
microflares at a time, making this the first to collectively examine a sizable
number of events. Additionally, this paper introduces an observed variation in
the NuSTAR gain unique to the extremely low-livetime (<1%) regime, and
establishes a correction method to be used in future NuSTAR solar spectral
analysis.
