10.5" Pistol Build Feeding Failure

[dpete]

My Son's BLK pistol has come up with an odd feeding failure.  The 2nd to last bullet in virtually any magazine used will hang in the feed ramps with its nose buried into the top of the chamber entrance and the bolt wedged into the top of the case.

 

After the jam.  Pull the bolt back, drop the mag, jammed bullet drops out, reload bullet into mag, slap the mag back into the gun, release the bolt, and it fires the previously jammed bullet and the last one in the magazine as it should.  It has done it once on the 3rd to last bullet, and it has done it using 30 round Pmags, 30 round colt mags, and 5 round ASC mags.  You can run a 30 round mag as fast as you can pull the trigger and it fires perfectly until you get to that 2nd to last bullet.

I have rounded the edges of the ejector before trying the 3 different mags and the malf continued.

I just finished weighing and measuring the buffer system parts of this BLK and my 8" pistol that works perfectly.  Tossing in weights of buffers of two other working guns.  The 16" BLK that works has the same buffer system as the 10.5" malfing gun.

10.5" malfing gun                                            8" working gun                   16" BLK working                 16" Colt Carbine

        4.6 oz                   buffer weight                     2.8 oz                                   4.6 oz                                  3.75 oz

         10"                       spring length                       10.5"

         34                         # of coils                             37

         .071"                      wire dia.                           .074"

 

My first thought after taking weights and measurements is this thing has too much buffer weight for the spring that is trying to push it.  I plan on doing a spring/buffer swap using the 2.8 oz buffer and 10.5" spring out of my 8" pistol and see what happens. 

Bullets used were 110 gr Nosler Varmageddons loaded over 19.0 gr of Lil'gun to 2.060", in a WCC cut and formed case and CCI 400 primer.  They have been universally accurate and functional in 4 different 300BLK rifles so far. 

Am I on the right track?  Any advice would be appreciated.

 

                                                     

 

[dpete]

I just got back from the range.  Thank God its only 10 minutes away.  I put the spring and buffer from my working pistol into my Son's gun.  Same malf with both 30 round Pmags and a 30 round Colt steel mag.  The 5 round steel ASC mag fed fine with no feeding issues at all.  Next step is to take some weight out of the 4.6 oz buffer and see if it will run with the original spring.  

I'm also considering the possibility that the bullet length isn't agreeing with the gun for some reason.  I've got some 110 gr Barnes Tac-Tx loaded up for hunting and I'll try those next.

[unforgiven]

Hiya doin brother hope and your family are doing well.

[dpete]

We're fine except for this foul up.  Original buffer is fubar.  Steel weights that act as if they were pressed in.  No way they are coming out.  Ran the 110 Barnes Tac-Tx through it this afternoon.  They ran in the ASC 5 rounder and jammed in the 30 rd Pmag.  This was with the spring and 2.8 oz buffer from my working 300 BLK pistol.   

 

[98Z5V]

This jam only happens on thenext to last round?...  Various mags?...

[dpete]

20 hours ago, 98Z5V said:

This jam only happens on thenext to last round?...  Various mags?...

Yes.  I've tried 3 different mag manufacturers, 30 rd Pmag, Colt 30 round steel, ASC 5 round steel, and most common jam is 2nd to last round.  It has happened on 3rd and 4th on one or two occasions.  Its weird as h*ll.

[98Z5V]

This is strange.  I want this gun to play with, just because.  This is some really weird shiit, brother.

[dpete]

1 hour ago, 98Z5V said:

This is strange.  I want this gun to play with, just because.  This is some really weird shiit, brother.

No kidding!  My 16" pistol gas carbine with the same buffer parts in it shoots anything I put in it flawlessly.  My understanding is that the 300 BLK was designed to use a carbine (3oz) buffer and regular carbine AR15 spring.     

[98Z5V]

This upper - on another lower...  does the same thing?...  no matter what mags? This "jam on the next to last round int he mag" is some weird shiit, man. 

 

Edited October 22, 2019 by 98Z5V

[dpete]

6 hours ago, 98Z5V said:

This upper - on another lower...  does the same thing?...  no matter what mags? This "jam on the next to last round int he mag" is some weird shiit, man. 

 

That is one thing I haven't tried yet.  But you can bet I will.

[RedRiverII]

I was thinking the floor plate retainer might have a deformity,  but then you said multiple mags.  Interesting challenge from this side of the problem,  best of luck.  What would make the second to last ass end of a bullet sit higher than normal?  Old springs?  Follower?  Gotta be gremlins.

[Sisco]

Thinking spring tension of the magazine might have something to do with it. Maybe combine that with the pistol length gas system and it causes a malfunction in a very narrow range of tension, that only shows up in the magazine loaded with two rounds. Try loading 3 rounds and see if it happens on the second when you do that. Like 98 said. Weird schit.

[dpete]

The last round of shooting I did to try to figure this out was using 5 rounds in a mag.  1, 2, 3 all went bang.  #4 which is 2nd to last went click and jammed.  In all the mags I tried.  The long nosed Barnes Tac-Tx worked when used in the 5 round ASC mag, all 5 rounds went off.   The bullets that jammed in all the mags were short nosed Nosler 110 Varmageddons.  Shorter nose than the Barnes with a steeper ogive angle. 

Edited October 23, 2019 by dpete

[98Z5V]

1 hour ago, dpete said:

The last round of shooting I did to try to figure this out was using 5 rounds in a mag.  1, 2, 3 all went bang.  #4 which is 2nd to last went click and jammed.  In all the mags I tried.  The long nosed Barnes Tac-Tx worked when used in the 5 round ASC mag, all 5 rounds went off.   The bullets that jammed in all the mags were short nosed Nosler 110 Varmageddons.  Shoter nose than the Barnes with a steeper ogive angle. 

So this is all the exact same brand/type of projectile doing this, that next-to-last-jam?  If so, that's getting even weirder - that the previous ones feed.  Any other ammo do this in that gun and those specific mags?...

This is a tough one, brother.  I like it. 

I'm leaning towards magazine springs, right now- even though it's different brands of mags.  They're losing tension/force as the mag gets more empty.  They're at max force in a full mag, just from spring compression - there's so much preload on that mag spring at that point...   Maybe they're "losing control" on that next to last round...  the very last round is gonna sit right in those feed lips no matter what, with nothing above it in it's way...

Try this - load 5, shoot 5 - verify that #4 hangs the gun up.  Do that again, load 5, shoot TWO of them...  #3 is in the chamber...  Gently remove that mag and look at #4, which should be on top now...  See if it's sitting in the mag all funky... 

Edited October 23, 2019 by 98Z5V

[dpete]

On 10/22/2019 at 10:20 PM, 98Z5V said:

Try this - load 5, shoot 5 - verify that #4 hangs the gun up.  Do that again, load 5, shoot TWO of them...  #3 is in the chamber...  Gently remove that mag and look at #4, which should be on top now...  See if it's sitting in the mag all funky... 

This is what I did and I think I found the problem.

 

I loaded 5 rounds, fired two, leaving the third in the chamber, then verrrrry slowly dropped the magazine out of the lower and this is where I found the 2nd to last bullet.  Not even IN the magazine, but sitting on top of the last bullet and the right side feed lip bridging them.  This was with the 30 round Pmag, and it happened more than once.  In a 5 round ASC metal mag and a 15 round C-Products metal mag, all 5 rounds fired perfectly with no sign of the 2nd to last bullet bridging out of the mag.  My buddy the gun pusher (yes I have one too) loaned me a 30 round Lancer L5  300 BLK mag to try and it worked perfectly too. 

Apparently this thing is allergic to Pmags and prefers longer nosed bullets.  Luckily I have 100 Barnes Tac-Tx bullets to load up to use for hunting and theres no need to use more than a 5 round mag in a hunting situation.  Hopefully mystery solved.

[98Z5V]

This is fuckin' AWESOME news, brother...  

[Cunuckgaucho]

[MikedaddyH]

This will help the timing... It will slow Down the pulse by a fraction of a second. 

[dpete]

11 hours ago, MikedaddyH said:

This will help the timing... It will slow Down the pulse by a fraction of a second. 

Ummm, what is Tom's favorite saying???   Don't complicate shiit with complicated shiit!

[98Z5V]

That fucking pigtail gas tube is crazy - but it's effective.  It turns a carbine gas system-barrel into (effectively) a midlength gas system.  That shiit is nuts!...  

I don't think this is a gas problem - I think you had a mag problem.  By the way, you send the documentation to MagPul, and they'll send you another PMag - they won't even want the old one back - they're that good about it.  If you call them on the phone, and talk to a human in Customer Service about the problem, brother - they'll probably send you one of these to replace it:

https://www.magpul.com/products/pmag-30-ar-300-b-gen-m3-300-blk

They know there's something up with 300BLK, or they wouldn't have made a special mag for the round. 

[98Z5V]

I documented the difference, and the mods you have to make to run 300BLK through a PMag - in some thread, somewhere here.  Kill those nubs at the top, that guide the 5.56 rounds - cut 'em out, file them down, whatever works.  Just get rid of them. 

[dpete]

10 hours ago, 98Z5V said:

I documented the difference, and the mods you have to make to run 300BLK through a PMag - in some thread, somewhere here.  Kill those nubs at the top, that guide the 5.56 rounds - cut 'em out, file them down, whatever works.  Just get rid of them. 

I was thinking of doing exactly that.  I eliminated the entire internal rib on two 30 round Pmags to run subs as long as I could in the mag.  Both of them work just fine.  I wonder how Magpul would feel about replacing the 6 Pmags my son bought to use with this blaster. 

[jtallen83]

Magpul is putting out dedicated 300 BO 30 rounders now, I have several and they have worked flawlessly  in three different pistols, Aero and Anderson lowers. Great fun using full mags to have dueling tree competitions, just gets a little pricey!

https://www.magpul.com/products/pmag-30-ar-300-b-gen-m3-300-blk?ProductColor=VO343

Edit; Guess I should have scrolled up before posting 

Edited October 26, 2019 by jtallen83

[98Z5V]

16 hours ago, dpete said:

 I wonder how Magpul would feel about replacing the 6 Pmags my son bought to use with this blaster. 

They would probably direct-swap you out, no questions asked - and not even want the old ones back.  I kid you not.  That's how badass their Customer Service is.  It's definitely worth a call. 

[dpete]

 

On 10/27/2019 at 2:16 AM, 98Z5V said:

They would probably direct-swap you out, no questions asked - and not even want the old ones back.  I kid you not.  That's how badass their Customer Service is.  It's definitely worth a call. 

Its not quite that good.  Send to Magpul the magazines that don't work.  They issue store credit.  We order new 300 BLK magazines and they send them to us.  For six 30 round .223 mags + $2 and change we are getting five 300BLK magazines back because of their price difference.  Now we're waiting for them to arrive.  Still not bad if it solves the problem.