While there is in most games various forms of bows, there are rarely covered
some unusual accessories that were reputed to have allowed significant
improvement.

Whether or not and how it improves the performance will be left to others,
rather I will simply describe the accessories and how they supposedly improved
them.  The descriptions are simple enough that they could be used in real life.

They were largely abandoned and forgotten and they require specialized training
and the development of crossbows which required far less specialized training.

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Two items are from the description of the "Turkish bow" from the "Book of the
Crossbow".

One is a "horn groove".  This is a ring worn on the index finger of the hand
holding the bow.  Attached to the top of the ring is a horn material shaped into
a groove where the arrow head will rest, and the extends about 4 inches to the
string (allowing the use of a arrow several inches shorter than what would be
needed for the bow).  On the front of the grooves is attached a string that the
fingers below the index finger holds down to hold the groove upright  by
counterbalance when the arrow's weight is on the groove.

The second item is a "thumb ring".  This is a thick and large band with a lipped
edge so that when worn on the bottom of the thumb of the string-side hand, the
lip holds and pulls the string and allows release with a little movement of the
thumb.  This allows the user to apply nearly full arm strength rather than
finger(s) strength and still have fine control over the release.

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The other items is from "From Stone To Steel", a game supplement that attempted
to chart the development of personal arms and armor from pre-history to the
1600s.

The "majra" and "solenarion" were similar in nature (possibly either independent
development or influenced by) to the "horn groove" ring of the Turkish bow.  It
was to allow small dart(s) to be fired by a bow.  There appeared to be several
types depending on the goal.  All were a hollow shaft (made of either wood or
bone) in which one or more darts would be inserted into the head of the shaft. 
The rear of the shaft was notched to be nocked by the string of the bow and also
had a string attached to the rear.  The other end of the string would be tied to
either the bow, the bow's string, or one the wrists of the user (the best is
either attached to the bow string or the string-hand wrist.  The ideal length of
the string is such that the the string would pull taut when the bow's string
reaches maximum down deflection when released.  When the shaft is pulled back,
the dart(s) continue forward.

This allows for a kind of either: triplex (actually up to 5 darts) round where
multiple darts are loaded into the shaft, or a discarding sabot effect.

The development of the solenarion just stopped short of a true discarding sabot
effect (due to losing favor to the crossbow).  If it was continued to develop,
the the shaft could be made smaller with a slotted holding area in front for a
single thin dart (say 3mm steel dart with a l:w ratio of 10:1 or whatever can be
used if Damage RC is allowed to exceed 1, which is not normally allowed).  The
shaft might even look vaguely like an extra long spindle-type discarding sabot
due to the front "petals" for holding the dart and the rear "petal" for nocking
the bow string against and thinning of the main part of the shaft to minimize
the mass to maximize velocity while still be strong enough to withstand repeated
use.

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While not great when compared against modern weapons, the above concepts cold be
used in either alternative histories or in Warp World situations where advanced
guns and other high tech devices are restricted for some reason and these
cunning arrangements of physical principles are still allowed.

I do not know if if anyone has tried to make a spreadsheet on VDS or other
systems for optimizing designs.  Here are some thoughts from the power plant
sections from the view of most player character interests.

The one key limit at any given Tech Level is the number needed to have 0 crew
(for constant attention) and to round down to the nearest 100 kg power plant
mass (due to the maintenance time).  The total Durability should be long enough
until you can have a technician look at it (generally months to years).  Next
would the number of days of cumulative operation of fuel that the fuel tank
would need to hold between refueling (generally weeks or years).  Adventurer
requirements generally need power plants to go for long stretches of time before
maintenance and refueling are needed.