From my experience, a bi-pod works best if you match it to your most comfortable shooting position. If you lay down flat to shoot, choose a short bi-pod. If you sit to shoot with your elbows on your knees, choose a longer bi-pod, etc.
Also, I agree with others who said a bi-pod should not be mounted to the barrel, as the rule of thumb is a barrel will be more accuarte with the less attachments connected to it. I have Harris bi-pods mounted to the swivel stud, and am very pleased with them.
All the best.,
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