AR-15 Stuuuuuff.

This is not going to be an article. I honestly just made this so i can go back to it as a reference and show it to others. There are hundreds of articles that explain all this shit. 

 AR gas systems. 

Pistol Length 

Barrels less than 9” 

Gas tube length 4.5” 

Carbine length 

Barrels 10-18” 

Gas tube length 7.5” 

Mid-Length 

Barrels 14-20” 

Gas tube length 9.5” 

Rifle length 

Barrels 20”+ 

Gas tube length 13” 

Carbine Length vs Mid Length Gas system.

What is the point? AR's with barrels between 14-18 inches can use either, which is better?

The short answer is Mid Length

The longer gas system is going to provide less pressure going back to the bolt, less pressure is better for longevity and will reduce felt recoil.

Mid length gas systems will have a slower rate of fire. measured on a 14.5 barrel using NATO M855

Carbine = 864 rpm

Mid = 737 rpm.

For more info this report from NAVSEA has a ton of info, even on shit that will never effect you.

https://partner-mco-archive.s3.amazonaws.com/client_files/1527866983.pdf

AR barrel twist rates 

1:10 twist rate or higher barrels should never use anything higher then 65g bullets. 

Buffers 

Carbine Buffer — 3 oz.

For a carbine-length rifle, 3 oz. proves useful with associated gas systems and various supported ammunition types.

Heavy (H) Buffer — 3.8 oz.

There is a wide range of heavy buffers available, starting with the 3.8 oz. model that proves effective for rifles experiencing erratic extraction.

H2 Buffer — 4.6–4.7 oz.

The second tier of heavy buffers is ideal for mid-length gas systems, though it’s common to find up to 4.7 oz buffer weights in pistol builds with under 16-inch barrels to reduce violent cycling. If you fire 5.56 NATO, a tier 2 buffer weight is ideal.

H3 Buffer — 5.0–5.4 oz.

If your AR platform fires 7.62x39, .300 AAC, or even .50 Beowulf, the heaviest buffer weight is your go-to, as it will help reduce the recoil and cycle the larger rounds.

Pistol Buffer — 5.0–8.5 oz.

You would think a pistol caliber build, which uses smaller rounds overall, would require a lighter buffer weight. The opposite is true, though. In a pistol caliber build, the traditional gas system of an AR is missing outright. Furthermore, the bolt carrier group is typically heavier. To smooth the action and ensure reliability, a larger buffer weight is a must-have.

Buffer springs  

A standard carbine spring will measure about 10.5” and have 37 to 39 coils, but a standard rifle spring has a typical length of 12.75 and boasts 41 to 43 coils.

As long as a rifle spring is more than 12 inches long, and a carbine spring more than 10 inches long, they are still serviceable. When they have shortened to those lengths or shorter, replace them.