The tunnel rats carried what ever was available, be it a .45 or .38 S&W. there were some special exotic weapons designed for use. Ruger .22 automatics with a silencer, large bore revolvers with special ammo ( one type I believe was a captured plunger that pushed the bullet, the gas never escaped the cartridge )but these were rare. The troops never knew when they would find a tunnel, most tunnels were in Chu lai district because the clay was perfect for digging. In other parts of Nam the tunnels were small and short because the ground would cave in on anything larger. One of the biggest problems the rats had was busted ear drums, and all the tunnel Rats were volunteers. If the NVA had the time and equipment they would build vast complexes using roof beams and other shoring. There was nothing special about the weapons used, again they used what ever they had on hand, which was normally the 1911. Most of the write up you read about using special weapons are one of a kind, short usage or test programs. If the war had lasted another 10 years I'm sure they would have come up with special issue weapons. One note, many times when the Viet Cong or NVA ( and after Tet 68, the local Viet Cong were no longer a issue, it was all NVA, ) realized they had been discovered they vacated the area. They knew that to stay in a tunnel was sure death. Once in a while they would leave a rear guard, but if they were able to, they would decide they needed to be somewhere else in a hurry.