Yeah, they can be found just about anywhere that sells cameras. They are priced anywhere from $1 - $15. The 'nicer' usually have a larger clamping area using most or all of the inside ring (not like the one in my picture, that was a cap from an old cheap spotting scope). The pinch area on mine that locks it in is very narrow. The larger should offer better protection and seal. Some are also sold with a tether, might be able to rig it somewhere.
And no, I haven't had any issues with the scope touching the barrel. The angle in the photo with the dime next to it isn't perfectly lined up... There's a little more room than it looks, not much though. I can slide the first 2 feeler gauges underneath. It's also a 17 hmr
Hope that helps-
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