So I wanted to figure this out for myself, and the easiest way to keep track was put it here.  I got my tech info from a website I lost.  I would reference it, but I cleared out my cookies.

OVER gassed from EXCESSIVE dwell time.  Proper port size tames things quite well.  Dwell times over 0.21 milliseconds.  
Pistol 10.5” – Lots of gas to the face even with the correct gas port.  Just get a Car for 10.5”
Car 16” – really bad (0.25ms)
Mid 18”
Intermediate 20”
Rifle 22” and up
I include 22” + rifles here for accuracy sake only.  I don’t think any rifle gas system has function or reliability troubles.  The problem with 24” ish barrels is the bolt can begin to move before the bullet gets well away from the barrel and affect accuracy for BENCH REST SHOOTERS WITH POWERFUL SCOPES AND HANDLOADS.  For precision long barrels stick to rifle buffers or buffers heavier than H2 and you should be fine.  26” barrels have double the dwell time of a 20”.

INDUSTRY STANDARD – Most likely to run any ammo with common industry standard gas port sizes, but often OVER gassed for NATO ammo.  Dwell times very near a rifles 0.18 milliseconds.
Pistol 7.5” – EXPLOSIVE and port size is tricky
Car 14.5” - Still over gassed due to long dwell (0.21ms) and high gas port pressure
Mid 16” – Good
Intermediate 18” – Good.  Emulates 20” rifle
Rifle 20” – Good.  The standard.

IDEAL – best case for soft shooting function with CORRECT gas port size.  Usually these can be chopped down from the next barrel length without gas port changes.  Good with silencers. Ideal for race guns with low mass components.  Dwell times between 0.12 and 0.15 milliseconds. Mid through Rifle gas runs at nice pressures and impulses.  
Pistol about 9” - gas port size is tricky (no LMOS)
Car 12.5” (11.5” is not bad either considering the high gas port pressure)
Mid 14.5”
Intermediate 16”
Rifle 18”

UNDER gassed from SHORT dwell time.  Small changes in port size make huge differences in ammo compatibility and function.  These combos are least reliable and hard to tune.  Adjustable gas is better.  Gas port size is critical.  If tuned for Wolf/Tul, will kick a little with NATO.  If tuned for NATO, may not cycle Wolf/Tul.  Silencers can cause violent cycling without proper tuning & gas reduction.  Dwell times around 0.08 milliseconds.
Pistol 7.5" - boom
Car 10.5” – half the dwell time of a 20” rifle at a very high violent pressure.
Mid 12.5” - half the dwell time of a 20” rifle at a usable pressure.
Intermediate 14.5” - half the dwell time of a 20” rifle at an ideal pressure.
Rifle 16” - half the dwell time of a 20” rifle at an ideal pressure.  Least gas available of any combination.  Note, even this worst case is still running at over 17,000 psi, but we need less than 1,000 psi to the bolt carrier.  Its all about the port.

Note: The pistol gas system is explosive.  Only use durable components and heavy carriers/buffers/springs…. and pray.  Pistol gas can eat adjustable gas screws like lollipops.  Also every time a 7.5” goes off, a kitten dies.

Now cycling is a balancing act.  You have the size of the gas port vs the reciprocating mass vs the spring pressure vs drag on your carrier and buffer vs pressure in the chamber causing the brass to stick vs hammer pressure vs magazine drag....you get the idea.   It aint simple.

Normal ejection is for the brass to fall about 4:30 a few feet away.  Anywhere between 3:00 and 5:30 is fine. If the rifle is not pulling the next round out of the mag you could be under gassed or have too much buffer/spring.

A rifle is usually over gassed if the brass is ejecting from 12:30 to 3:00.  This is because the bolt and brass is slamming to the rear before the ejector can kick the empty case outside the receiver.  The bolt violently bounces off the back of the receiver tube and punches the brass forward.

A dangerously over gassed rifle could eject anywhere from 12:30 to 5:30.  It may not load the next round.  It may not eject the fired case.  It may not lock back on an empty mag.  In other words it may trick you into thinking it is under gassed.  Too much gas is unlocking the bolt early, but there is still too much pressure in the chamber pushing the case against the chamber wall.  The bolt is drug by this case/chamber resistance and if the extractor holds, the bolt will be slowed to the point the gun does not cycle right.  You could see extractor pull marks on the brass, ejector pressure marks, blown primers, flattened primers, or heavy scrape marks on the case walls.

The weaker the buffer spring, the less recoil you feel, but you must add adjust mass or reduce gas or both.  You may have failure to feed issues with weaker springs.

Lighter BCG's and buffer and reduced power springs generally take less gas to function.

If a gun is slightly over gassed, try going up to a heavier buffer.  Car – H – H2 – H3 – etc.  Then try an extra power spring.  Adjustable gas is better.

An ideal gas port would be such that you can run cheap steel cased ammo with a Car spring and Car buffer, but not an H2.  NATO spec ammo would then run reliably with the Car buffer.

If your gun is showing pressure signs or ripping/pulling signs from the extractor pulling hard on the case, you are over gassed and in danger.  Even if the next round is not getting pulled from the magazine.  Reduce the gas or shorten the barrel.  Other options are M16 BCG and the heavier buffers with stiffer springs (these are patch jobs).  Cutting the barrel back or a Govnah are better options.  

Screw adjustable gas block work best on longer gas systems.  The Govnah is best for Pistol and Car gas.  Switchblocks are nice too.

If your gun has an H2 and runs NATO fine (no signs of high pressure), but short strokes on steel cased ammo make sure your buffer spring is not an extra power spring.  Then clean the whole rifle.  Then try a Car buffer.  If that does not work try pulling the weights out of the car buffer or get the TACCOM lightweight buffer.  You could also just try a reduced power spring first, but these are not recommended on defensive rigs.  For range toys they are fine.  Once you have got to the TACCOM or pulling the buffer weights out you should really be evaluating your junk ammo.

One thing of note about competition rifles, they seem to go for adjustable gas, rifle or intermediate gas systems, shorter dwell time (.12 - .15ms), and light weight BCG’s.  Reduced power springs are common as well.  All this reduces felt recoil.  As you go to LMOS and lightweight buffers, you should consider barrels with less dwell time and MUST use adjustable gas to turn things down to sane levels.

The one constant and the one with the greatest effect upon function regardless of barrel length or gas system length is a properly sized gas port ?

What would be the correct size for gas ports

I don't mind the 16" with the carbine gas system


I seem to be the only person who doesn't have issues with tulammo lol

There is no exact and correct answer, only a "within a certain range". Too many variables to deal with although all gas systems have a recommended plus or minus spec.

Every example listed above will vary and no two shooters will ever agree on the perfect combination.

Excellent info here:


Gas system info