I had a gunsmith drill and tap my smooth bore Mossberg 500 many years ago and I've been shooting with a low-power scope ever since. Identify the scope you want to use first, so he can put on the exact mounts you need to hold that scope.
if you find a local gunsmith this should be very affordable. it's touchy work if the receiver is aluminum, but it still shouldn't be expensive.
If you mostly hunt in the woods, I would recommend a red dot. you can also get a 2-power red dot.
otherwise just a low-power scope. make sure you get a big eye relief. a smooth bore shotgun can hit you with a scope, and I've done it. there are muzzleloader and shotgun scopes with eye relief of 5 inches or more. also, trust the eye relief measurement of an expensive/good scope much more than the eye relief measurement of a cheap scope. i've learned that too; cheap scopes that say 5 inches rarely are.
or use a red dot, which has unlimited eye relief but won't give you the power of a regular scope. again that depends on whether you're going to shoot far. from 50-75 yards, it is nice to have a 2 power scope or a little more.
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