Blowback AR 40 S&W / 3 round bursts

[Albroswift]

My carbine 40 S&W shoots flawlesly from the bench, but out at the gravelpit yesterday had several 3 round bursts. I've shot full auto AK's and this had to be cycling twice as fast, 1000-1200 RPM, which is really awsome until the local LEO shows up. So thinking, is it possible that the spring/ buffer weight combo (EA1095/ 10 OZ buffer/ 7-5/8" tube) strokes back far enough when gun is fully restrained but when running and gunning it doesn't quite get back far enough to catch the disconnector, but far enough to strip a round and slam fire? Mil Spec trigger group. Any one experience this with their 9MM blowbacks? With the "Extra Power" carbine spring that is recommended with the 7.5 and 10 oz buffers I see the buffer bumper getting a lot of deformation from bottoming out, which is why I went to the EA 1095 spring.

[98Z5V]

Too much recoil timing - there's too much time allowed for the BCG to travel rearward, then forward.  It's the 7 5/8" extension.  These things were designed around the 7.000" extension.  That extra 5/8" each way is another 1 1/4" of BCG travel - and time.  You might need more than a 10oz buffer in there, too.  Which buffer is it, exactly?  It's gotta be pistol-specific, and longer than 3.250" carbine buffers are.  .40 is a mean caliber, it's "10mm Lite..."     I'm running 8oz pistol-specific buffers in the 9mm and .45AR, and the .45 used to double on me once in awhile because the buffer was too light (ran an H3 buffer and $1.75 in stacked quarters before), and the BCG was moving so fast.

Blowback pistol caliber ARs are hard on hammer pins - get some KNS Precision anti-rotating pin sets in these things. 

Edited May 19, 2019 by 98Z5V

[Albroswift]

Yes, mean caliber, getting 1500 FPS w/ 155 gr pills out of a 16" barrel!

Got the KNS pin keepers

 KVP 10 oz buffer pistol length 4-1/8"

spacer in to set the travel to .125 +/- past bolt lock.

7-5/8 tube/ EA1095 spring

Spacer is a steeped setup to allow correct spring length for and correct rearward travel of buffer. I changed things up this afternoon put the slightly shorter spring and a 7" tube, same as in my AR 40 Pistol. I think I could have had too much  spring and buffer, the 7.5oz buffer runs well in the pistol with the 4-1/2" barrel was trying to balance the higher pressure loads and the 16" barrel.

 

Edited May 20, 2019 by Albroswift

[98Z5V]

29 minutes ago, Albroswift said:

 KVP 10 oz buffer pistol length 4-1/8"

Ah, you got this.  That pistol buffer is 7/8" longer than a 3.250" AR15 carbine buffer. You're on it. 

[Albroswift]

Yes, 4-1/8" 10 oz tungsten. Who makes a heavier one?

[98Z5V]

1 hour ago, Albroswift said:

Yes, 4-1/8" 10 oz tungsten. Who makes a heavier one?

Is that one marketed specifically for .40 cal?   That's a pretty hard hitting pistol round - let alone in a blowback AR platform.  What's that buffer sold as, man?

[98Z5V]

2 hours ago, Albroswift said:

You have a close-up of that spacer you made?  How much length did it add, in total? Got a pic of what it looks like installed on the spring end? I'm trying to figure this out...

Edited May 20, 2019 by 98Z5V

[98Z5V]

Also, I don't know if this matters yet, but what BCG did you use? KVP has conflicting information on their website about BCG weights, between 9mm and .45ACP - and this is not one of those - it hits harder.

[Albroswift]

I use a Stern Defense BCG with the added center weight left in. About 16 oz total.  

All the spacer was for was to set the travel in the 7-5/8 tube. Slips inside the spring, makes up about 5/8". I'll shoot a closeup tonight with the spring. 

I think the issue is short stroking/ slam firing, I played around last night if I was real carefull I could eject and feed the next round without getting all the way back to the disconnector.  Probably off the bench the carbine is rigid enough that I get full stroke, but loosly held in the gravelpit enviornment may have made a difference. Like I said above, I yesterday I changed out the 7-5/8 tube and the EA1095 with a 7" tube and an extra power carbine spring, next step down. That was the combo that I staered noticing buffer bumper deformation with the 7.5 oz buffer.  

[392heminut]

Interesting post! I'm following this one closely. I just got my .40 carbine put together (THANKS Albroswift!) but haven't been able to try it out yet. The wife had some surgery and I'm stuck at home taking care of her. Maybe this weekend!

[sketch]

ordered up a 10oz. buffer and spring last night! and another lower.. Albroswift you're a bad habbit 🍻

[Albroswift]

OK, so what works: 

7.5 to 10 oz buffers 4-1/8"

extra power carbine spring in a 7" tube

Ea1095 spring in a 7-5/8" tube, but, not with the 10 oz buffer. Need a spacer but not a stack of quarters, don't want to space the spring, just the buffer. See image below. 

Factory glock mags. There is a 22 rounder that we have tried out works flawlessly, also the 15 rounder.

Also the Korea RWB 31 rounders, GL40-31MAG, but not the SGM's,  SGM-40G31R

 

We have built 3 so far, two 4-1/2" pistols and one 16" PCC. I believe they can be over buffered/ over springed, if you start getting double and triple bursts that may be the culprit. Had an EA1095 in a 7" tube/ 7.5 oz buffer and it doubled up several times, went to the EP carbine spring, fine since.  The PCC was doing that Saturday at the gravel pit but has never done it from the bench, close balance 10oz buffer EA1095 spring 7-5/8" tube and spacer. Changed it out yesterday. 

IMAO the BCG can travel back far enough to eject and feed without going back far enough to set the disconnect, if you don't believe it, pull the firing pin and try. This is what I did: Pulled the charging handle slowly back until it picks up the round, feed it in, and pull the trigger. No Click! Hammer had  followed the BCG back down. Do it fast and BURRRRP! We run a stock MilSpec FCG, and a Stern BCG. A little taller hammer like maybe some of the heavier 308 type might fix that, or a slightly steeper ramp on the BCG, or a lighter hammer spring. Fine balance. My old AR15 has a cut BCG and a notched hammer so it locks up if the disconnect is damaged or disabled to prevent slam firing, but as far as I can tell, these are just waiting to go full fun mode. 

 

Sample buffer for photo, not the 4-1/8" KVP we run with

IMAO,  you could basically use any length buffer with the correct spacer length. Probably could be said for any AR platform.

[Albroswift]

OK, tried the carbine today with:

Extra power 308 carbine spring

7 in tube

10 oz KVP 4-1/8" stainless steel buffer

Glock gen 4 22 round and 15 round mag

155 gr  Ranger HP ammo / 1500 fps

Flawless operation off shoulder, fed perfectly, no double/ triple bursts, LRBHO  BUT a little more felt recoil then the 7-5/8" tube and the EA1095 spring. Buffer bottoming out for sure. Still peppered the 100 yard target. Seems like a reliable combination, just be replacing buffer bumpers occasionally. 

[Albroswift]

* 10 oz Tungsten 4-1/8" buffer

[98Z5V]

I just found this info - did a little re-research on Quarter Circle 10.  I've had a 9mm Colt-mag/UZI mag lower from them since 2012 - back when they were making all of DDLES stuff for them. 

They're recommending the Sprinco RED spring as the preferred spring for PCCs.  The Sprinco Orange spring is a heavy duty .308AR spring, for .308ARs using the 7.000" extension and a 2.500" buffer length.  The Sprinco Red spring is a heavy duty .308AR spring for use with 7 5/8" extensions and a 3.250" H3 buffer. 

I don't know if that spring is heavier than the Armalite EA1095 spring - but that's what Sprinco makes as an upgrade to that system.

https://www.quartercircle10.com/products/parts-accessories/lower-small-parts/buffer-spring-xp.html

This is also something I mentioned to Larry earlier - I brought up the Sprinco Adjustable buffer that I saw at SHOT.  QC10 makes their own version, just for PCCs. It adjusts from 5 to 11 ounces, and is 4 1/8" long.  This is badass.  The price seems crazy, but it reflects all the tungsten weights you're getting - that ain't bad.

https://www.quartercircle10.com/adjustable-pistol-buffer.html

In a .40 PCC, I'd be running that buffer at 11oz with that Sprinco Red spring, just to start off...  

[Albroswift]

That buffer is badass, probably the 120.00 could be better spent elsewhere though. In 2 different builds this year the EA1095 spring was just a little bit heavy even with 7.5 oz and 10oz buffers ($34.00-$54.00) The heavy duty 308 carbine spring (KVP not Sprinco) is maybe just a little light, excellent function but the buffer bumper takes a bit of a beating and can be felt in the recoil. If the red spring was half way between it would probably be about right. Free shipping, no tax, got one on order right now! Let you know...

[Albroswift]

Got the red spring, it is longer then the extra power JBO/KVP 308 spring,  shorter then the EA1095. Got it in the carbine with the 10 oz buffer, report back tomorrow hopefully. 

[Albroswift]

Bloody range report AR40 Pistol and Carbine

First tried red spring/ 10 oz buffer in the carbine. Flawless. Even a couple of the iffy mags functioned well. Recoil was sweet. Then took the EA1095 and 7.5 buffer out of the pistol which had been working well with the factory mags, and put the 10oz/ red spring combo in that. great results. Then put the EA1095 + 7.5 buffer in the carbine, had a couple of jams  even with the glock mags, and like an idiot I kept going, until I had a lawman 155gr HP go off out of battery blowing one of the cheapo mag into its basic components and taking a chunk out of my chin. Blood everywhere, had to finish up the 2nd project of the session (springs and red dot on a glock) periodically blotting blood out of my beard and off the bench. 

To summarize, in 7" tubes, 15.5 oz Stern Defense BCG:

RED Sprinco/ 10 OZ buffer excellent functioning combo for pistol, carbine

EA1095/ 7.5 oz buffer good in pistol, although same combo in the other pistol we built this year did have some double taps

EA1095/ 7.5 oz buffer NO BUENO in carbine. Curious why worked with 4.5" barrel with 180 gr IPSC loads  but not 16" barrel with 155 gr +P,  all other things equal.

Forgot to try the red spring with the 7.5 oz buffer, a little out of sorts after the blow to the chin.  

 

Looking back from the 50 yard berm

[jtallen83]

Gives me great pause on building a PCC. I must not have a grasp on the system because I thought it would not be possible to have an out of battery ignition without a BCG issue? Scary schit!

[Belt Fed]

WOW, sorry to hear. I hope the wound isn't too bad. glad it wasn't worse. 

Edited June 9, 2019 by Belt Fed

[unforgiven]

Damn brother glad it wasn't worse. Fuk

[Albroswift]

Mere flesh wound 

good data on the springs 

[Albroswift]

4 hours ago, jtallen83 said:

Gives me great pause on building a PCC. I must not have a grasp on the system because I thought it would not be possible to have an out of battery ignition without a BCG issue? Scary schit!

pretty sure you can get anything to go off out of battery if you fuk around enough with it...

[shooterrex]

I had a 9mm carbine fire out of battery. I tried lots of different things to get it to run but never was right (midway stoner brand). Swapped barrel and bolt to a 6.5 Grendel. Now it runs.  I just purchased a Palmetto 9mm pistol kit that runs like a champ. These  pistol caliber guns are like the 308's no Mil spec and the manufacturers seem to be making up things as they go along.

[sketch]

damn,  what does the mag look like? how did your chin catch parts? are you a left handed shooter? glad you have a tough chin!