M-16s are pretty common for NE terrorists. Some are original US made, some others are Chinese copy of M-16s. The old shipping route through Vietnam, Cambordia and Myanmar has been clamped down on but not completely shut down.
found some thing related to this.
Drawing from Yun Sun's take on China's Strategic Assessment of India , it would be prudent to delve intio aspects other than Doklam which impact Sino-Indian dynamics.
1. The resurgence of Indian covert capabilities have gradually deleveraged the Chinese MSS in the North East.
2. The MSS used a network of human assets , training camps and ports in Bangladesh , Myanmar and Thailand to facilitate insurgency in the North-East.This included sheltering key leaders like ULFA's Paresh Barua and NSCN-K's S.S.Khaplang inside Chinese Territory.
3. Gun running by the MSS involved Chinese commercial trawlers and Thai drug dealers.The trawlers would offload arms off the Bangladesh coast on speed boats and smaller fishing boats.These were then passed onto insurgents in the North-East through Cox Bazaar and Chittagong
4. A part of the consignment was sold-off to Maoists in India using an elaborate chain of student couriers based in Delhi and Bangalore.The Nepalese Maoist resurgence was fuelled by similar consignments passed on by militant organisations in the North-East.
5. India used a three pronged approach to combat this growing menace which commenced with RAW's Bangkok Station mounting intense surveillance of MSS's gun-running Ops in the Bay of Bengal and NE-Militant leaders which led to the Chittagong Arms Haul in 2004.
6. Covert Operatives sieved through Dhaka , Cox Bazar and Chittagong in search of MSS assets inside and outside the Khaleda Zia regime and some key assets were neutralised.During this period , Paresh Barua of the ULFA had a lucky escape managing to flee from a RAW Hit Squad.
7. This culimnated with the dethroning of Begum Khaleda Zia in 2006 as the prime minister of Bangladesh.The overt and covert actions which led to this landmark event set in motion the wheels of long term change
8. In the Mid-90s , India decided to recalibrate relations with Myanmar and followed it up by toning down support for the Arakan Army and Karen Rebels as a direct overture.India followed this up with tacit silence against the Military Junta's alleged atrocities at the UN
9. Responding to these measures Myanmar shared limited intelligence on the presence of militant camps on its territory but did not show any alacrity to tackle them as the Junta was busy fighting insurgencies in the North
10. With a friendly regime in Bangladesh , India managed to stem the flow of arms to the North-East effectively.This despite the ISI Station in Dhaka collaborating with the MSS to smuggle arms into Bangladesh via Pakistani Merchant Shipping and Air Cargo.
11. The ISI took over the mantle of gun-running and training with help from the Jamaat underground but this faltered as its assets were mysteriously neutralised and its reach within the government machinery waned after the BDR Mutiny of 2009.
12. The BDR Mutiny was the ISI's last hurrah before a long period of relative peace.But its ferocity contributed to the vengenace with which Sheikh Hasina went after Islamic Radicals.This gave India unlimited leverage inside Dhaka's Anti-Terror framework with fantastic results.
13. A series of economic measures were undertaken to connect India's North-East with Myanmar and Thailand to improve regional connectivity and trade.Military relations improved and the Border Roads Organisation undertook repair and maintenance of multiple roads within Myanmar.
14. This led to wider co-operation between both Militaries and India extended Lines of Credit to Myanmar for purchase of Defence Equipment. Exchange of personnel for training commenced and mutual trust improved rapidly.
15. India combined diplomatic and militray co-operation to make the Junta see reason and co-operate in destruction of insurgent camps inside Myanmar.This included tacit approval for cross-border Ops as the Myanmar Army did not want to lose personnel in the effort.
16. NSCN's repeated insistence on Greater Nagalim which included parts of Myanmar increased the Junta's wariness and there was wider acceptance within the Myanmar Army of the need to root out foreign insurgents on its soil.
17. The Surgical Strikes inside Myanmar in 2016 and co-ordinated cross-border combing ops knocked the wind out the NE Insurgency as they had no choice but to surrender.The Chinese in the interim were looking at status quo as they had an economic corridor at stake.
18. China was now worried about the safety of the China Myanmar Economic Corridor (CMEC) and was wary of Indian covert capability inside Myanmar. In August 2019 , four major bridges along the route to the two most important trade hubs on the Myanmar-China border were blown up.
19. Co-ordinated attacks by an alliance of the Ta’ang National Liberation Army (TNLA), the Arakan Army (AA) and the Myanmar National Democratic Alliance Army (MNDAA)—in Mandalay Region’s Pyin Oo Lwin and Shan State’s Naung Cho Township shook Beijing
19. The MSS was administered its own prescription and had no choice but the lie low. It had been gradually defanged and could exercise almost negligible control over the NE-Insurgents.A significant erosion of capability which would be very difficult to rebuild.