i heard they have an adjustment for high rate and a low rate. is this right?
There's no adjustment switch or dial, however, by changing the booster from 8mm to 308 (or vice versa), the type of ammo, and/or the caliber, you can change the rate of fire a bit. The heavy MG3 bolt (IIRC 950g?) in 308 will definitely slow things down a bit, but to my understanding, you do have to be careful swapping out other parts (MG3, MG74, or M53) parts into an MG42 as you can cause reliability problems and possibly accelerated wear and damage to the receiver if everything is not in balance. I keep mine set up as a mock MG3 (standard, not heavy bolt) in 308 and never shoot 8mm out of it. I've never tested it, but compared to other machineguns on the firing line, I'd guess it to be running in approximately the 900-1000rpm range.