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What a coincidence, assault grenadiers DID receive a similar buff to rifles.
Wehrmacht - Strategic Reserves
Assault Grenadiers
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MP40 Changes
MP 40s are having their mid-range power increased to match SMGs like the M3 Grease gun used by Assault Engineers. This change also affects Assault Officer and Volksgrenadier MP 40s, but not other MP40 variants.
MP 40 Mid range cooldown from 1.25 to 1
MP 40 Mid accuracy from 0.24 to 0.375
MP40 reload frequency from 4/5 to 6
MP40 reload from 2.8/3.5 to 2.8
Essentially you have an increase of about 40% DPS at range 17. Rifle buff was an increase to near range of 3 which increased their dps at range 6 by about 10%, dropping off as it reaches the midrange of 16(shorter then the mp40).
Combined with the stock buff to RA+a previous buff to vet RA, and reduction in reinforcement cost, assault grenadiers have been solidified as a very decent early game CQC squad.
And yes, like most other SMG infantry, they do fall off in the late game, though the 6man upgrade does help them scale and retain their usefulness more then most other SMG infantry.