It's still 22 caliber, but there are two different sizes from NSA. .217 and .218 diameter. One might give better results over the other.
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That is the problem here that the dimensions given are different from the actual ones.
I tried both .217 and .218 in 23gr About the same results but the .218 did a little bit better. I was told to try heavier slugs. I have some 27gr in .217 and .218 I’ll try those tonight and post results.
In a recent hunting video posted by Matt, I think he uses the .217 @ 950fps and is getting excellent result.
At the STX Slug liner listing on Utah airguns website they have mentioned the .22 slug liner for 21-28gr slugs sized .218
Fx in an announcement regarding slug liners have mentioned .22 slug liner for 21-28gr slugs sized .217
Pretty confusing stuff.....