Sorry, but that question is impossible to answer without more specifics. What is you budget? What is the intended purpose? These are just two questions which have a huge effect on the answer you're going to get. Please be more specific in what you're looking for. THIS LINK will give you a whole lot to think about.

Placeholder.

There are four types of AR15 gas systems, Pistol, Carbine, Mid length/Intermediate, and Rifle. Pistol gas systems are dependent on the barrel length.
The AR15 .223/5.56 was designed with a 12" gas system for a 20" barrel, therefore a 20" barrel with 12" (rifle) gas system performs optimally, & gives an approximate 7.5" of bullet travel from the gas hole to the end of the Flash hider, around 0.200 ms referred to as Dwell Time. Increased Dwell time will allow for more gas to be forced back through the gas tube into the gas key. Target rifles will usually have adjustable gas blocks.
Carbine length or 7" gas system is the gas system used in the M4 Carbine with a 14.5" barrel & yield 7.5" of travel dwell time. Measured from gas hole to end of flash hider. Uses a gas tube that is approximately 9.75" total length.





The carbine length 7" gas system is also used on 16" barrels and have 9.5" of travel dwell time.
Mid length or 9" gas system is a commercial solution designed to be used on 16" barrels with an approximate 7.5" of travel dwell time. It is visually different from the Carbine having the front sight base 2" further down the barrel. Uses a gas tube that is approximately 11.75" total length.
Intermediate gas systems are also available by two manufactures, one(noveske) is 1.5" shorter than rifle length approximately 13.6 " in length. The other(knights armament company)  is approximately 13.2" in length the same gas tube on all SR models.
Rifle length or 12" gas system is the gas system used in the M16 series of rifles with 20" barrels, and can be found on barrels of 18" to 24" in length. Uses a gas tube that is approximately 15" total length
Carbine buffers: the H stamped buffer is issued with the M4 14.5" barrel and is optimal for the 16" barrel with either the 7" carbine or 9" mid length gas systems, however some will use the H2 buffer. Most lowers with a collapsible stock come with the standard carbine buffer (no stamp).





Examples of buffer weight in ounces No Mark 3.0 / H 3.8 / H2 4.6 / H3 5.4 (actual weight will vary by manufacture)
Increasing the buffer weight will slow the auto cyclic rate and increase time for the bolt to unlock, the brass has more time to shrink away from the walls of the chamber and slightly reduces the felt recoil, but puts more mass in motion, which may or may not affect the follow up shots depending on the shooter. Increasing the buffer weight too much will result in a short stroke or fail to function. A lighter buffer has less mass in motion, which lessens the chance the sight picture will be disrupted between shots.
Different results can be obtained by changing of gas port size, adjustable gas blocks. Timing is also at play. (Not discussed here)





-contributed by FeuerStolz
*It is important to note that there are 3 main stream choices in carbine, mid-length and rifle.  They are all basically the same size and interchangeable from brand to brand.  When looking at pistol length different companies offer different lengths and even different configurations such as ones that circle around the barrel like a pigs tail.  It is valuable to know exactly what length is being sold compared to what you need to save headaches.  The other alternative is the intermediate which is growing in popularity amongst SPR type rifles.  -nick

We get many threads that are something like this....I am looking for information about reloading any help would be great.  Below are a hand full of questions or topics of discussion and a thread or section that covers that type of info.  Click on the topic or question to be redirected to the information.  If it takes you to a whole section of the forum first look over the frequently asked questions if it is not covered you may need to ask.

Placeholder