There is a difference between cyclic (250rpm) and practical (150rpm) rate of fire. Cyclic is how fast the weapon is actually capable of firing, and practical is how fast the crew decides to fire the weapon. If you want the weapon to fit more along the lines of practical, just make it a non burst weapon with a short cool down. I chose cyclic because you can have a weapon that fires in short bursts, with a cool down in between bursts, which would equal practical rate of fire.
Think of it another way: the MG 42 has a theoretical cyclic rate of fire between 1200rpm and 1600rpm (or 20rps and ~26rps). The practical rate of fire is much less. However, you wouldn't change the animation speed, you would just make shorter bursts and add a cool down between them to simulate the gunner allowing time to cool.
If you want to slow down the animation in OE, just change the animation speed. I believe I set the FlaK 43 to play at 278% speed (which acts as 1.5 * 2.78, or 4.167rps).