i'm no fluid dynamics engineer, but that sounds about right, to me.

the gas velocity generated by the much higher velocity 5.45 round is far above the .30 calibre round. the pressure generated has got to be higher, also.

while it can safely be said that the ak-74 brake is not, by any means, optimized for the .30 calibre ammunition, it should easily be able to accomodate the volume increase caused by the larger cross section of the 7.62 pipe.

the volume of a 16" 5.45 pipe is approx. .613 cubic inches.

the volume of a 16" 7.62 pipe is approx. 1.215 cubic inches.

the increase is almost double, not taking into account the gas blead off to accuate the action of the rifle.

what must be also factored into the equation is the spead at which that volume of gas is moving.

common sebnse and real world observation tells me that the ak-74 brake does, indeed, reduce felt recoil for the 7.62 kalashnikov.  real world observation also shows no degredation in accuracy.

as far as 7.62 "flash hiders" (suppressors), they were few and far between. while a few countries kitted out their rpk's with a flash supressor, the ak-47 was not issued with one and the akm was most often issued with a muzzle nut or slant compensator.