Technical Question about bolt lug wear/stress between piston and DI gun AR15's

[variablebinary]

I need some clarification on the operation of direct gas AR15's compared to piston AR15's and the HK416.

On a direct gas AR15 when the bolt carrier is going backwards, gas actually pushes the bolt forward, helping to unlock the lugs, rather than have the lug and extension grind together until they are fully unlocked. Correct?

On LWRC, and basically all other piston guns, there is no gas entering a gas key, therefore, the bolt carrier is being forced backwards, along with carrier tilt, which actually places more stress on the bolt lugs and barrel extension while they unlock compared to a direct gas gun.

On the HK416, the bolt is under constant spring tension, so when the piston rod strikes the notch on the bolt carrier, the spring pushes the bolt forward, helping the lugs unlock, taking stress off the lugs, similar to a direct gas gun

Is this accurate?

[MistWolf]

I need some clarification on the operation of direct gas AR15's compared to piston AR15's and the HK416.

On a direct gas AR15 when the bolt carrier is going backwards, gas actually pushes the bolt forward, helping to unlock the lugs, rather than have the lug and extension grind together until they are fully unlocked. Correct?

This is incorrect. The highest the pressure inside the carrier could possibly be is equal to the pressure inside the bore. I am told the pressure inside the carrier is actually less. This makes sense to me as the gas tube being a smaller diameter than the bore would act as a restriction before dumping the gases into an expansion chamber (the carrier) which causes pressures to drop. The carrier also has vent holes. Unless the pressure in the carrier is greater than the pressure in the bore, it cannot push the bolt forward. There is also the "sticktion" of the expanded case to take into account. Even so, the pressure would not push the bolt forward as it is pushing the carrier rearward as there is much less resistance in that direction


On LWRC, and basically all other piston guns, there is no gas entering a gas key, therefore, the bolt carrier is being forced backwards, along with carrier tilt, which actually places more stress on the bolt lugs and barrel extension while they unlock compared to a direct gas gun.

Possibly but doubtful. Again, the "sticktion" of the expanded case helps transfer some of the backthrust applied to the bolt face during firing. The only added stress to the bolt lugs would be if somehow the carrier tilt would cause the bolt to tilt and cause one or more bolt lugs to experience more of the thrust than the others. This is unlikely as to do so would likely cause the carrier to drag along the bolt enough to hamper function.


On the HK416, the bolt is under constant spring tension, so when the piston rod strikes the notch on the bolt carrier, the spring pushes the bolt forward, helping the lugs unlock, taking stress off the lugs, similar to a direct gas gun

Is this accurate?

I am unfamiliar with the details of the HK416. However, in order for the spring to push the bolt forward, it would have to be of sufficient pressure to overcome any rearward forces on the bolt or of those holding the bolt in place

[Pax]

The Adams Arms piston kits and uppers also feature bolt springs. The Spikes/AA guns probably do, but no confirmation on that yet. Other piston manufacturers may also include this part.

[Evil Bert]

Once gas enters the carrier's expansion chamber it applies pressure both forward towards the bolt pushing the bolt forward and the carrier rearward beginning the unlocking process. Now for a very, very short time, the rearward and forward pressures are equal. However, the bolt starts to unlock a bit at this time and the carrier begin its rearward travel, further unlocking the bolt. This is when the cam is engaged and the bolt rotates, etc.

I know nothing of the HK416.

But for the LWRC, while it is identical, it does not utilize the expansion chamber, due to the piston system, equalizing the pressures and unlocking of the bolt, etc. So I think you are wondering if the LWRC puts undue force on the rearward facing portion of the lugs during cycling. I tried to find a reliable picture of an LWRC bolt with a spring on the back like AA uses. I cannot find it. I also tried a search for a POF as well, and I could not find anything. This does not say they do not use a spring, but rather I cannot find evidence of it right now.

I would say yes. This is one reason I have not embraced the Piston design in the AR platform. Perhaps the 416 has addressed this. I do not know.

[Evil Bert]

The Adams Arms piston kits and uppers also feature bolt springs. The Spikes/AA guns probably do, but no confirmation on that yet. Other piston manufacturers may also include this part.

More springs and more parts increases chances for failure.

[MistWolf]

The bolt does not start to unlock until after the carrier is moving. The distance the carrier moves before unlocking is determine by the cam slot in the carrier. By the time the bolt begins to unlock the bullet has left the muzzle and pressures are rapidly dropping.

Gases do not enter the carrier expansion chamber until after the projectile has cleared the gas port. By the time that happens, the bolt has already carried the maximum stress load from the peak of the time/pressure curve

[montrala]

On the HK416, the bolt is under constant spring tension, so when the piston rod strikes the notch on the bolt carrier, the spring pushes the bolt forward, helping the lugs unlock, taking stress off the lugs, similar to a direct gas gun


I do not believe this spring is strong enough to counteract chamber pressure.

Look at G36 bolt - multilug as AR15, but op-rod driven - no spring here and it seems to keep up with forces put on the lugs during unlocking (actually HK416 bolt is designed after G36 bolt).

This spring seem to have more to do with firing pin safety - it pushes firing pin back and makes it engage safety. It may also help on chambering, but this is just wild guess.