Their are alot of things getting in hands of NE terror group, Beside Tacticool looking M4s, They are also getting HMGs and RPGs with ammunition and IED explosives & Motars both in 81mm and 51mm ..
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Most militant outfits in India operate with (or despite) a shortage of weapons. And yet, for some 'inexplicable' reason, the NE militants seem to have an oversupply of modern firearms, despite their funds being amongst the lowest in India. Also interestingly, the weapons recovered from the militants are almost always of Chinese, or American origin.
The Chinese angle is easy to understand; there definitely exists a certain degree of support for the NE militants from Chinese intelligence. The American angle is interesting, however, because that implies either Thailand, or non-state actors from Thailand are involved in gun running operations in NE India.