Yes and no.
The MIL-C-63989C says that M855 shall not be less than 12,000psi average at the port on an M16A2.
The bore pressure near the muzzle is nowhere near the same as the chamber pressure, so the pressure the pin and weld job would be far, far less then 50k PSI. See the chart below for bore pressures at the port for various length gas systems.
Gas port pressure by system length.jpg
That being said, thin walls will expand and contract more as the bullet passes and attempts to push the barrel material out of the way so deformation, cracking, flaking and eventually material removal will happen relatively quickly at that spot. This could end up marring the bullet as it passes that point and will also lead to excessive erosion at the muzzle, both causing accuracy issues. Best not to run this way with a can.
I've had a smith accidentally drill all the way through into the bore on an MR556 pin an weld job nearly a decade ago. I had them counter bore the muzzle to 6mm ID and 5mm in to remove all contact and stress from the bullet, basically making a 90 degree crown set back by 5mm into the end of the muzzle.
Ended up being a tack driver and could print sub 1/2 MOA groups with the right ammo, trigger, and optics. Suppressed just fine as well.
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