Flattened primers

[bwaites]

The CWS helps, but I'm also running a stiffer spring in my 28" with rifle gas system and it still isn't enough. There is this option, too: http://www.brownells.com/rifle-parts...prod71557.aspx

[Bwild97]

I'm gonna go out on a limb and predict that the addition of adjustable gas keys will result in more broken bolts. From my understanding of the DI gas system (to me) it just seems to be an inappropriate place to restrict gas.

[montana]

I'm gonna go out on a limb and predict that the addition of adjustable gas keys will result in more broken bolts. From my understanding of the DI gas system (to me) it just seems to be an inappropriate place to restrict gas.

With less gas entering the inside of the bolt carrier I fail to see how that would be harder on the bolt. Were you thinking there would be excessive gas pushing on the restricted gas key?

[Bwild97]

I wouldn't say that the gas entering the key and Carrier are excessive, the parts were designed to handle the pressure and heat.

What "IS" happening though; is the force of the gas entering the restricted key will push the carrier rear ward (forcing the bolt into the barrel extension) before it has a chance to act on the bolt to push the bolt forward into the breech face) until gas pressures bleed past the restriction in the key to act as it should.

Under normal operation (unrestricted key) the gases are directed from the gas tube to the gas key, into the carrier's expansion chamber. The gasses then act upon the bolt and carrier simultaneously, pushing the bolt forward and the carrier rearward (because the bolt has less mass than the carrier, the bolt moves forward first) as the carrier continues it's path rearward it forces the cam pin to rotate the bolt, unlocking it and allowing the carrier to continue rearward untill the buffer spring forces the carrier forward to complete the cycle.

This is a great little example gif (if it works) to illustrate the sequence.

[rushmc1a]

The CWS helps, but I'm also running a stiffer spring in my 28" with rifle gas system and it still isn't enough. There is this option, too: http://www.brownells.com/rifle-parts...prod71557.aspx

Have you had much experience (you or anyone for that matter) with the JP systems adjustable set up? I am running that with the second to the strongest spring and it seems to work well

Just curious about as to whether anybody else had run this set up.

[montana]

Yes, that is what I was asking if you thought the restricted gas key would have excessive pressure pushing on the restricted gas key. There are some on this forum who think the adjustable gas block is a choke point and not a good idea. I have had nothing but good experiences with the ones I have used. I use adjustable gas blocks on most of my AR rifles. I observed a JP gas block fail on a friends rifle when the adjustable screw fell out of his gas block during a competition. It was a new JP rifle and wouldn't function with his ammunition. He claimed he didn't mess with the adjustable screw but I think he may have backed it out too far. I have Syrac gas blocks but favor the SLR on all of my new builds. I'm not totally dis agreeing with you on your assessment with the adjustable gas key since I have no experience with adjustable gas keys and was just trying to understand your concern with them.
I'm not sure I agree with your assessment of the bolt being pushed forward from the gas tube gas since the recoil of the fired cartridge would push the bolt back into the barrel extension locking the bolt and expanding the brass long before the gas reaches the gas port. Only after the bullet leaves the barrel and the pressure drops will the case relax allowing the case to be ejected. This is why it is critical to have the correct barrel gas port location on different length barrels and the correct gas port size to time the system. I only use adjustable gas blocks to fine tune my rifle to my ammunition. I have one 18" AA barrel that came with an over sized gas port that needed an adjustable gas block to function.

[Bwild97]

montana

the fired cartridge would push the bolt back into the barrel extension locking the bolt and expanding the brass long before the gas reaches the gas port.

That is exactly why the bolt must be pushed forward, before the carrier is pushed rearwards rotating and unlocking the bolt. To relieve some of the tension between the bolt locking lugs and the extension lugs.

[montana]

So if I'm following you correctly you think the pressure on the carrier and bolt will still be too high for the timing if the gas is adjusted at the carrier key instead of a gas block. The gas pressure on the key will still be too high even when the gas delivered to the bolt- carrier chamber is lowered causing undo stress to the bolt when un-locking. The bolt carrier moves rearward approximately 1 quarter inch before the bolt begins to rotate. This is why the mid length gas port location is preferable on a 16" barrel as opposed to a carbine length gas port . The mid length gas port allows the chamber pressures to drop quicker before extraction occurs. If this is a concern with an adjustable gas key it would only be speculation with out testing. The gas port location would be more of a concern to me than the excess residual pressure from the gas tube pushing on the carrier key regarding bolt rotation and extraction.

[Bwild97]

montana

So if I'm following you correctly you think the pressure on the carrier and bolt will still be too high for the timing if the gas is adjusted at the carrier key instead of a gas block.

Not quite... Just the carrier, not the bolt "yet". What I'm trying illustrate is that; because there is a restriction in the gas key, the gas forces will act upon the carrier before they should, forcing the carrier to move at an inappropriate time (too soon). This in turn, will cause the carrier to attempt to unlock the bolt early, until the gas forces have bleed past the restriction in the gas key to act on the "bolt and carrier" as they should (forcing the bolt and carrier in opposite directions). This event will become more extreme with more restriction with in the gas key.

Now, after some more thought on the matter; the cam pin is really going to take a beating with this type of setup.

Port position will have minimal effect, except where dwell time is increased, lengthening the amount of time the carrier is held back, against the cam pin at which time will be trying to unlock the bolt, applying an undue amount of stress on the cam pin until the bolt is unlocked.

[NugginFutz]

bwild97 - thanks for the nifty GIF. I'll have to save it for when I try to explain to some of my bolt gun friends how the DI system works.

So, if I follow correctly, your concern is that the forces of impinging gasses are going to be reduced in the BCG piston area, where the bolt is used to initiate carrier movement and, instead, be transferred to the gas key, imparting an impulse to the BCG directly, essentially causing the bolt to be dragged out of the extension. The consequence of this being that the bolt doesn't benefit from the initial push from the gas forces prior to extraction. True?

If so, I would ask what the distinction would be between an adjustable gas key and an Adams Arms Gas Piston conversion kit? In the Adams Arms kit, as with other Piston guns, force is developed at the gas block and transferred to the BCG via an op rod at the point where the key assembly exists in the DI guns. With a choked down adjustable gas key, the impulse are still applied at the key via the gas tube, but a significant portion is vectored to the interior of the bolt carrier, imparting forward motion to the bolt.

Even with the adjustable gas key at full restriction, (which I personally don't see why anyone would need such an extreme setting), I don't see how the forces at the carrier assembly would be as great as those a Short Stroke Piston setup would impart.

All this would suggest that Piston guns should be exhibiting a higher than normal degree of wear and breakage in the areas you mentioned.

Your thoughts?

[Bwild97]

NugginFutz

So, if I follow correctly, your concern is that the forces of impinging gasses are going to be reduced in the BCG piston area, where the bolt is used to initiate carrier movement and, instead, be transferred to the gas key, imparting an impulse to the BCG directly, essentially causing the bolt to be dragged out of the extension. The consequence of this being that the bolt doesn't benefit from the initial push from the gas forces prior to extraction. True?

You've got the idea, yes.

NugginFutz

If so, I would ask what the distinction would be between an adjustable gas key and an Adams Arms Gas Piston conversion kit? In the Adams Arms kit, as with other Piston guns, force is developed at the gas block and transferred to the BCG via an op rod at the point where the key assembly exists in the DI guns. With a choked down adjustable gas key, the impulse are still applied at the key via the gas tube, but a significant portion is vectored to the interior of the bolt carrier, imparting forward motion to the bolt.

Even with the adjustable gas key at full restriction, (which I personally don't see why anyone would need such an extreme setting), I don't see how the forces at the carrier assembly would be as great as those a Short Stroke Piston setup would impart.

All this would suggest that Piston guns should be exhibiting a higher than normal degree of wear and breakage in the areas you mentioned.

Don't they?

Unfortunately I have little experience with AR piston systems, so I can not offer much in the way of comparison to gas key modification.
I do know that early AR piston kits offered many lessons in futility, and improving the technology they have come a long way. But as I understand it, they still just don't work very well in an AR.

[bwaites]

Unfortunately I have little experience with AR piston systems, so I can not offer much in the way of comparison to gas key modification.
I do know that early AR piston kits offered many lessons in futility, and improving the technology they have come a long way. But as I understand it, they still just don't work very well in an AR.

Most of the guys repping for the companies selling piston systems run impingement systems in competition when it is serious. That makes me believe that they still aren't as reliable as some would have you believe.

[m796rider]

Most of the guys repping for the companies selling piston systems run impingement systems in competition when it is serious. That makes me believe that they still aren't as reliable as some would have you believe.

Yep.

As Bwild97's excellent GIF illustrates, the AR15 BCG is designed to allow gas to work in BOTH directions for proper unlocking and timing. Gas pistons in an AR just punch back on the bolt carrier, doing nothing to manage the bolt's unlocking other than ripping it out of the barrel extension. This in turn does beat the ever-lovin' crap out of the cam pin and thus you have products coming out like the POF roller cam pin to alleviate issues specific to gas pistons in an AR.

[NugginFutz]

Well, in an attempt to get this train back on track, I would submit that the adjustable gas key may be an viable means to change the unlock timing, but adjustable gas blocks are certainly well proven.

In the meantime, NoOne, factory ammo is almost always a good, safe starting point, as well as a good reference. In other words, what do the primers look like when factory ammo is fired through the same equipment? Were it my setup, I'd be curious to know if it was an ammunition thing or an equipment characteristic.

[NoOne]

I shot some factory ammo with my first (of many) Grendel about 10-15 years ago. I haven't tried any since. Maybe it would be worthwhile to get a box just to check out the hypothesis of ammo vs equipment.

The discussion about adjustable gas blocks vs adjustable gas keys was quite interesting. To my simplistic way of thinking though, does it really matter whether the bolt and bolt carrier get X pounds of gas pressure whether the regulation of that pressure comes from the key, or the gas block? Why would it matter how/where the X pounds of gas pressure is regulated prior to it's arrival at the bolt/bolt carrier to operate the system?

I keep thinking about vehicle cylinders, and how a turbo or a blower can increase pressure inside the cylinder to boost horsepower, but both do so through vastly different methods. In the end, the cylinder still gets X amount of additional pressure prior to the spark plug igniting the air/fuel mixture.

Has anyone done any actual "side by side" comparisons between adjustable gas blocks, or adjustable gas keys to moderate gas pressure in order to determine whether there is actually a difference in the rifle's operation?

I think the statement about people who sell piston guns using DI rifles in competitions says a great deal. Although I had always heard it was a "given" that the piston guns had a more difficult time achieving the accuracy of the DI rifles. As I have only run DI AR-15 style rifles, I can't give any input to piston rifles.

Just to make sure there was nothing odd about my buffer, I took all my rifle length buffers apart, and they all have the five steel weights, so they are all stock rifle weights. I did notice that the recoil spring in the rifle with the flattened primers was about a half inch shorter than a couple of the others. So I put the longer spring into the rifle showing flattened primers. As odd as some rifles can be, I wouldn't be surprised to learn that the half inch shorter spring was the cause of everything.