▁cost ▁of ▁ 3 D ▁printer s ▁has ▁reduced . ▁This ▁has ▁enabled ▁faster ▁turn ▁around ▁of ▁such ▁scale ▁models ▁and ▁allowed ▁a ▁steady ▁increase ▁in ▁the ▁speed ▁of ▁production ▁and ▁the ▁complexity ▁of ▁the ▁objects ▁being ▁produced . ▁ ▁Const ruction ▁ 3 D ▁printing , ▁the ▁application ▁of ▁ 3 D ▁printing ▁to ▁fabric ate ▁construction ▁components ▁or ▁entire ▁buildings ▁has ▁been ▁in ▁development ▁since ▁the ▁mid - 1 9 9 0 s , ▁development ▁of ▁new ▁techn ologies ▁has ▁stead ily ▁gained ▁pace ▁since ▁ 2 0 1 2 ▁and ▁the ▁sub - se ctor ▁of ▁ 3 D ▁printing ▁is ▁beginning ▁to ▁m ature . ▁See ▁main ▁article . ▁ ▁Fire ar ms ▁▁ ▁In ▁ 2 0 1 2 , ▁the ▁US - based ▁group ▁Def ense ▁Dist ributed ▁dis closed ▁plans ▁to ▁"[ design ] ▁a ▁working ▁pl astic ▁gun ▁that ▁could ▁be ▁downloaded ▁and ▁reprodu ced ▁by ▁anybody ▁with ▁a ▁ 3 D ▁printer ." ▁Def ense ▁Dist ributed ▁has ▁also ▁designed ▁a ▁ 3 D ▁print able ▁AR - 1 5 ▁type ▁rif le ▁lower ▁receiver ▁( cap able ▁of ▁last ing ▁more ▁than ▁ 6 5 0 ▁r ounds ) ▁and ▁a ▁ 3 0 - round ▁M 1 6 ▁magazine . ▁The ▁AR - 1 5 ▁has ▁multiple ▁rece ivers ▁( both ▁an ▁upper ▁and ▁lower ▁receiver ), ▁but ▁the ▁leg ally ▁controlled ▁part ▁is ▁the ▁one ▁that ▁is ▁serial ized ▁( the ▁lower , ▁in ▁the ▁AR - 1 5 ' s