Edit: thanks to shortyusa.com, airosft man and motorbreath for pictures.
Step 1: BB's and Guns
$17.99 will get you 4,000 .2's or 3,000 .25's
also, if you really want to minimize your impact, stop using gas guns. I konw that CO2 is a greenhouse gas, but I'm not sure about green gas, TOP gas, propane, or any of the other ones. I had previously done all the calculations, but then I hit the spellcheck button. For some reason, when I end one internet window, it ends all of them. So ending the spellcheck ended the page without saving it. Because of this, I will skip the calculation and just give you the end result. If 4,000 people use CO2 guns, and on average they use 2 cartridges per week, that ends up being about 1650 pounds of CO2 a year. Not too much on a large scale, but worth worrying about to some people. However, this is only CO2, and is probably underestimated as to the number of people who own CO2 guns.
Step 2: Your Playing Field
Option two is like a shed, but has no roof and lower walls. It works in both a square shape and a ring/octagon shape. You can build one or two foot high walls all the way around, then dig a trench behind it to give you more to hide behind. Uses less material, and is easier to build. However, provides slightly less protection.
If you want to make it extremely natural, you can do the following to create some protection in the middle of the field instead of the "forts" listed above. Take four somewhat long sticks ( the length should be however high you want the cover to be. )
Then put them in the ground in a rectangle formation, like this
The space in between the short side of the rectangle should fit more sticks. Then simply put more sticks inside fo the rectangle lengthwise. Needs no ropes to hod it, but you may need to put something in the ground to support the sticks that are standing upright. If you do not understand the way I worded this, then message me and I will explain it to you.
Step 3: Other Small Hints.
For targets, you can use soda cans and two liter bottles, along with milk jugs and other things. no matter how much they've been destroyed, these can be recycled. for guns with 300-400 FPS ( with .2g BB's ), try putting different size cardboard boxes inside of each other. This will hopefully stop the BB's for 380 FPS and up, you might want to put something of thick wood or metal inside so it completely covers the entire target. Not only will you be recycling old materials, you will be able to catch the bb's as well. A notice to my first step: while these BB's are technically biodegradable, they are only 50% so. the first 50% will degrade in a few years. the other will take a while. Even so, it does still help. This is because they are made in Japan. In Japan, anything even 50% biodegradable can be labeled as so.
Try putting pine needles, leaves, and soil on top of the wood if you have a shed on top of you bunker. This will encourage bacteria to break down the wood if you stop treating it. As long as you treat it or cover it with certain types of finish, you will be fine. But if you stop airsofting, or move a way for whatever reason, it will break down sooner. It also helps camouflage it from a distance.
If you need some cover for a sniper, dig a hole a foot or two down. Sit, crouch, or kneel in the hole, and cover yourself with dead foliage. Make sure you wear stuff over your entire body, as you will get ticks on you. This is the case no matter, how much you wear, but you can minimize this. Also, you may get the occassional spider on you, so you should carry some kind of antivenin or spider bite treatment with you. If you get bit by something, let your team know so whoever has the kit can treat you or help you with it.
Please comment, and keep in mind its my first instructable. pictures soon to come.