AR gas key design question

[DoubleTapDrew]

I watched an episode of Tac-TV and they were going over the HK 416. One of the features was an integrated strike block (or something like that) for the piston rod to hit, in the place where a DI AR would have the gas key. It was touted for it's superior strength vs a removable one. Other piston rifles may be similar (I think the ruger SR556 is).

It got me wondering, since it's important to keep gas keys tight (and everyone wants to make sure theirs are staked properly), why are they a separate piece screwed into the BCG in the first place rather than integrated? Are they supposed to be user-serviceable? Is it stronger than having a one piece bolt carrier? Are they just cheaper to make the way?
I've never heard of someone replacing a gas key before, just the entire BCG.

 

[LJ-MosinFreak-Buck]

Cheaper to manufacture, I would think.

 

[matrem]

Seems to me that making the gas key integral would sure slow down and make for a much higher scrap rate when machining the carrier.
I can't think of a disadvantage of owning one built that way though.

 

[TurtlePhish]

I bet it could be done custom for a lot of money. Then again, a cheaper route could be welding.

 

[briansmithwins]

It would be bloody hard to manufacture the gas port thru a one piece bolt carrier/gas carrier key.

BSW

 

[wally]

it would be bloody hard to manufacture the gas port thru a one piece bolt carrier/gas carrier key.

bingo!

 

[mjsdwash]

the bolt carrier has a very long life. the gas tube will eventually wear down on the key. The bolt could be salvaged after that. compare a new key to an old one to see. but think that any parkerized part that looses its finish with use, will wear at the metal over the life of the rifle.

 

[DoubleTapDrew]

It would be bloody hard to manufacture the gas port thru a one piece bolt carrier/gas carrier key.

BSW

Oh good point BSW. I totally forgot about that aspect in a DI gun.
So in a piston AR what causes the bolt to extend out of the BCG?

 

[wally]

So in a piston AR what causes the bolt to extend out of the BCG?

The gas that flows in through the carrier key pushes the carrier back, its why the gun will become unreliable when the bolt gas rings get worn out.

 

[C-grunt]

Oh good point BSW. I totally forgot about that aspect in a DI gun.
So in a piston AR what causes the bolt to extend out of the BCG?

Usually a spring.

 

[WNTFW]

Post #9
I don't think that answer matches the question.
He wants to know about a piston AR.

 

[matrem]

It would be bloody hard to manufacture the gas port thru a one piece bolt carrier/gas carrier key.

Sure would, but carriers are interchangeable and that has little to do with how they're prototyped or manufactured.

He wants to know about a piston AR.

Guess I didn't get that?

 

[DoubleTapDrew]

Well I was originally asking about why DI guns can't/don't have an integrated gas key in the BCG, then got sidetracked wondering how piston guns do it (extend the bolt), since they do seem to have an integrated strike plate or whatever they call it on the BCG.

 

[wally]

Post #9
I don't think that answer matches the question.
He wants to know about a piston AR.

Oops I didn't catch the context switch.

In a piston AR the operating rod is pushed by the piston and the rod hits what would be the carrier key on a DI carrier. This pushes the carrier back while the bolt is locked in the barrel. The cam pin then rotates in the carrier slot as the carrier moves back causing the bolt to rotate with it to unlock. When it unlocks its been pulled forward, no extra springs required for the bolt, although there is usually an extra spring for the operating rod or piston, designs vary a bit, I've not yet needed to take apart my Ruger or CMMG piston guns to see the actual piston/oprod detailed arrangement.

 

[PPS43]

In a direct impingement AR gas key is not load-bearing. The force that pushes back bolt carrier is exerted on the bolt carrier itself.

In a piston AR the part that replaces gas key is load-bearing, it's getting pushed by the piston rod and drives the bolt carrier back.

 

[Gtimothy]

Oops I didn't catch the context switch.

In a piston AR the operating rod is pushed by the piston and the rod hits what would be the carrier key on a DI carrier. This pushes the carrier back while the bolt is locked in the barrel. The cam pin then rotates in the carrier slot as the carrier moves back causing the bolt to rotate with it to unlock. When it unlocks its been pulled forward, no extra springs required for the bolt, although there is usually an extra spring for the operating rod or piston, designs vary a bit, I've not yet needed to take apart my Ruger or CMMG piston guns to see the actual piston/oprod detailed arrangement.

I have Adams Arms piston systems on my ARs and along with the spring on the drive rod there is a spring on the back of the bolt that heeps the bolt pushed forward in the bolt carrier assy. The gas rings are removed from the bolt as well. The gas key is solid with no opening for a gas tube.

 

[wally]

I have Adams Arms piston systems on my ARs

The designs vary, there is no bolt spring on my Ruger SR-556. My CMMG has a spring that pushes the bolt about half way forward when the bolt and carrier is out of the upper.

Its not really needed, but I can see how it might help keep the bolt centered in the carrier after the gas rings are removed.

How clean the bolt stays on a piston gun is most of their appeal to a guy like me that hates cleaning guns.