Just because the rifles look similar does not mean that they're
copies. The similar look is mainly due to the Bullpup
configuration.
Here is the definition of the Bullpup configuration in simple terms
(from Wikipedia):
Bullpup refers to a rifle
configuration that increases the barrel length relative to the
overall weapon length. Thus it permits shorter weapons. The design
was invented at the Royal Small Arms Factory at Enfield in the
United Kingdom, where several years of research led to the
construction of the EM-1 and EM-2 assault rifles.
In a normal rifle, the barrel lies at the very front of the gun,
with the firearm action (or mechanism) behind that, and finally the
stock at the extreme rear. The trigger is located directly under
the mechanism, with the magazine placed in front of it.
In the bullpup configuration the magazine is instead located behind
the trigger, which allows the entire mechanism to be moved
rearward, reducing the length of the weapon some 20%. The trigger
mechanism of some bullpup weapons uses a flexible rod or similar
system, which has the disadvantage of a heavy and sluggish trigger
pull, but the SAR-21 uses a stiff sliding plate.
In comparison, just because most pistols look the same does not
mean that they were all copied from each other.
