The problem with "heavy" bullets in little cases is that, particularly for hunting, the down-range performance of any given bullet is dependent on "terminal velocity" to achieve the right balance of penetration and expansion.

The relatively small 7.62 x 39 case just does not have enough capacity to launch a bullet that is much heavier than the original, at a velocity that will deliver the full potential of the bullet down range.

If your magazine will not allow seating out of a longer, heavier bullet, you are effectively wasting powder space, and there isn't much spare in this compact cartridge.

Furthermore, most .303 / .311 bullets in the 150gn range are built to be launched by .303 class cartridges, and thus have fairly "robust" jackets.

HOWEVER, if you want to go the other way and play in the field with the .300 Whisper, the twist rate (1:10") is too SLOW for "big", eg. 220gn. bullets launched at such low velocities. .300 Whisper / Blackout runs in 1:7" twist, just to stabilize the long bullets at low speed.

I have heard of folks splitting the difference and using 170-180 gn FLAT nose bullets designed for the .30-30 etc. These are relatively short for weight, have thinner jackets and are designed to "perform" at .30-30 velocities. How they perform on game at subsonic velocities is unknown to me.

How they feed from various mag. systems is subject to experimentation.

Have at it!