I am thinking a 600 mm M4 in .177 will have a lot of fun in it, but as mentioned in the pea caliber choice is limited.
I would shoot .22 if my masters would allow it, but i can not have such awesome powers..
My Maverick with the original 600mm barrel would not shoot well ( FAC ) ( bad luck in barrel lottery )
But the second i got a 700 mm barrel on the sucker, and later a heavy barrel it was good times, and i am many many 10.000 shots into that rifle.
And i have been having great fun out to 130 yards with slugs i have predominantly been shooting, but pellets have also worked even in the heavy liner CUZ in .177 the 2 liners are the same when it come to twist, so at least with lighter stuff like 13 grain slugs, i be hard pressed to tell a difference.
.22 on the other hand real difference in barrel twist rate, and on the ammo side far more choice, even different sizes in a given weight.
So if no one hold dominion over you like me, i would for sure go for .22, you can also shoot it relaxed if you want to, no saying just the heaviest of stuff as the fastest of speeds will be fine.
