Biggest thing, IMHO, is a good staple gun and good ear protection. You don't have to drop a lot of money on the latter- a bag of 50 pairs of earplugs runs like $15 at the hardware store, or $25 at the gun shop. Same plugs. 32-33db is usually what you'll find. I have fancy electronic ones that I wear hunting, and I've used reusable ones, but the disposables are use once and toss, and I find I get fewer ear infections since I've started using those almost exclusively. They are more comfortble than muffs, and if you wear glasses, muffs lose a LOT of their effectiveness. But if you'll be shooting indoors, I'm going to suggest adding a pair of muffs as well. When the guy next to you lights off a hand cannon heavy enough to hunt T-rex, you'll be glad for both.
The staple gun is becuase you can spend a lot of money on cheap $12 staple guns before you just admit you've been a dummy and buy a $30 one and never buy one again. Again, hardware store.
Shooters aren't as gimmick-obsessed as golfers or fishermen (but we come close
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BTW, I have the same locker. The plastic shelf kinda sucks, I'm not going to say how I found out, but there was a broken scope and a scarred pistol involved. Use the holes, but buy some plywood and some secure bolts and some small 90 degree brackets (or scraps of wood) and build one that won't break and goes all the way to the back, to.
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