It said they used knives, a special revolver utilizing silent ammunition (in very small numbers only;
Most, I would say a huge majority of tunnel rats as you call them were nothing more then the smallest GI standing around when it was determined someone needed to check out one of those mud pits.
At 130 lbs (in 1967) I got "volenteered" a time or two. Much more then I desired.
There was nothing exotic about their weapons. They consisted for the most part, a 1911a1, a L-shaped flash light with red lense and **** poor batteries, and a M7 bayonet.
The bayonet was used for digging to make the hole a bit larger because as small as we were, those little buggers who made the tunnels were smaller.
Its not like this was an MOS or something, just some poor shumck who didn't have enough since to find somewhere else to be.
It always struck me funny that it seemed the smallest guy in the squad carried a '60. It didn't take me long to realize there was a reason. If you got the "gun" you're normally out of sight, pulling security down the trail.
Certainly nothing exotic about wollering around in the mud like a pig, and that's what you're doing.
But, you did find out there was nothing better then a USGI 1911a1 when it comes to working after crawling on our hands and knees in elbow deep mud with your pistol still in your hand.
Anyway that's what was really like, not what you see in the movies or what you read on the internet.
The thing is, you don't want exotic guns, you don't need fancy sights, (can't see them anyway). You don't want something that requires you pack some wierd ammo, weight was a problem as it was with issue equipment and ammo.
Best Vietnam knife made was the M7 bayonet. I could did a hole just as fast as with an entrenching tool (which you threw away) and a pocket knife.
Knife fighting was left for the movies.