So here's my advice with slugs in any gun, especially ones with unchoked barrels. You can try a ton of different slugs in different sizes till you find one that's the most accurate. Nothing worng with that method. Or you could slug the barrel and measure the slug with a micrometer to find a good starting point in the right slugs size to buy and try.
I found alot of chineese barrels are oversized. I sluged a Umarex zelos .25 barrel and it sluged at .2541. Also vulcans, uragans, leshy and few other run oversized barrels.
Example a vulcan .30 cal shot ok/good with .300 size slugs. But it was stupid accurate with .3063 size. The right size as the barrel sluged at .3061.
I noticed oversized bore are particularly common on chineese barrels. With the zelos oversized at .254, it made sense why .249 were terrible and .250 slug weren't particularly accurate. Some were ok or possibly acceptable at shorter ranges like 50 yards. But at long range it will become more apparent.
In addition it's possible to be loosing 20 to 30 fps with the same grain slug that is undersize to the grove of the barrel. I ordered .254 slugs to try. In general you want the slug to fill and seal the grooves. Just something to think about with unchoked barrel and slugs.
Zelos .25 cal measurement
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