Question about AR systems

[avs11054]

First, please excuse the lack of technical terms, as I do not know a whole lot about what I am asking about, which is probably why I am asking it in the first place.

I recently read a post on THR, which stated some ARs use direct impingement, and other ARs use a different method of gas opperation (possible gas piston, but it could be something else). How do I tell what ARs use what system? I am asking this question because I was issued a Colt 6920 at work a few months back. I clean my guns religously after shooting them,. I would sometimes go to the range and shoot only 40 rounds out of the 6920, and when I would clean it, the chamber was absolutely filthy (expected from an AR). I just recently bought a Colt 6940. Saturday I took it to the range and shot 40 rounds. When I cleaned it, there was hardly any residue in the chamber. Today I took it to the range and shot 60 rounds. Once again, hardly any residue in the chamber.

I highly doubt Colt would use different gas opperation systems in guns that are virtually identical except for a few details, however, it blows my mind that there could be such a huge difference in the cleanliness of the two rifles.

 

[benEzra]

The overwhelming majority of AR's, including both the rifles you mention, use direct impingement. Also, note that chamber fouling isn't caused by the DI system (the DI puts gas into the back of the bolt carrier and it is vented out the gas vents in the side, out the ejection port). In my own experience, my direct-impingement AR (16" midlength) doesn't seem to get much dirtier than my gas-piston Romanian AK does.

The difference you noticed may have to do with different ammunition, with one rifle being better lubricated than the other, etc. But they are both DI guns.

 

[Good&Fruity]

Heavy lube does help keep it cleaner.

 

[madcratebuilder]

What Ben said. Ammo makes a huge difference in how dirty the rifle gets.

 

[mljdeckard]

That's what I was going to ask, were you using imported steel-cased ammo when it was dirty?

 

[bbuddtec]

One way to tell is the usual gas port volume option, on piston machines, located on the front of the gas block. Or if the gas block is the sight post, you definitely have GI

 

[Tirod]

Having owned and shot a HK91 for almost 20 years, it's not the action that dirties it up. The HK is mechanically roller locked - no gas at all. Shooting commercial hunting or new handmade reloads, it would get somewhat dirty. ALL self loading actions open the chamber with some pressure left in the barrel, and it dumps into the gun.

Pakistani .308, what a mess. I still have some boxes of it from the '80s, it blackened the bolt in ten shots.

Powder is what makes the mess, not the action. As said, DI directs a gas pulse through the tube, into the key, then inside the bolt carrier, where it expands and presses against the back of the bolt. THAT IS THE PISTON IN DI. While pressurizing the BCG, it's the only thing that can move - as it moves back, it rotates the cam pin, unlocking the bolt. As the bolt carrier group continues 1) the BCG ports start exhausting gas out the ejection port, 2) the brass is pushed back by residual pressure in the barrel, which escapes around it as soon as it moves.

Only when the BCG moves far enough can the gas tube separate from the key - the pressure has dumped into the action from the chamber by that point, the tube is practically a non contender in comparison. Look at where it's located - high and back of the chamber. It can't dump gas in large quantities because it's metered by a .065 to .095 hole, it doesn't turn 180 degrees and dirty up the chamber, and it won't because it's not moving against the pressure dumping around the case.

Point being, it's not DI making brass and chambers gritty, it's just normal barrel gas. Second, compare the DI piston to a barrel mounted piston - they both get dirty. Your choice if you want to ignore it, they will run anyway. All properly designed guns take into account gas residue and allow for it. Military guns tolerate a lot. Some tolerate 50K rounds with nothing but a wipedown and lube, and have never seen a bore brush. Others have gone 2,400 rounds with no lube at all - about ten basic loads of ammo - which no soldier would do in combat. It would get wiped down and lubed daily regardless, not every ten days of heavy combat.

Just some food for thought on the AR, stuff that isn't talked about even in the 22 years I served in the Reserves.

 

[Bartholomew Roberts]

There shouldn't be any major difference in how dirty the chamber is between a gas piston rifle and a DI rifle shooting the same ammo. DI vents gas well behind the chamber. The entire upper receiver and bolt carrier group area in the DI rifle might be a little dirtier; but there shouldn't be any difference in the chamber.

My guess is ammunition is probably the explanation for the difference.

 

[avs11054]

Well, I know it's not the ammo. Only ammo I use in either one is Federal American Eagle. Maybe I'll check the ammount of lube I've been using.

Thanks for the responses.

 

[Shawn Dodson]

How do I tell what ARs use what system?

Simple. With the bolt in battery and the ejection port cover open look through the ejection port. If you see two holes in the bolt carrier, one on top of the other, then you have a direct gas impingement system. The holes are vents for propellant gases. If there are no holes in the bolt carrier then it is a piston system.