Why, because bumping can be fun.........

I have had a few 9mm carbines in the past and they all seem to bump very well. My current 9mm AR build is as follows:

CMMG dedicated lower
RRA un-ramped bolt
Non-notched "Sporter" style rounded hammer (have used these on all my pas 9mm ARs)
RRA Colt 9mm buffer
JSE Surplus SS 9mm barrel
Metalform mags
DSA upper receiver

Ammo thus far has been my reloads (plated 115 gr. bullets over a range of hogdon tite group. 4.5, 4.6, 4.8 and 5.0....loaded for carbine only)

I can shoot it all day long slowly or as fast as I can pull the trigger with no issues. I bump it, and it has an OOB within 5-7 rounds. This happened this last weekend every time i tried to bump it. Which was 3 times.

Short answer......stop bumping it!

Any input? I haven't tried other ammo, and am thinking that the barrel chamber could be on the tighter side, and the plated bullets could be on the slightly larger side???

Thanks

No answer for you. I had the same experience, bulk 100 round box from Walmart with my Oly 9mm upper. Last round first mag of 25 OOB.
Next to last round next mag of 25 OOB took out the extractor. Never bumped it again.

I've heard its pretty common.   What's happening is that you're trigger releases during the bump are timing so that the hammer's reaching the FP right at the peak of bolt rebound after chambering a round.    You can try a faster (or slower) lock time FCG....

You might try running a heavier buffer.

I also knew a guy who had his smith install a couple cross pins in the bolt, so as to  capture a piece of tungsten rod, shorter than the captive ara, with rubber bumpers on each end... he said it provided an anti bounce effect (his gun was registered FA).

I agree that it is pretty common, but every other AR 9mm I have had (3 others) would do it once in 350 rounds or more. That is why this one baffles me.

Maybe I need a DI 9mm with locking/rotating bolt. That would eliminate the problem.

I may try a heavier buffer.....

Thanks.

P.S. I never thought about bolt bounce.....I just assumed the hammer was hitting the firing pin before the bolt was fully closed....maybe it is closing and bouncing open a tad.

HMMMMMMMMMMMMMMMMM???

The nature of the 9mm bolt means that the firing pin can indeed reach the cartridge when it's out of battery, unlike the 5.56 configuration with the rotating locked bolt.  So it's just a matter of the timing of the function cycle of the firearm compared to the trigger pulls.

Curious if you're running a bolt travel shortener or not.  If not, maybe try a Q buffer or an insert to shorten the bolt travel... this would tend change the timing of the cyclic vs the trigger pulls, so might solve the problem.

I am not running a spacer or longer buffer. I may try this.

Get it figured out, man.    Remember, the big risk with OOB in a 9mm AR is that somehow the burst case gets ejected but with a bullet lodged in the bore, and another one feeds and fires and KABOOM.

I don't think that would happen, if it's THAT far out of battery the hammer couldn't reach the firing pin.  Also consider that most open bolt 9mm SMGs use advance primer ignition, so they "out of battery" discharge by a controlled amount with every shot.

The quickest way to try this is to use some quarters as the spacer in the buffer tube.

It has happened to me while bump-firing a 40cal Oly Arms gun.



Luckily, I was shooting all lead bullets and the second shot pushed out the first!



I believe that a jacketed or plated bullet would have caused a KABOOM!
HK_dude, would a Wolff spring and standard buffer help you think?

echo-5, I am not sure....I haven't tried.

Update....I am using the SS R-Guns lower parts kit. I have used about 5-6 of these before and all have been good.

When I went to take it apart last night (I haven't noticed this before...and don't know if it means anything) if the hammer was in the forward, non-cocked position, it was very hard to push out the take down pin....meaning that the hammer was pushing on the bottom of the bolt. If the hammer is cocked, this isn't a problem.

Would this have anything to do with it? If the hammer falls during bumping and the top of the hammer is slowing the bolt down due to friction, could it make it fire before fully going into battery?

I am just thinking out loud.......?????

With an Olympic-pattern, yes that could happen. I'm not sure what a Colt-pattern BCG looks like.



With Olys, the bottom of the BCG does not fully cover the firing pin, allowing the pin to be hit while very much out-of-battery.

I have a very long update of things I have found and what things I have changed.

When breaking the gun down to see if there was anything odd....I first found that the hammer pin was broken in 2 pieces. I could also see a relatively deep "V" that was in the tail of the disconnector where the back of the hammer was hitting.

The other part, which I think could be the largest part of the OOB issues was:

I built this with a stripped upper (I can't remember if it was DSA or Spikes) When I first got it together it was hard to close the upper to the lower and get the take down pin in. Due to this the bolt and charging handle would bind when it was about 3/4" from completely closing. It turns out the inside of the upper at the top/back where the charging handle slides in and out has a raised rib on both sides that ran parallel to the sides of the receiver, they only ran for the distance of the side cuts for the charging handle, but they were causing trouble. I first removed a bit to get it to work correctly (what i thought was enough) It turns out I hadn't removed enough.....when it would get the least bit dirty (9mm is DIRTY) the bolt started binding and was slightly stiff to go into battery.

What I changed (Not knowing exactly which change fixed it....):

1. Changed out the FCG group with the same style hammer, they just weren't the R Guns SS parts)
2. Removed all of the excess aluminum in the rear of the charging handle area of the upper (Smooth as silk, no drag/bind and it's easy to push the take down pin in/out)
3. Added 10 quarters (Plan to machine the same length spacer out of nylon or aluminum)

I only had it out 1 time, but with same ammo and mags, I was bumping 3-4 round bursts (this is what i like) throughout the entire mag with no problems at all.

It was too bad the problem happened in the first place but must have something to do with Murphy's law. I remember from your original thread that you were trying for an inexpensive 9mm build and all seemed OK till the first bump firing. I was kinda wondering how that inexpensive ProMag magblock was going to work but it got blown to smithereens by the first OOB episode. I'm glad you got the whole thing up and running just fine and you didn't get hit by flying magblock parts.

If I didn't already buy a Colt 9mm buffer i would.

I plan to machine a spacer and use with my Colt buffer........for the small amount that I have ran through it now, it seems to be fine. I couldn't even get through 1 burst before, and I did 5-6 bursts with the latest set up.

It can happen with any pistol caliber blow back AR's even the 22's and the stock colt 9mm's.

Bolt bounce is one of the reasons for the oobs, the bolt hits the back of the barrel the same time the hammer is on its way to the firing pin then the bolt bounces back the same time the firing pin is hitting the primer, you could call this bad timing in a way.
I made some longer extra heavy buffers filled with tungsten powder instead of using a spacer for all my pistol caliber blow back AR's.
A extra power buffer spring is another added bonus to help keep the bolt forward with less bounce.
With the colt style 9mm's bolts I replaced the weight in the rear with a catridge filled with tungsten powder and it
got rid of the clack and now sounds like a thud when the bolt hits the barrel.
The tungsten powder acts like a dead blow hammer with sand in it and removes the bounce effect.
.

After having many oobs with a stock colt 9mm and a couple other pistol caliber AR's I started doing some testing
and fixing as the oobs used to scare the shit out of the wife when she was shooting.