So the raging debate between AR-15 / M-16 / M4 family-of-weapons shooters is which operating method to use.  The bottom line to most people is that it won't matter much either way, but lets go through some of the details and arguments.

[contentheading]How do the two systems work?[/contentheading]

Direct Gas Impingement (DGI, or DI for "Direct Impingement"):  Have you ever been at a restaurant, and shot the wrapper off a soda straw?  That's basically how DI works.  There is a tiny hole in the barrel, under the front sight, and a metal "straw" tube that runs all the way back to the bolt.  The bolt has a metal "wrapper" tube that fits over the "straw".  When you fire, the bullet passes the hole, gas is released into the tube, and the bolt is blown back.

Piston:  In a piston driven gun, there is a small piston somewhere along the barrel of the gun.  A small hole in the barrel, similar to the DI set up, allows gas into the piston.  When gas enters the piston, it kicks it back, and a long metal "operating rod" runs from the piston to the bolt, and that operating rod kicks the bolt back.

[contentheading]What are the arguments on both sides?[/contentheading]

There are a number of positive and negative effects of each of the systems.  A factor that is a positive for one system, is generally a negative for the other, and vice-versa.

Arguments for DI:

• Simpler operation, less moving parts.
• Less moving parts, so there is less mass moving around to throw off your accuracy.
• All the top AR shooters use DI.  The piston guns have yet to win.
• "It's the way it was designed originally.", or "It ain't broke, don't fix it."
• Cheaper (less parts again).
• Standardization - all DI guns are basically the same, while almost every piston system is proprietary to the manufacturer.

Arguments for Piston:

• Increased reliability, and fewer malfunctions.
• Easier to clean, since the dirty gas is largely restricted to the piston.
• Receiver stays considerably cooler, since hot gas is not being blown back into it.
• Most successful military rifles have been piston driven.  M-14, M-1, AK-74, AK-47... all piston driven.
• Does not necessarily need a buffer tube (some piston designs do).  This allows a folding stock, like on the FN SCAR.

[contentheading]What's the reality of the situation?[/contentheading]

My opinion, is that for most people, the arguments are too polarized, and you'll never likely see the benefits of either system.

There is one big exception, cleaning!  A DI gun gets pretty dirty, even after just a few rounds fired.  If people have to invent special tools, just to clean the carbon-roasted parts of an AR platform gun, any arguments to the contrary are silly.  I like keeping my guns clean, and clean them after every outing, and I generally have better things to do than scrape carbon off the bolt with my knife.

There also use to be some issues with piston guns, specifically the piston conversions to guns that were originally DI.  The physical strike of an operating rod, instead of the gas push of a DI gun, could "tilt" the bolt and wear the bottom rear of it out over time.  There were some other problems, but realistically, the modern piston guns have essentially eliminated them.  Most piston guns now aren't even conversions, since they were designed from the ground up to run with a piston, which means they function as intended.

Beyond that, the pro's and con's are legitimate, but generally too "edge case" for the average person to experience them.  Few of us are such excellent shooters, that we will notice the trivial motion of a thin operating rod knocking our aim off.  Nor do many of us fire hundreds of rounds a minute, where we will witness the heat buildup in the receiver, and get the gun dirty enough to see a rapid reduction in reliability.

DI AR-15s range around $600-1500, while piston guns range around $1000-2700.

Forced to recommend one, I'd say go with the piston gun, if you can afford it.  The ease of cleaning is pleasant in our busy lives, and having a rifle with a folding stock is very handy.  Get one that was designed to be piston driven (e.g. FN SCAR) However, you can still be successful with a DI gun, so there is little harm in that choice.  Choose the one that works best for your needs and budget.