The cock on opening conversions usually don't work very well as they have only a short firing pin fall. Better just learn to use the cock on closing.
Frankly, I would not put money into that Oviedo. They are OK if the barrel is good and they are left in 7mm, but there are much better rifles available.
If the rifle has been converted to .308, which operates at 15-20k pounds of pressure over what the rifle was designed to take, leaving it alone is an even better idea. They won't "blow up", but the receivers are soft and will eventually stretch.
Jim
