Archive for the 'AR-15s' Category

My only comment is that trigger break weight will be a function of where you put your finger because the trigger itself is a lever arm.

Joefour, this one is for you.  It’s you and me, Stoner versus Kalashnikov. Eugene wins every time.  Stoner rules, Kalashnikov drools.

Brownells on AR gas and buffer systems.

Eh, okay, whatever.  I accept what they’re saying for nominal applications, but for fine tuning I think there is more to it than that.

On another but very interesting note, Brownells is assisting in the Covid-19 analysis.

Brownells, an 80-year-old, Iowa-based online firearm, and firearm accessories supplier has joined forces with Folding@Home to lend its excess computing power to help combat the COVID-19 virus sweeping across the globe.

Folding@Home is helping with an international effort to understand the molecular structure of the virus in hopes of finding ways to defeat it. Folding@Home has called on individuals and companies with excess server space and computing abilities to help execute computer modeling simulations to help speed up an otherwise huge and lengthy task.

“Our IT team learned about the Folding@Home project and their work using computer modeling to better understand the molecular and protein structure of the virus,” said Brownells IT Director Curt Graff. “We are committed to helping protect our country by virtue of our personal protection and sustainability products, but we see this as a way to support the international community in a time of significant need.”

Brownells anticipates it will donate at least 1,300 hours to the project and run approximately 200 simulations on behalf of COVID-19 research.

Okay, so this is really cool, and some of you may be confused, so I’ll do my best to explain this.

Processor speed hasn’t significantly increased for many years, but the ability to utilize threaded calculations has.  Many PhD theses have been written on “massively parallel” computing, and most high performance computer codes today (that require billions of calculations) are written to be able to utilize thousands of cores (written, of course, for HPC, or UNIX High Performance Computing clusters).  In my recent work, I’ve used more than 1400 cores for approximately 20 wall clock-hours for each simulation.  That requires a lot of computing power.

Why on earth would Brownells have this kind of computing capacity, you might ask?  I suspect, but I don’t know with certainty, that Brownells has these capabilities because of CFD (computational fluid dynamics) and FEA (finite elements analysis) for ballistics and bullet design.

Someone could prove me wrong by calling Brownells and discussing this with them, or I can.  It might be interesting to find out.  If they don’t have it for those reasons, they have it for some other reason, and it would be interesting to know.  A number of large corporations who do advanced computing have access to thousands of cores, as do the national laboratories (who have the largest computers).  It was a bit surprising to me that Brownells has access to the kind of computing power that these computer codes will need.

But I think it’s cool.  Does some reader want to call Brownells and let us all know?

Either way, it’s nice to see that a firearms giant is helping in the battle against Covid-19.  Hey, if Brownells helps find a treatment, do the gun controllers promise not to use that treatment?  After all, they wouldn’t want to be in bed with the gunners, would they?

Ammoland has the details.

Coming in at 6.8 pounds, I really like the lightweight handguard.  The price point seems right too (MSRP: $1100), and I think it’s a good addition to their stable given the usual unavailability (and high cost) of 458 SOCOM and the much higher availability (and lower cost) of 450 Bushmaster.

I’ve never had any of these things happen to my guns, but then again, I’ve never had to put 500 rounds downrange in ten minutes.  So there’s that.

I think Chris does a really good job explaining this.

American Rifleman has the scoop, but Ammoland does a little better job by giving stats.

Actually, neither had the scoop on me.  I had discussed this very gun with Steve Mayer at RRA back in December of 2019.

My only complaint about Rock River Arms guns is that the ones that I’ve had seem to have weighty front ends.  I had always thought they needed to be a little more on the cutting edge for reduced-weight hand guards.

With the unloaded weight of this gun coming in at 6.8 pounds, it seems like they’ve taken up the challenge.

Steve also points out that if you don’t want to buy the whole gun, they sell the upper separately.

Shooting Sports USA.

Armalite’s original blueprints specified a 20-inch barrel, 12-inch (measured from the upper-receiver face) gas-port location and a ballpark 12,500 PSI gas-port pressure level. Those are the “rifle” specifications, which work just fine. Therefore, the problem isn’t in the design. Rather, it’s in the redesign. The shorter the front end is, the more redesign has its influence. Carbine-length barrels and especially pistol-length barrels create a condition where using the same ammo there is higher gas pressure at the gas port.

An AR-15 has a “direct impingement” system. Propellant gas is bled off through a gas port in the barrel, this gas goes through the gas manifold or “gas block,” through a gas tube, and into the carrier key atop the bolt carrier, and that’s the end of the line—the resultant force deposited into the key starts the bolt-carrier assembly in motion.

Think of this gas system as a pressure chamber with two valves—one at the gas port and the other at the muzzle, and the moving bullet opens each valve as it crosses each plane. Therefore, the spacing of the valves matters greatly.

The better way to think of this is each is an orifice (even though an adjustable gas block is an adjustable orifice), not a valve.

As the bullet enters the barrel bore and moves forward, the space (volume) behind the bullet is increasing, which lowers the pressure of the contained gas behind it. More available barrel-bore volume before the gas port translates to lower pressure at the port. So, post-gas-port barrel length influences how long the system is “sealed” under full pressure, and the shorter the length, the shorter time. Again, the bullet is serving as a plug that’s sealing all the pressure in the system—until it exits.

[ … ]

Approximate figures for M855 (genuine 5.56 NATO) chamber pressure is about 60,000 PSI; pistol-location gas port pressure, 50,000; carbine-location, 33,000; mid-length, 27,000; rifle-location, 19,000. Thus, 5.56 NATO is hot and getting hotter, and it has been for years.

Installing a heavier buffer and also a stouter buffer spring buys time. Both increase resistance to the bolt unlocking, thus delaying it from moving. The heavier buffer better resists movement and moves slower. The stouter spring increases in-battery load against the bolt carrier, increasing its resistance to initial movement, and the carrier also then moves slower coming back against the buffer.

“Just put an adjustable gas block on it” is also routine advice, and, yes, that helps, but ideally such devices should be used to tune function. If it’s needed just to make the gun run, then the chances are certain something else was missed, most likely in the architecture. That’s where we’ll find the cure.

[ … ]

That’s easy enough with a carbine-length front, 16-inch barrel. A mid-length gas port is located 2 inches farther ahead of carbine-standard. It effectively also shortens the post-port distance by the same amount meaning lower pressure getting in and a shorter time the system is under maximum pressure. Both are good things.

If you’ve got a hankering for an AR-platform pistol, choosing a 10.5- or 11.5-inch barrel makes it possible to get a carbine-length gas system affixed, and that is a 3-inch additional length over the common 4-inch pistol-port, and a significant reduction in post-port length. That really tames a little gun.

I think it’s becoming fairly routine advice to choose the right gas system for your gun as a first step to correct over- or under-gassed systems, rather than tinkering with the gas block.

This is what the author is calling the “architectural” solution to the problem.  This isn’t exactly the same thing as having an adjustable gas block on a new AR-10 6.5 Creedmoor with a fixed 20″ barrel for folks who want to shoot hotter loads or those who want to hand load specialized cartridges and need to make slight adjustments on the block to ensure reliable operation.