What maker and model or custom build? there's so many short stroke ARs nowadays.
It's a Ruger SR556, factory configuration.
Do you anticipate using an AR-15 in harsh environments?
It's not so much harsh environments as
any environment. Depending on certain qualities of the ammo you're using (even actual quality ammo!), you may have to break down your AR for cleaning in the middle of a static range session because the weapon has jammed.
You know the joke about 'direct exhaust injection'? Well, the regular AR-15 uses something not unlike DEI to run. The hot gases that are used to propel the round down the chamber are tapped off the barrel and routed back to the bolt carrier. The gases literally blow the bolt carrier backwards and cycle the rifle. This means that the hot gases, burned powder remnants and everything else that's in the gas cloud that is generated by firing a gunpowder firearm are in part vented
into the rifle. This has lead to many, many ARs jamming over the years, not to mention making it much more difficult (but even more necessary) to clean frequently and making the entire weapon very, very warm to hot after even a short shooting session.
Even if you clean and lubricate the weapon properly and frequently enough that it doesn't jam due to 'crapping where it eats' if you are in a dusty or sandy environment, the proper level of lubricant can and does attract said dirt and dust which can cause the weapon to jam. Even when just on the range.
Most competing weapons platforms today (and indeed all the subsequent systems designed by the AR's creator) use a gas piston and rod of some sort to move the bolt or bolt carrier. This means that the gases tapped off the barrel hit the piston and drive it backwards, but they don't continue on into the receiver. The rest of the weapon doesn't heat up, crud isn't automatically blown into the workings and the necessity of sudden cleaning and maintenance is greatly diminished. This is important; when your gun goes down and needs to be cleaned in the middle of the action it really sucks, whether it's in combat or the middle of a civilian timed shooting competition.
Here is a link to a Flash animation showing the differences between the two types - gas piston is represented by the HK416 and the M4 represents direct gas impingement. You can switch back and forth between the two in the animation by clicking the 'COMPARE TO' button in the lower right corner of the graphic.
http://www.armytimes.com/projects/flash/2007_02_20_carbine/
Isn't that basically the same thing HK did with the 416 a few years ago, or is it some different kind of piston operation?
Similar idea. The HK416 is basically a G36 gas system (itself a derivative of the AR-18) tacked on to an AR-15 type upper receiver with a more traditional HK-style barrel attached. However, it's recently been reported as having some severe problems: it doesn't like cold weather (the gas system literally freezes up and takes down the weapon) as well as having issues with the upper receiver cracking - it's so bad that USSOCOM, who had helped develop the HK416, gave up on it and went back to the M4A1 (and later moved on to the FN SCAR). Also, while it jams less than the parent AR-15/M4, the HK416 still jams more than the SCAR or the XM8. Civilian versions are, as is usual for HK, ridicu-priced for what you get (or rather,
don't get.)
The Ruger uses an all new two stage gas piston system, which seems to resolve a lot of the problems with piston-type ARs, and it seems to vaguely resemble the FAL's gas system, user-adjustable gas regulator and all.
