I love mousetraps, here's a way to chase bears with them.
Every fall several animals, but especially a bear, kept eating all the fruit on Barbara's trees and breaking off limbs. Then he leaves big insulting piles of half digested apples in the road. So I made a bear chaser from a mouse trap and a piece of iron pipe. It seems to work ok, the biggest problem is locating it so the bear will trip it but the wind won't.
It uses a .22 cal bank like carpenters use to fire lug bolts into cement. Loudness depends on the size of the cartridge used, lumber stores have them in about 5 sizes for various cement hardness and bolt length. Some brand names are Ramset (HD22, RS22, 721, 4170, M70), Remington (476, 490), Simpson Strong Tie (P22AC4, P22AC4, ), and Hilti (DXE37, DXE72). They are color coded and ranked by number, 1 (gray) is the lightest charge and 5 is the highest (I haven't seen #5), 4 (yellow) is next to highest. Here's some from Home Depot in boxes of 100 for $7. The smallest charge seems to work ok but ours isn't a very big bear. I've been attaching them to the trunk so they'll be at ear height when the bear is standing on his hind legs, but it's probably better to mount them up high so it won't fire at eye level. Wear hearing protectors or earplugs when you set these traps. Some eye protection might be a good idea too.
Step 1: Some Considerations
These photos show a much shorter wood stock than is practical. A long stock allows it to be fixed to the tree with more options, like with rope or vinyl garden tape. If it's simply hung on the tree, the wind knocks it around and sets it off. If it's mounted with the trap on top, birds can light on it & get smacked.
A number of strings can be tied to the wire loop. The other end of the strings are fixed to branches or whatever he has to climb through to get to your apples, chickens, pets, livestock, or garbage cans.
The mousetrap's mouse bopping wire fires the .22 rimfire cartridge by just hitting the end. The bopping wire can become bent after a few firings but it's easy to straighten. If the trap has a weak spring or the shell has a tough primer, wrap some wire around the mouse bopping wire to give it some weight. I just bought a new pack of Ramset #4 blanks so I can hear them from indoors when they go off. Sadly, the primers are harder than my l'il mousetrap can dent even with added weight. The makers mark on the primer end says Super X. I had this problem with my first one and solved it by switching in a rat trap. I don't remember what kind of blanks I used, they came in a black and white box. Cartridges with higher power (above #2) might blow out of the breech. That sounds scary but it's only dangerous if someone standing up close gets hit in the eye. Solution: devise some kind of breech lock or wear eye protection while strolling in your orchard on windy days. Advise kids not to mess with it too ("It keeps bears away". "... RRight Dad, camels too?" "Sure, why not").
Crows and coons also raid trees and trash and though they don't take as much, they can spring the trap so it won't be ready for any bears. A family of coons can take a week to eat all the apples but a bear can do it in one night. Hang it on the trees that ripen earliest, say peaches first, plums, pears, then apples. Grapes in there somewhere too. One or 2 bangs will sometimes protect them all for the year. It's prudent to check it every day when the apples start to ripen in case there are several gangs. Raccoons have big families.
This one is mounted on a bolt once used to re-attach a limb that the bear broke. The tree began to heal but he came back the next year and took it off completely. The bolt had already grown in so now it's solid.
Step 2: They Have to Climb This Net to Reach the Fruit.
A net thingie can be attached to the bear banger so the weight of anything climbing the tree will pull it down and spring the trap. It's made from a hoop of #8 or #9 wire and heavy weight plastic bird net. It helps to have a vise and large pliers to form this weight wire. It still weighs practically nothing, but light weight netting is impractical. Heavier plastic nets tend to have wide slick "cord" surfaces which slides across tree bark where light weight net hangs up every time. I suspect the net alone gives them some pause though because the trap is seldom fired even when windy, and the tree is still protected. They may still remember getting banged a few years ago though. Or it may seem too rickity.
The dark band on the tree is tanglefoot to keep ants from farming aphids & spreading mold on their nasty little feet.
The design of these traps blocks the barrel so only gas & noise get out, but a slug can't.
Historically, Trap Guns (for shooting skeet) aren't the same thing as Gun Traps (for shooting anything that trips them). Gun Traps (AKA Spring Guns) are a totally bad idea, irresponsible, dangerous, indiscriminate, illegal as hell, and I recommend that you never build one.
Step 3: Be Like MacGyver
This is another version using a 3/4 inch iron pipe nipple and a yellow brass 3/8 x 1/8 pipe hose barb wired to a piece of wood I found on my junk pile. The "3/8" size of the hose barb refers to the inside diameter of the hose that fits it, not the inside of the barb. I suspect the wall thickness might vary between manufacturers so measure it before you chose a firing chamber. The English Navy mandated Red brass cannons (copper-tin, aka bronze) instead of Yellow brass (copper-zinc) because red brass will split when it fails and yellow brass shatters. I appreciate that, but the relative charge to ball weight is what causes cannons to explode and we don't have any ball weight because it's a blank. So I still feel relatively safe.
I chose a brass hose barb because I was wandering around the hardware store with a .22 casing looking for an appropriate size hole and that one fit. If you don't have a drill, I suspect a lot of available metal things have a hole that works. Just look around. I threaded the hose barb into the pipe to keep with my design philosophy that these are noise makers and not gun traps, because I had the tools to do it and it was cheaper than buying a 1/8 inch pipe Tee to do the same thing, but that's a possibility too. Or heck, just use a lot of glue and bury the un-threaded metal parts inside the wooden stock for strength. I recommend Household Goop because it's rubbery, sticks to everything, and resists shock. However it's rubbery nature can cushion the blow of the bopping wire if the chamber isn't massive enough, or is loose, so the trap won't fire. Clean it off your hands with gasoline.
This one uses a 3/4 inch pipe nipple attached to the wooden stock with crab pot wire. Bailing wire or string is probably good too. Be like MacGyver.
The metal trigger of this style mousetrap is more sensitive than the yellow plastic kind in the upper photos. The yellow plastic (less sensitive) kind are better for this gizmo because they need an authentic push to trip them. Since we're hunting bear instead of mice, this shouldn't be a problem. Both are made by Victor. There are some Chinese traps available also that use plastic triggers and cost less. I haven't used them yet.
This one also has a breech lock of sorts to keep the casing from popping out when the gun fires. It also holds the bullet in when the trap is inverted. It's a washer with a notch that turns to allow loading (there's a 5/16 inch nut beneath it used as a spacer to set it at the right height). The problem with breech locks is sometimes the expanded casing remains stuck in the chamber and needs some kind of prying device to pull it out. It helps to rub some paraffin or candle wax on the casing before it's loaded. That doesn't happen with an open breech, the shell just blows out when it fires, like an automatic.
Drill a 3/8 inch hole through the mousetrap beneath the big fender washer, so the trap can be moved around to strike the cartridge in the best spot. Then tighten the screw.
Step 4: Inverted With Lever. a Spring Holds the Nets Weight
This is the latest version as of 16 july 2015 except it now uses a rat trap. The color looks odd because I dipped it in copper-green and Linseed oil to protect it from rot & weather. Paint probably works too. It's meant to be mounted with the trap on the bottom so birds won't trip it. But upside down requires a lever to invert the downward pulling force of the net to the top side. A lever (unlike a string) doesn't have to slide over random stuff with variable friction which might prevent firing. A spring holds the weight of the net and zeros the trigger so it is less likely to be jiggled inadvertently. It works well & I'm happy with it, but there's probably room for improvement, so go crazy.
It's a good idea to put a warning sign or bright plastic tape on the trees to warn people. I don't guarantee that animals won't figure it out too from a lot of signalling. So be subtle, think of something only people would believe (and let me know what it is).
Step 5: Add a Flag That Shows It's Been Fired
This is a view from below showing the long stock, a rat trap in place of the mousetrap, and a folded red flag that drops out when the trap is sprung to show where the action was and which tree needs to be reloaded.
Instead of a .22 blank, a somewhat safer noise maker is an air horn (amazon search) like boats use to hail each other. A problem with some hand held air horns is that reliable ones cost more and the air leaks out after they've been used so it's empty after a few months. There are refillable tanks for bicycle air horns that cost $30. A person could buy a lot of apples for $30 but it's the principal of having opposable thumbs which demands that we build it rather than look for other food sources like amoebas [do] (don't eat amoebas). It could be connected to the hanging net and work for the entire picking season without reloading.
Here's a youtube link for a DIY pvc locomotive air horn. 100+ db at 100 ft, 180+ db at 3 ft (130 db = immediate hearing damage). Here's a simpler version, not so loud, won't wake neighbors in the next farm.
Step 6: A Last Resort
If none of the above work (because maybe the bear is already deaf or has
the hormones of a Tyrannosaur): a third alternative which leaves a more convincing impression is an automotive air bag mounted in the tree so it physically pushes the SOB out with enough boost to land him a few feet away.
Please note that I haven't resorted to this yet because the mousetrap works fine, but there are places in western Canada with much bigger bears and otherwise really polite people who still have limits to their exasperation. This plan is for those situations and needs careful thinking because the tree will absorb an equal and opposite push which could break sizable limbs (um.. not to mention bears limbs too which is unnecessarily cruel, and if he dies from his injuries, you'll have to go through the whole thing again in a year or 2 when another one (possibly less civilized) moves into that unoccupied territory). Set it up on a well rooted mature tree, so that the bear doesn't have too far to fall. It will also require a series of flashlight batteries that total 12 volts to set it off. Air bags like the one in this picture can be found in wrecking yards*, or your friends junk car. There are some wreckers who won't sell them. They can't be shipped through the USPO because they contain solid rocket fuel. They can't be tested before use. It would be a waste of money to use it on coons, and it might knock him through your window. If they go off in your face, they might break your nose, so disconnect the battery before you remove or work on them. Therefore: arrange the trigger to require a heavy hand which only a bear can provide. They should also have a waterproof cover such as a plastic bag.
*Car-part.com is a wrecking yard hotline listing the highest prices on the first page, the lowest are at the bottom of their listings, and marked with an asterisk. Sometimes yards don't remove the items from the list after they sell, and they just don't answer those emails.