For the sake of argument, let's say you've shot your first 3-gun match, and now want to make your rifle more competitive. I'm going to use the AR-15 as an example here, as it's pretty much the ubiquitous platform for 3-gunning. We'll delve into the world of piston driven guns later.
The basic M4 pattern carbine suffers from several drawbacks for competition use, though, it's come a long way since Eugene Stoner designed it, and the basic operation is still the same. This means that it's over-gassed, lacks an effective muzzle device and has a crappy trigger. The stock handguards also mean that POI can shift dramatically if you make effective use of a barricade, so clearly there is room for improvement. The good news is that all of these shortcomings can be overcome relatively cheaply, mostly using tools found in the average garage, without shelling out for an entirely new gun.
The parts required to turn a stock, off-the-shelf carbine into a match winner can be found at any one of a number of online vendors, but to save you the hassle of ordering parts individually, Wheaton Arms puts together a basic kit that gives you pretty much everything you need. Crisp, single stage trigger; check. Ambidextrous safety for weak hand shots; check. Adjustable gas block to control bolt carrier velocity and a big 'ol compensator to drive the muzzle forward and down; check and check.
In order to deal with the problem of barrel deflection due to contact with cover, most competitors use a free float tube to isolate the two. The number of available free float tubes has exploded in the last few years, but Samson Manufacturing's offering is one of the best. It's very lightweight and available in a number of lengths, including the increasing popular, monster 15-inch version. Samson also has delved into the 3-gun kit market and now offers a bundle of products including their Evolution rail thrown in with a number of other goodies. They're loyal supporters of the sport, and you'll probably see more of their fore-ends in use at a major match then any other brand.
For this project, we'll use a Bushmaster MOE carbine as the test mule and go through the upgrade process step-by-step as we break out the gun plumbing tools. But, before we get to install our new goodies, we're going to remove the old parts.
Stripping your AR-15
First off, clear the weapon and separate the upper and lower receiver. Remove the handguards by pulling back on the delta ring and lifting them clear. Place the upper in a vice upside down, taking care to clamp around the front sight tower, not the barrel - this part will be discarded. Unscrew the A2 flash hider from the muzzle. Drive out the taper pins holding the front sight tower to the barrel and tap the tower off the barrel towards the muzzle. Drive out the roll pin that secures the gas tube and then remove the gas tube.
Take the lower receiver in a firing grip and using your support thumb, and lower the hammer. Using a punch, press out the hammer and trigger axis pins and remove the hammer. Press firmly on the right side of the safety pivot, where it comes through the RHS receiver wall while rotating the safety lever; this will cause it to pop free. Remove the safety. Remove the trigger and disconnector, along with their springs.
You should now have a pretty sorry looking carbine and a pile of spare parts. Here's where it gets interesting.
Installing the New Kit
The first part to be installed is the gas tube to the gas block. Make sure the gas tube orifice lines up with the corresponding hole in the gas block, then drive in the roll pin to secure it. Place the gas tube onto the barrel and line up the gas tube with the barrel's gas port, then tighten up the set screws.
Place the Samson thermal bushing over the barrel nut. Slip the fore-end down the barrel and tighten up the set screws. For a full demonstration of the process, go here.
Screw the muzzle brake onto the muzzle and torque it down until it indexes with the 12 o'clock position. One of the easiest ways to screw up accuracy is to over-tighten a muzzle device, as this will stretch the barrel and constrict the bore at the worst possible place, so if the comp doesn't line up first time, you can file a little off the convex side of the crush washer. Alternatively, use a peel washer and adjust the number layers to achieve a perfect fit. I usually just put a 3/8-inch punch through the compensator and use that to tighten it, holding the upper receiver in my left hand. A dab of loctite makes sure it stays there.
That's it for the upper. Let's finish up the lower & take this thing to the range.