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Reloaders Corner: Pressure Curves and Port Pressure – Part 2

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The following is a specially-adapted excerpt from the forthcoming book,” Top-Grade Ammo,” by author Glen Zediker, owner of Zediker Publishing. Click here to order.

Last time I gave a caution about respecting one of the differences between semi-auto and bolt-action rifles, and that was with respect to propellant burn rates. The summary reason for that is that different rate propellants will “peak” at different areas as the expanding gases and the bullet travel through the bore. Slower-burning propellants peak farther, and that means more pressure is available at the gas port location in an AR-15, for instance, as the bullet passes it. If the system is oversupplied, then the system is overworked.

Compared to ideal function when gas supply is delivered as engineered, mistimed peak pressures can result in the bolt unlocking too quickly and excessive bolt carrier velocity rearward. The system just gets hit too hard. The extractor tries to yank the case out of the chamber too soon, before the case is released from its grip on the chamber walls (from being expanded through firing). Spent-case condition shows a measurably more abused hull. Probably the worst popular example of these effects is the M1A. I’m doing an entire column or two on reloading for this beast. Essentially, a spent case from an M1A will show dimensions that don’t seem possible. These come from the bolt unlocking too quickly. AR-15s actually handle excessive pressure better than some other designs.

Always keep in mind that this is all happening in about 2 milliseconds. Average time a bullet spends in the barrel, for most modern centerfire rounds, is 0.002 seconds. Timing is everything.

Keeping in mind the behavior of a pressure curve, which is like a wave cresting, factors that influence the amount of gas-port pressure, using the same load, include barrel length, gas-port size, and gas-port location. When the bullet is sealing the bore, the longer the barrel, the more pressure is contained for a longer time. The smaller or larger the gas port size, the slower or faster the gas enters the system. The farther back or forward the port is located, the sooner or later. Bullet weight is a factor also: heavier bullets accelerate more slowly (and also the reason heavy bullets erode the chamber throat more than lighter bullets).

And, the amount of volume inside the bore has a huge influence on all this. That matters when we’re using another caliber than .224 in an AR-15 or .308 in a big-chassis AR (like an SR-25). For instance, in that rifle chambered for .243 Win., but retaining the gas system specifications (gas port size and location) of the .308 Win.–chambered rifle, there’s way more pressure only because there’s less space, less volume, in the bore. The opposite is usually true when we’re running an AR-15 with a larger caliber bullet.

Selecting a propellant with a suitable burning rate, which, again, is something in the vicinity of H4895, is really the only thing we can do on the loading bench to ensure that we’re not contributing to these symptoms. Beyond that, dealing with excessive pressure gets technical.

All my NRA Match Rifles, which usually have 26-inch barrels, get their gas ports moved forward one to two inches. These, of course, are custom-barreled. I also usually install an adjustable gas manifold.

Moving the port forward effectively delays the wave of gas moving through the bore, kind of repositioning its peak with respect to its outlet; there is more space available for expanding gases. It also allows a little slower-burning propellant, which can take more advantage of the longer barrel. It’s common in a similarly constructed AR-10 to get a port moved as much as 5 inches forward to accommodate a .243 Win. or .260 Rem. chambering.

The adjustable manifold allows some tuning. There are essentially two forms these take. One way is to restrict or limit the through-flow; the other just bleeds it off. I like the first kind the best.

Also, I have searched far and wide for a consensus on gas-port sizes, and came up empty.

All this changes with different chamberings and rifle configurations. Carbine-length barrels are particularly sensitive to port pressure because the port is located farther back.

There are a few surefire things that will alert you when your rifle is exhibiting “over-function” symptoms, such as spent-case condition showing excessively blown (extended) case shoulders, extractor marks on the case rim, and a generally explosive sensation in functioning.

In a more extreme circumstance, an over-accelerated carrier can “bounce” back from its rearmost travel so quickly that a round can’t present itself in time to be picked up by the bolt, or the bolt stop can’t engage quickly enough to hold the bolt carrier.

Sometimes what appears to be a “light” load is actually not. I’ve seen excess pressure leave a spent case in the chamber because the extractor lost its grip, and I’ve seen chunks pulled right off case rims. That’s severe. That’s also another cause for the “short-stroke” appearance of over-function: the extractor issue has slowed the carrier.

If you’re having any problems with “over-function,” solutions include retrofitting an adjustable manifold, increasing carrier mass, installing a stouter buffer spring. I do all those things on my rifles. Keep in mind that I am primarily a Service Rifle shooter, and I am trying to push an 80-grain bullet as fast as reasonably possible from a 20-inch barrel that can’t get the modifications mentioned. I know a thing or three about delaying bolt unlocking — I’ll cover more on this topic if you all want to know.

 

Sources:

Sun Devil Manufacturing

663 West 2nd Ave., Suite 16

Mesa, AZ 85210

(480) 833-9876

 

Medesha Firearms Ltd.

10326 E. Adobe Rd.

Mesa, AZ 85207

(480) 986-5876

 

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11 thoughts on “Reloaders Corner: Pressure Curves and Port Pressure – Part 2”

  1. I really enjoy these excerpts from the new book. I’m really looking forward to getting a copy as soon as it is available. I’ve read through the previous reloading book many times and even written to the author. I’m a big fan.

  2. Great articles. The problems you have mentioned were unknown to me and now explain some of the problems I was seeing in some of my semi-auto builds.

  3. I read the article and have been experiencing the symptoms you described referring to case damage. I built a custom 18″ bbl. AR10 in .308 and find ejector swipes and case rim damage with most rounds. I have tried different powders. I consider my loads mild by most standards. I have tried different bolts, added a H4 buffer, Syrac adjustable gas block, but still have this problem. Sent bbl. back to McGowen with fired case and bolt and they could not answer my question about cause of this condition. I have friends that shoot factory SR25/AR10 in .308 and they don’t experience this problem with my ammo. Any suggestions?

    1. First, I apologize for being so late in answering these posts. Holidays… Sounds like you pretty much ran the range of solutions the I would have suggested, especially the gas block. One of the most effective means to delay the bolt unlocking (which gives a little more time for the case to return to closer to normal for easier extraction) is the CWS accessory from Superior Shooting Systems LLC. It’s a weight that installs back of the bolt carrier. They also make a Flatwire buffer spring for your gun that’s got a stronger load in battery. I run these in all my carbines and Service Rifles. I don’t know that this will fix your problem, but it’s a strong influence on bolt unlocking, and that is the the source of your troubles.

  4. Great article, but did we miss something ? When the .243 and .260 ar’s were designed, weren’t they set up using factory ammo – what most people would use ? It is doubtful factory’s would use a fast burn rate powder. Would that mean we would have more latitude in our powder selections ? I would guess something around 4350 ?

    1. Every stock rifle I’ve seen just mimics the .308 setup. I admit I haven’t seen them all… I honestly think the smaller caliber retrofits were not well thought out. They should have had the gas ports relocated at least a couple inches farther forward. Carrier weights and adjustable gas blocks, and the like, are about the only solutions that don’t involved a new custom barrel. A lot of factory .260 Rem. and .243 Win. ammo will have a little slower burning propellant than would be ideal.

    2. hello again. following up, loaded shells for my 260 rem, 20″ dpms hunter with tac, arcomp, and imr 4064, all near max. result, none would fully cycle the action. most would eject, none would load next shell. on my gun, full operation didn’t occur until use of h414. currently using hybrid 100v with excellent results. thanks

  5. I have an AK variant chambered in 223/556. I always get over function with this rifle , an Arsenal SLR-106Cr, in the form of the extractor pinching into the case rim a visible amount upon ejection. Would the same principles apply such as adding a heavier bolt assembly or a stiffer recoil spring? Or is that platform just prone to over gassing in 223 ?

    Ejected cases all experience a dent in the case wall perfectly perpendicular to the case rim just above the case web to right below the shoulder. I only shoot my reloads so this kind of makes me hesitant reloading these cases too many times. I have with no issue to this point , but discard the cases sooner than typical also. I believe the dent is from the case hitting the receiver cover upon ejection…. Although it is difficult to believe it hits in the exact same place, the exact same way every single time. Would slowing the action maybe change the ejection pattern?

    223 is an oddball in AKs. For a $750+ rifle it doesn’t function that way. Glad I got it for 5! Still, any help would be appreciated.

    Great article also.

    1. I admit to ignorance when it’s necessary, and I honestly don’t have much experience with AK-platform rifles. The only one I have is a Galil in .223 Rem. and it’s the most ridiculously cycling rifle I’ve yet encountered. It throws cases like nothing I’ve ever seen. If there are retrofits for stouter springs or heavier carrier assemblies that would no doubt help, but I honestly don’t have an answer I can warrant.

  6. I encountered such problems several years ago when I began working with 7-RSAUM on the Armalite AR-10 platform. The gas port position is 2′ farther out on a 24″ bull barrel. The system works fairly well, but with only a couple of powders–the slow ones are best, Win 780 Supreme and VV N560 in particular with the 180 Berger VLD bullet. I lengthed my throat with a PTG throat reamer. Before reaming the throat I was having a lot of trouble with pierced and blown primers especially with the (inferior) Remington brass.

    Using Quickload software shows clearly the pressure peak location. The optimum seems to be a near 100% case fill, with all powder burnt before bullet exit. That will give something like 2700 fps for the 180 VLD

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