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Old August 11, 2013, 03:22 PM   #1
Sabre9mm
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Bumpfire tuning

Starting a new thread on this because it it different than the original question on wear and tear.

VIDEO OF FIRST TRIP TO RANGE

As you can see this is second time ever using this device, definitely without question does what it says, easy to use, and very controllable (surprisingly so)

Only issue was the jamming about twice per mag, they were all failure to lock forward fully into battery.

Buffer weight? Spring tension? Tips on determining cause and correction?

The DefendAR15 does have its own buffer tube with a spacer to take up slack space between additional stock length and STD tube length, and never have this issue, same ammo, with stock tube.

I figure there are many here with the knowledge of the system to better help me understand what may be the malfunction, and I get to learn more about it in the process.
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Old August 11, 2013, 03:55 PM   #2
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A little research has led to possible consideration of high ejector causing same issue, could this be an issue that would be exacerbated by rapid fire? I would think if it were that it would be present in any rate of fire...
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Old August 11, 2013, 07:25 PM   #3
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Second guess would be possible round pressure, I have found several discussions concerning failure to lock and round pressure, these are .223 rounds (Federal AE bulk)...


Will try ASAP a mag of the M855 Lakecity stuff they are 62gr vs 55gr, and I assume loaded to a slightly higher pressure.

Side question on that, is if the built locks on the return stroke, I would assume round pressure being a factor would be not pushing it back as hard, therefore not as much forward momentum on the return stroke?

This is me learning more about the weapons function, troubleshooting and repair, historically all I have done is shoot them, but I am finding the function aspects to be fascinating the more I learn about them.
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Old August 12, 2013, 09:37 AM   #4
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Are you sure it wasn't fully in battery? I ask because the rounds following the stoppage seemed to be extracted just fine.

The most common issue with bump firing (whether traditional or with a stock made for it) is that the hammer will not be caught by the disconnector or sear and will follow the carrier down (too slowly to hit the primer hard enough.)
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Old August 12, 2013, 10:11 AM   #5
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Thank you,

I assumed not, but I am no expert on failures in this gun, learning as I go.
To me that is what appeared to happen because the trigger would not engage, but I guess it could have simply not been set, and the next round manually charged would have corrected that. That actually makes more sense....

Forward assist would not help, no trigger click, so your suggestion could be right on point.


They did extract without issue and no change in feel or additional resistance, with no signs of issue with the round, likewise same rounds had no primer dimple so I reloaded them and they fired without issue on subsequent runs.


So what would be the solution, or mitigation per se, heavier ammo, heavier buffer (My assumption is this would slow it a little, give it more time?), lighter buffer (My assumption here is that it would cycle further? I have a 3oz carbine buffer/spring)
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Old August 12, 2013, 12:40 PM   #6
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It's a timing issue in a gun that doesn't have the parts necessary to make it time consistently while running in that fashion. There really isn't much of a fix, that's just the nature of bump-firing. You *MIGHT* be able to help the hammer engage a stop by running a lighter buffer, buffer spring or hammer spring, but I doubt it'll be worth the tradeoff and other issues you might induce.
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Old August 12, 2013, 01:12 PM   #7
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That is cool, I was hoping there might be a fix that would not be too much of a modification to hinder its normal performance.

As it was a novelty to begin with, I would not want to go all out there just to make it work as this is not the primary purpose of this weapon.

I will just experiment with different ammo I suppose and see if I get any varying results, maybe the buffer or spring as they are cheap and no biggie, if they do not work I just have spares.

If not, then the occasional failure to fire on the range is no huge burden for the fun it is to shoot it like that.

Thanks for the input.
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Old August 13, 2013, 05:06 PM   #8
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Hmmm, what about a lighter/faster hammer, I see that this is an option, is it a practical one? And would it cause any foreseeable negative affects as obviously people do this that do not use the gun for this purpose...

Although I am not sure why a faster lock time outside of this type of scenario would be of any benefit.

Just a thought, opinions?
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Old August 14, 2013, 11:52 AM   #9
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I spoke to the Mr. Foster at FosTech directly.

He said the the issue was almost certainly the trigger pull weight, that the stock trigger on that particular line was generally 5#+ range, which cause the user to have to pull harder in the forward motion to fire. I agree the thing does have a very heavy trigger. This is a sort of known issue per se it appears.

The additional forward force counters too much recoil causing it to move forward again faster than the action can cycle, effectively the reciprocation is TOO efficient and I need more of the recoil delivered instead of countered, so....

It sounded like he was reasonably sure, and I would consider him to be the most likely authoritative source on his own product (At least the one that has logically used it the most and troubleshot it the most)

It is currently at local gunsmith having a polish / spring job to lighten it up to around 3#.

Either way I get a lighter trigger out of the deal, which was not an unreasonable modification to have made anyway bumpstock or no, we will see if it fixes the problem and I will let others know / post video of it working ASAP.
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Old August 14, 2013, 02:16 PM   #10
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Good update thanks.
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Old August 14, 2013, 06:18 PM   #11
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Success!

Trigger pull did it without question.

Installed a Brownells AR15 reduced power spring set (Yellows), and had it polished, cost me $35 total.

Dumped three mags, with sufficient cool down in between, no issue at all.

Only warning the smith had was that the lighter strikes could misfire on some of the cheap foreign ammo because of harder primers, so just for grins I bought 40 rounds of tulla steel to test that theory...

Sure enough 1 misfire, dimpled but not ignited.

M855 and M193 blew through it like a breeze.

( Could not video, was at range by myself, could not hold it and phone at the same time )

Now to clean the tulla poop out of my gun, man that stuff is like nasty dirty...
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Old August 15, 2013, 07:25 AM   #12
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Quote:
To me that is what appeared to happen because the trigger would not engage, but I guess it could have simply not been set, and the next round manually charged would have corrected that. That actually makes more sense....

Forward assist would not help, no trigger click, so your suggestion could be right on point.
This doesn't jive with your guy's diagnosis. Was the hammer still cocked (giving you a live trigger) after the stoppage or not? If your trigger were too heavy, you could have simply pulled the trigger again without having to cycle the action resetting the hammer.
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Old August 15, 2013, 10:21 AM   #13
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No the trigger was not set, pulling it had no affect in the previous malfunction. I had most likely erroneously determined that the faulty cycle was not in battery because it would not *allow* me to fire, but I believe now it was that the trigger was not set and therefore could not allow me to fire.

He said that it was caused by the fact that because I was having to pull too hard forward to trip the trigger, I was absorbing too much of the recoil with my left arm, which got me back to the trigger too quickly, this was causing the bump cycle to occur too fast and outrunning the action on the weapon.

By lightening the trigger, the required force to hold forward is lessened so the felt recoil was higher. I am assuming this also increases the stroke length of the gun reciprocating, distance from finger bridge to trigger for each cycle, etc... Slightly decreasing time between shots and allowing the action to function properly.

It did work, and sounds completely reasonable, so it is hard for me to argue with the logic overall. Since the only issue I had was with the Tulla, the gunsmith warned me this might happen after the trigger job, and since after cleaning up after a meager 40 rounds of said Tulla my hands looked like I had just shined the whole company's boots... I do not see myself having that issue in the future as I do not believe I will ever feed it anymore.

So now I feel fairly certain any time I want to I can just flip a switch, and dump whatever is in or remains in my mag for the fun of it, so mission accomplished.

Thanks all for the input.
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Old August 15, 2013, 12:56 PM   #14
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Eh... ok, I'm done. Have fun with it.
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