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There are a number of differences between the Model 750 and Model 7400.
On the Model 750, the gas orifice hole is drilled at a 45 degree angle, instead of a 90 degree like the Model 740, Model 742 and Model 7400. This does 2 things; it eliminates the 90 degree turn at the gas port, and it eliminates shaving off copper in the bore from the projectile. The copper shavings in the gas port of the Model 7400 would build up over time, and cause the gun to not cycle reliably.

The gasblock on the barrel has been moved rearward and therefore the gas orifice hole has been moved rearward. Moving the gas hole back we can capture the gases sooner, and under greater pressure. This will greatly increase reliability by giving more consistent bolt velocities, as well as allow us to offer carbine models in calibers not possible in the Model 7400.

The barrel extension is now made as a cast part instead of a machined part. By casting, we can better control internal geometries, and eliminate machining burrs. This again, translates into better reliability.
We have added a nickel/Teflon coating to the bolt head. This allows the bolt to move and cam into place more freely.
Since the gas system is greatly improved, we can now use the R3 pad, and not have to worry about functionality.
We lowered the drop at the comb so that you can use the rifle sights easily.

Here some info https://support.remington.com/General_Information/What_are_the_differences_between_the_Model_750_and_the_Model_7400%3F