If I upgrade to 308 ammo on the (og) will my rifleman perk be useless?

No. Why would it be useless? It's still a rifle.

Rifleman will still work as long as you keep the Overseer's Guardian as a semi automatic.

And for that matter, the Overseer's Guardian as it sits when you buy it is a full-auto. As a full auto, Rifleman doesn't apply to it anyway. Commando is the perk that applies to automatic weapons. Rifleman only applies to semi-automatic rifles (and shotguns). So unless you put a semi-automatic receiver, such as the .308 receiver on the Overseer's Guardian, Rifleman is irrelevant to it.

Overseer's Guardian with the .308 port and modded up the way you like it is a beast.

I thought it would be useless as 308 is sniper ammo and mabey the game would not let it work on rifleman! Thanks for your replies?p.s iam a noob

Note that two shots is also an very good mod for hard hitting automatic weapons as in combat or assault rifles. The second shot is base damage and is not hit by the nerf you get converting to full auto, the extra damage will also benefit from commando perk.

For miniguns or machineguns exploding or bleed is better as its fixed damage. \

It works. In .308 with Rifleman 5 and no other damage perks it's 180 damage for me.

If you mod this gun will it cancel its bonus?

I like your avatar

well thankyou, I'm a cat person so love yours too! haha

Nope. Sniper rifles are still rifles. The perk also still applies to .50 caliber sniper rifles (upgraded from the base hunting rifle).

Nope. You'll get the bonus regardless of the mods you put on it. The Overseer's Guardian has been my primary weapon for a long time now. It's an excellent gun.