Always start with the lowest gas port setting, any time you shoot any MG with an adjustable gas port.
I always shoot my own 308 reloads in it.
The gun was a registered dewat that I bought back in the mid 1970's. The barrel's chamber was plugged and the plug was welded in place, then the barrel was welded to the receiver. It was easy to remove the weld holding the barrel to the receiver. I filed the Form 1, and paid the $200 get the OK to reactivate the gun.
Installed a spare barrel and I was off to the races.
Still the plugged barrel sat in the spare parts foot locker until the later 1990's, when I decided to rehab it.
That was not a easy thing to do, as you need a good lathe and to know how to use said lathe.
The work includes: drilling , boreing, internal threading, fabricating a filler plug, external threading, installing the plug, pilling the plug in place, then more drilling, rough reaming the new chamber, then finish reaming the new chamber.
There will likely also be some milling work needed at the chamber opening.
The hardest thing was the magazine, as just stuffing 308 cartridges in the T99 mag did not work reliably.
It was suggested I try a M14 mag, and I figured out that just adding a block to the rear of the mag, would allow it to fit into the T99 LMG magwell.
As to the ammo, as I said I reload my own. Mild (starting loads) are best, and flat base bullets are a must.
I normally use pulled 7.62x45R bullets, as they are flat base and normally run .309 to .310 in diameter.
Yes, they fit into the MILITARY 7.62x51 chamber OK, can't speak for a Commercial 308 chamber.
Converting a T99 LMG to 7.62x39 is very much the same.
