First I'll star by giving some specs/components


Lower is a Joe Bobs Spartan Billet lower

Spikes Tactical lower parts kit

Thordsen Customs ENHANCED AR PISTOL BUILDERS PACKAGE:

• Carbine Buffer spring


• 3.0 oz. standard recoil buffer

VLTOR MUR-1A Modular Upper Receiver w/Forward Assist

Hardened Arms 10.5 inch 300 Blackout:

• 10.5" HBAR Profile barrel with M4 Extension


• .750" barrel diameter at gas block


• Low profile gas block


• Barrel dimpled to secure gas block


• Pistol length gas system

WMD Guns Nickel Boron X Bolt Carrier Group

30 round PMAG Gen3

Now on to the problems... I took it to the range today to get it sighted in. The range I go to only allows you to load a maximum of six rounds at a time, I however only loaded five at a time. The very first time I loaded the mag into the gun I had the bolt back, loaded the mag, hit the bolt release and right off the bat it had a double feed. Cleared the double feed and proceeded to fire. Fired all five rounds but the BCG failed to lock back after the final round.

Next set of five, bolt road home with no double feeds but after the second shot I had a FTE which caused a live round to get jammed up in the weapon with the spent casing. cleared that out. Also had one lite primer strike which resulted in a FTF. Out of the 5 sets of 30 rounds I fired the BCG failed to lock to the rear 3 times.

My guess is right off the bat to take a look at the buffer spring/buffer then move onto the gas block, gas port. Since it's only a 3.0 oz buffer I am wondering if I need a heavier buffer? Let me know what you guys think, and I appreciate the help!

Failure to lock on last round is typically an under gas issue. Check to make sure the bolts are tight on the gas key, look for blow-by between the key and carrier. Make sure the gas block is lined up correctly. Did you buy the short barrel or cut one down? What size is the gas port in the barrel?  Did you rest the magazines on the bench while shooting?



Oh yea, have you scrubbed the chamber REALLY well?



 

Barrel was bought short not cut down. The mag was not resting on anything. Chamber is clean as a whistle. I just ordered a different gas block because I wasn't a fan of the one provided, it seemed really cheap so I had planned on replacing it anyway. So I will replace that and try a couple different buffer weights and see how it goes. Just gotta wait for the stuff to get here.

The .300blk was designed for an H2 in a short barrel config (with pistol length gas).  However, I know many people that run the Spikes T2 since it is between an H and an H2.  I would check gas block alignment first and also replace the buffer.

What is gas port diameter?

What ammo are you feeding it, reloads or factory?

Well I have no real way to measure the diameter, and I don't have drill bits. A 7/64 Hex Key (Allen Wrench) will fit in the gas port almost perfect but 1/8 is too large. I have already ordered a new gas block and a new gas tube and I'm going to switch out the 3.0 oz buffer for my spikes ST-T2 buffer and see how it goes.





http://www.ozarkordnance.com/300-AAC-Blackout-147gr-FMJ-Qty-500_p_12.html

That's the ammo I am using, same ammo I use in my 16 inch 300.

As the others have said, look at the gas system and the buffer.

But also, have you tried a different magazine?

Gas port seems to be too big (.108 ish) if anything.  http://micromoa.com/?page_id=496



Magazine may be having issue and causing lock back issue. Try a different mag. Actually try a different lower if you can.



Overgas may be cycling too fast and causing feed issue.



It is a little frustrating, I just went thru a 'development' cycle with one of mine (10.5 barrel). I had an adjustable gas block with a .128 port and allowed me to solve gas issue. When I finally got it to run it was 'OK, now what do I do'.  

Just pulled this from their website for my barrel

Barrel Specs:
10.5" HBAR Profile
Chambered in 300 Blackout
4140 Chrome Moly Steel
1 in 8 Twist Six Groove Button Rifled
Pistol length gas port
Magnesium Phosphate Finish
M4 Feed Ramps
5/8x24" Threaded muzzle
Manufactured in the US
.750" at gas block
.125" gas port

Something that is throwing me off is the bolt not locking to the rear on the last round, classic sign of being under gassed yeah? I'll probably end up going Monday after I install the new gas block and gas tube. I'll check every variable, use different mags, different buffer weights, couple different buffer springs. Check to see which direction the brass is ejecting, check the casings for signs of shearing.

I think the bolt carrier is moving so fast it is already on its way back before the follower can come up and lock it open. Start with your heaviest buffer.



 

Well I tried a few different buffers and buffer springs. 3.0 oz buffer and a Spike ST-T2 buffer and tried a carbine length buffer spring and a rifle length buffer spring with both buffers and still the same problem locks back sometimes and fails to lock back others, almost seems like a 50/50 mix, also tried 5 different mags

I have two photos and two videos showing when the bolt locks back and when it fails to lock back.

Bolt locking back


Bolt failing to lock back.


And videos

Locking back.
https://www.youtube.com/watch?v=czZjk14QvVQ&list=UUHmh7PDNCr_owuBBa9kqZ-Q

Failing to lock back.
https://www.youtube.com/watch?v=nmkreowgt28&list=UUHmh7PDNCr_owuBBa9kqZ-Q

Well I just bit the bullet and bought a Syrac Ordnance Gen II Adjustable Gas Block. Hopefully that fixes the issue once and for all....

Exactly.  Try Spike's T2 or a H2 buffer before anything else.  With the gas port at .125 LEAVE IT ALONE.

Not if it's cycling too fast which is where I was being lead to as well...  FTEs leaving the brass in the chamber can be caused by overgassing as the system tries to cycle before the brass has time to return to it's original size and lest go of the walls of the chamber.  You'll often see torn rims with this where the ejector grabs the brass but tears it because the brass isn't ready to come out of the chamber yet.

Failures to lock can also be caused by the system moving too fast because the bolt has already started heading home before the spring in the magazing can raise the bolt catch and engage the carrier.

If it's too fast then the heavier carrier and/or a reduced gas volume (read adjustable block) are the fixes.

Thanks, I ordered a Syrac Ordnance Gen II Adjustable Gas Block on Monday. I figure it's the better way to go, less wear on my carrier and bolt, less recoil, and it will run cleaner.

Well after installing the adjustable gas block and still having the bolt failing to lock back on multiple occasions. I did what I SHOULD have done in the first place.. I took the upper off the lower and started inspecting the lower slapped an empty mag in and found the problem really fast. Now I feel kinda dumb for not having done it in the first place

Live and learn I guess! Although it was really over-gassed as well so I did need the adjustable gas block anyway.

Here is the problem



On the left is my Spike's Forged Lower and on the right is my Spartan Billet Lower



You can also see the dents on my mag where the bolt was being stopped by the mag instead of the bolt catch...

I have already contacted JoeBobs to see what can be done. Seeing as all the reviews for this lower are shining and no one has mentioned this being a problem I can only assume I got a bad one.

I contacted Joebob's



So that's a good thing and I had also ordered an AR-15 9mm bolt catch just in case.

That stuff in the receiver is grease they had put the springs on a pre assembled lower I got awhile back.

Well after receiving the new lower it had the same issue so I just did the 9mm bolt catch fix... Took it to the range, dialed in the gas block and no more issues!

http://i60.tinypic.com/24mrme9.jpg

I'm in love