AR15 Short-Stroking.....Need help problem-solving

W

willywonka519

CGN Regular
Rating - 100%
141   0   0
Location
Alliston, ON
Need some help from those of you who are more mechanically inclined than myself..

I may be way out of the ball-park on some of this or overlooking something obvious, so let me know!

Objective info/equipment:

Upper #1: 15.7" IUR with Spikes full auto NiBoron BCG
Upper #2: 20" SA20 with CC BCG (?semi auto)
Lower: SA20
Ammunition: Remington UMC 55gr
Magazines: Magpul and Colt Canada Mag
Lube: both BCGs were lubed with white lithium grease at contact points

The Exam/the short-stroking:

15.7" IUR with spikes BCG - must be manually cocked after every round (did not feed next round after firing), did not hold open after last round, no effect with changing mags
15.7" IUR with CC BCG ----- functioned perfectly with both mags
15.7" IUR with spikes carrier and CC bolt assembly-must be manually cocked after every round (did not feed next round after firing), did not hold open after last round, no effect with changing mags
SA20 with CC BCG-----------functioned perfectly with both mags
SA20 with spikes BCG-------no noticeable short-stroking, fired all rounds without issue, did NOT hold open after last round, no effect with changing mags
Total rounds fired during session = 50

My impressions:
1. I'm unsure if 50 rds is enough to make a fair evaluation. However, that is all I had with me at the range

2. From what I've read, short stroking is most often a symptom of under-gassing or a heavy buffer spring. What I don't understand is why the spike's BCG performed better in the rifle length gas system (less gas) using a lower that was built from factory to function with a rifle length upper (lighter buffer spring).

3. I have also read that over-gassing may cause the case to become resistant to being extracted resulting in the BCG losing energy in extracting the stubborn brass. This is reportedly associated with brass deformation, which I did not see, but is more consistent with the issue occurring more severely in the IUR.

4. The Colt Canada BCG functioned 100% in both rifles. Perhaps being a semi (?lighter) rather than full auto (?heavier) made it less prone to short stroking as it may require less gas to move it the appropriate distance.

5. Perhaps the spikes BCG is losing energy (too heavy, not aligned to gas port, leaking gas, or creating excessive friction)

Further Investigations:
1. Test with another full-auto carrier in the IUR
2. Use different ammunition
3. Test spikes carrier in a non-CC ar15

My Differential:
1. BCG dysfunction ie. BCG out of spec, leaking....any other full-auto bcg should function properly
2. Ammunition
3. Rifle dysfunction ie. rifles are not able to reliably run a full-auto bcg, buying any other f/a bcg will result in the same dysfunction


Thanks!
 
As you said, most issues of this nature come down to balancing the gas and the spring/buffer weight.

If you have an adjustable gas block, you can turn up the gas. If not the. You'd have to reduce buffer weight and or go to lighter springs.

One issue with severely undergassed rifles can be if the gas block is not aligned properly. But that doesn't really sound like the issue.

You get different results with different bcgs, are the significantly different weight? Have you tested the rings on all of them?
 
I still need to weigh the BCG's with a reliable scale to detect a difference. I think the buffer weight and springs should be light if they were meant to cycle loads from a rifle length system. Right now I'm not thinking it's an issue with gas delivery with the rifles as they came from the Colt Canada factory...

How do you test the rings on the BCG?
 
Extend the assembled bcg, then stand it on its bolt face.

If the bolt collapses under its own weight, rings are worn out. If it stands extended then the rings are still good.
 
Seeing that both uppers seem to have issues with that Spike's BCG, I think you may have narrowed down the source of your problem.

Check the gas key on the Spike's BCG to see if it isn't loose.

If you work the charging handle, do you feel any noticeable resistance with either CC or Spike BCG? I had a DD BCG that was getting held up, turns out one side on the top of the cam pin was too wide and I had to file it down.

I'd suggest trying different ammo to see if the problem persists. Note where your spent cases are ending up with both CC and Spike BCG. If your brass are ending up around the 12 to 4 o'clock position, the theory is you're good for gas. If they are ending up from 4-6 o'clock position, you're running undergassed.
 
Last edited:
The spike bolt doesnt like your iur. It functions fine in the sa20. It could very well be that both parts has a super tight tolerence, and the spike bcg in sa20 has a looser tolerence.

If the spike botlt is fine running in the sa20, then gas ring, gas key has no leakage, otherwise it would've done the same thing.
 
I would have to concur that the issue is the Spike BCG. The only gas related issue not mentioned so far is dwell time. That is the time from which the bullet passes the gas port until it leaves ghe barrel. You are correct that the rifle system is operating under lower pressure than the carbine system. Additionally the carbine has more dwell time than the rifle. Both of these factors should contribute to better reliability in the carbine than the rifle. But you have found the opposite to be true. You need to get an accurate weight of the BCGs. Your stated weight difference of .1lbs is 1.6 ounces which is the weight difference between an standard weight carbine buffer and an H2 buffer.

My custom built 7.62x39 AR wouldn't reliably run Czech surplus with the supplied H2 buffer but runs it with 100% reliability once I switched it out with a standard weight carbine buffer. 1.6 ounces can make a difference.
 
One of my AR's with a match barrel and chamber needs to be run significantly wetter than my others. If I get a short stroked cartridge I give it a light spray, and all is good for another couple hundred rounds or so
 
Try useing real 556 ammo. If that works. Then it means your buffer is likly a H2. And is to heavy to shoot low power 223.
My KaC even after 8000 rds will not cycle on cheap stuff like the russian 223.
 
So, everything works, unless the Spike's carrier is installed in the IUR.
Sounds as if you have isolated the problem.
 
I would suggest not lubricating the BCG with grease as a starter. 1-2 drops of oil per contact point on the BCG is all that is needed and is used by the military for good reason. Have you checked the gasway on the spikes BCG (key, rings, obstruction)? Do you have a standard size Cam Pin in the Spikes? The difference in BCG weights inconsequential and will not cause issue.
 
r_taylor said:
I would suggest not lubricating the BCG with grease as a starter. 1-2 drops of oil per contact point on the BCG is all that is needed and is used by the military for good reason. Have you checked the gasway on the spikes BCG (key, rings, obstruction)? Do you have a standard size Cam Pin in the Spikes? The difference in BCG weights inconsequential and will not cause issue.
Click to expand...

The key seems to be solid, tested the rings with no failure, will check for obstruction when I get home. The CC BCG was lubricated the same as the spikes and the spikes is supposed to be able to run without lube due to its "magical" nickel boron coating...

I'm not sure if I can blame the ammo or the buffer system as the CC BCG functioned 100% in all configurations (unless there is a dramatic difference in weight of the BCGs).

I will give spike's a call today regarding their carrier.
 
I'm not sure what buffer it ships with....or even if the SA20 buffers are different than the SA15 ones.

I just ordered a semi-auto stag BCG from arms east. Thanks to Brownell's awesome return policy, the spikes is going back for a refund.
 
willywonka519 said:
I'm not sure what buffer it ships with....or even if the SA20 buffers are different than the SA15 ones.

I just ordered a semi-auto stag BCG from arms east. Thanks to Brownell's awesome return policy, the spikes is going back for a refund.
Click to expand...

Both The SA20 and 15.7 ship with HH buffers.
Same as the C7 and C8.
 
really, I would've thought the sa20 would run a rifle buffer? which will be about 5oz vs a carbine H2 which is at 4ish oz
 
You must log in or register to reply here.
Back
Top Bottom