A recent blog post was an illustration of how easily firearms such as sub-machine guns can be manufactured. Where local laws have created a sufficient market criminal organizations may readily act to meet the demand. One of the weapons I showed had been designed and constructed in Northern Ireland in what was then one of the most heavily policed areas of the United Kingdom. Where there is a will and a need a way will always be found.
What I did not state in my earlier post was that the Irish SMG shown had been built by a Loyalist Irish faction. As far as I know the republican terrorists did not produce a significant number of homemade SMGs. The PIRA/IRA etc were well supplied with small arms from both Libyan and American sources. Where the IRA were very creative was in the field of heavier weapons. Yesterday I came across this interesting home-made weapon. It is a recoilless bomb thrower designed to attack unarmoured and lightly armoured vehicles. The projectile is made from a tin can containing a shaped mass of explosive. The weapon works on the Davis principle and uses two packets of biscuits as counter-shot. The local pigeons must look forward to a firing!
The firearmblog webpage does a fine job at describing the device, so I will direct you there.