Never been a fan of them, much prefer a pack, rock or my knee. The Harris type that attach to a sling swivel I find make carry with a sling uncomfortable then there's the sticky out bits that always seems to get tangled up and they never seen to be that quick to set up for opportunistic shots. The magnetic attaching ones seem a good idea but I'm too cheap to fork out and they don't do try before you buy.
I will admit they are good for target and "sitting stalking" type shooting if they clear the natural cover.
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