You have to slide them slightly along barrel while checking groups. Yes they do improve groups slightly if you can stand to have to look at them on the barrel. They work best on thinner barrels.
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Sometimes you earthlings REALLY amaze me!
I have a 300 weatherby mag, it is well worth the 16.00 dollars.
I have a very good shot placement, the simms made it alot tighter.
all you gotta do is try it and you will see its worth it.