I think the high-end 77 Mk IIs have an adjustable trigger, but I won't swear to that. The standard trigger is rather primitive. It can be cleaned up, but it's a job for an experienced smith.
I bought a Mk II in .223. I installed a Timney trigger; it's a piece of cake to do if you have a bench grinder or a Dremel tool. The safety spur needs to be ground to fit, but it's easy to do and only a little care is needed. I tweaked the trigger pull to about two pounds.
I paid about $70 for the Timney. I think they're available from Midway, but I haven't checked the price...
Art