QUESTION: 11.5" CAR w/Tubbs Buffer Spring Double Feeding (UPDATE 100% RELIABLE NOW)

[Evan201]

Ok guys,

I've done my homework and still need a few view points here.

My setup:
Sig PM400 AR Pistol 11.5" Carbine

Swapped factory parts for:
KAK SB15 Dedicated Pistol Buffer Tube
Tubbs Flatwire enhanced buffer spring (not trimmed, yet)
Have tried multiple weighted buffer including 2.9oz CAR, Spikes ST-T1, Spikes ST-T2, and have ordered but not yet received or tested BCM H3 buffer.
Upgraded my extractor/ejector springs to Tubbs enhanced in all my BCG's.
Most commonly using 55gr FMJBT. (Wolf Gold, PMC, Federal 193 etc..)

Having double feed issues. Recently on a hot day in the desert I was running the Sig 11.5" and noticed that with One round in the few different mags I used, the bolt would lock to the rear. So I am assuming its not under gassed.
The spent casing would just sit in the upper disconnected from the bolt just laying there. The other more common failure at this point is the double stack after the FTE. Empty casing not ejected jamming up against the next round trying to be fed into chamber.

So I have done my reading on Dwell Time and that is why I purchased the BCM H3 buffer to test out in with the Tubbs Flatwire Enhanced Buffer Spring.
I have seen some things online about trimming down the Tubbs spring, but I am skeptical because I have the Tubbs Spring in my 7.5" combined with ST-T2 and it's running great. Also have the Tubbs spring in my 16" middie and 18" middie and never had to trim a thing with the other three.

I'm wondering if its a combination of the KAK Buffer Tube I bought with the Tubbs enhanced spring not allowing for good ejection/extraction.

I always assumed with shorter barrels that slowing things down was good thing, but at this point my tests will be Tubbs spring with H3 buffer and changing back to the standard MIL CAR spring and testing the different buffer weights to see what works best.
I do like my Tubbs springs because of the snappiness, enhanced life of spring, and the less Spong sound inside the tube. As I said, the Tubbs springs are working great in my other 3 rifles.
Another note I have read across my internet travels is to apply the O-Ring to the extractor spring which I have done now but have not yet tested. I find it hard to believe that the enhanced Tubb Extractor+Ejector springs aren't enough on their own to hand it, but who knows. So I threw an O-Ring on their anyway.
During my tests in the desert, I did switch out the entire BCG to see if it was something with the BCG going wrong. Same failures between my NiB BCG and my TiN BCG which is currently in the Sig 11.5"

Any further info you guys may need, let me know. Would love if someone else had the same experience with the same parts who could help me out.

[FeuerFrei]

Double feed is 2 rounds trying to share the chamber. Mag problem.
You describe a gas or extraction problem.

Does it run with all OEM parts?
Put it all back to factory config and then start swaping out 1 part at a time until it gags.

[Evan201]

Thanks FeuerFrei,

I am going back to the factory buffer spring to give that a try, but I really would like to keep the KAK buffer tube because it stretched out my Sig SB15 arm brace and If I put the original buffer tube back on, the arm brace will have major slop and play in it. Its on their stiff right now. Bolt locks back with single round loaded, but casing does not make it out of the upper. With only one round loaded, it comes out of the chamber but not out of the upper.

[Evan201]

Also note,

I just cleaned the gun very thoroughly because I read that a dirty chamber can cause some issues too.

[Evan201]

UPDATE: Upgraded to BCM H3 Buffer and BCM extractor spring upgrade with plunger and o-ring.
Going to go to the range and keep the Tubbs spring in it first. If it fails, will change buffer spring back to standard GI spring that came with the gun. Will also be testing out multiple weights in buffers.

I believe this could be a dwell time issue as I have been told that Sig's barrels can be over gassed.

Will report back after the range.

[Press Check]

I've heard good and bad things about Tubbs springs, and the bad things were fairly alarming.

[HondoRoll]

post a picture of your brass please.

im thinking broken/underpowered extractor. maybe it doesnt have the rubber bumper and o ring. or maybe your chamber is bad.

what youre describing is a failure to extract rather than a double feed, but even if this was a double feed you should have mentioned what type of magazines you are using at least 27 times since most true double feeds are caused by magazines. by the way you can probably find out everything i just told you in 5 minutes on google. but since your gun isnt working, and ive sworn the spartacratic oath to arm those in need, lets get you squared away.

but for real post pics.

dammit man, im a gunsmith not a psychic!

[Evan201]

UPDATE: 100% RELIABLE NOW.
Range report after upgrading to Bravo Company H3 Buffer and Bravo Company Extractor Spring Upgrade Kit in bolt.

TEST ONE Sig 11.5" Carbine Shorty.
H3 buffer w tubbs buffer spring and BCM extractor upgrade.
Federal 5.56 XM193
REPORT
Reliable, no failures at 40 rounds. Ejected at 4 o'clock.

NEXT TEST Same setup with PMC .223
REPORT:
Last round of first magazine failed to lock back bolt. No jams.
Second test with PMC .223. Changed tubbs spring for standard spring. One round in and bolt locked back.
50 rounds no problems after buffer spring Change with .223 PMC. Ejected at 4 o'clock.

NEXT TEST: wolf gold .223
with H3 and standard GI spring.
REPORT 20 rounds No failures ejected at 4 o'clock.

NEXT TEST H3 with standard GI spring and federal 5.56 xm193
REPORT
20 rounds No failures ejecting at 4oclock.

CONCLUSION: Bravo Company extractor spring upgrade kits will be going in every one of my bolts on all my AR's.
The H3 buffer felt amazing on the recoil and seemed to be harmonious with the standard Mil Spec buffer spring.
I now have an 11.5" AR pistol that runs like a sewing machine and will eat anything I choose to put in it.
So happy.