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Heavy barrels help if you are shooting long strings of rounds. A stonger barrel will take longer to heat up (and to cool) and will bend less as it heats.

On Wallaby's I would not go below 50gr so...

Different twists have different effects on projectiles. I will stick to 223 as an example as that is what I am familiar with. the slower twists (like 1:12) will stabilise lighter projectiles like the...