AR15 9mm buffer

[Sid T.]

I acquired a 9mm Olympic upper with a 16" barrel and fitted it on a standard carbine lower.
I've found that it runs fine with 124 grain ammo but fails to cycle fully with 115 grain.
With 115 grain it will fire and eject the first round ok but the bolt doesn't come back far enough to pick up the next one from the magazine.
All empty cases look good with no signs of pressure.
I'm thinking that I should fit a lighter buffer, but I weighed the one in it and found it to be 3oz and that seems to be the lightest of the standard buffers.
So what do you folks suggest?

 

[sniperdoc]

Maybe a weaker Spring would help

 

[A&S Conversions]

Something that came to mind would be is there a drag on the bolt? Unless the bolt has a tungsten weight or the spring is super strong, a standard carbine buffer should be too light for blowback 9mm. Yet there is no sign of over pressure from the bolt moving back too soon.

Scott

 

[rentprop1]

This doesn't make sense 9mm buffers are a lot heavier than the standard 3 oz.....most 9mm ARs need a spacer or some use quarters in the back of the buffer tube as well
Are you sure it's not coming all the way back or is it possibly slamming into the rear end bolt bouncing forward

 

[OldBesty]

The 16" Olympic 9mm uppers were engineered to be "plug and play" on standard lowers having standard buffers. Extra weight was added in the bolt carrier.


Before going route of a weaker spring, I would assess possibility that something might be dragging on the bolt, even if only slightly. I would first focus on evaluating whether the magazine and well block are too high in this particular lower and causing interference.

 

[Sid T.]

The 16" Olympic 9mm uppers were engineered to be "plug and play" on standard lowers having standard buffers. Extra weight was added in the bolt carrier.


Before going route of a weaker spring, I would assess possibility that something might be dragging on the bolt, even if only slightly. I would first focus on evaluating whether the magazine and well block are too high in this particular lower and causing interference.

Thank you, that somewhat makes sense. I can only assume that it will "run in" with use and maybe eventually run 115 grain rounds too.
Waddya think about shortening the spring?

 

[OldBesty]

Do you have another lower available you can test on?

 

[Sid T.]

Do you have another lower available you can test on?

Oh sure, ask the obvious!
Yes of course I have other lowers I can try it on, I'm so embarrassed I didn't think of it.
I'll give it a try.
Thanks again.

 

[OldBesty]

Take your favorite oil with you. If you still have trouble with the 115gr not wanting to cycle, try running the bolt & carrier "wet" if you did not already.


Also, did you check using more than 1 magazine? Am I correct in presuming you are using Sten mags?


A cursory check you can try before heading to the range to see if the magazine is possibly interfering due to misalignment of the magazine feed lips or from to presenting too high:

(1) with your firearm unloaded, assemble your upper on the lower (while having the buffer and spring removed from the lower)

(2) use your index finger through the ejection port to push the bolt carrier back and forth. there is a groove cut on the side face of the carrier that makes this easy to do. confirm the bolt travels easily in and out of battery with only minimal force/pressure applied from your finger. (CAUTION: you will want to be very careful not to push the bolt too far back over the hammer or it can become a challenging dilemma to some, as they will encounter difficulty returning the bolt to forward position that allows take down)

(3) check again with your mag well block and mag installed. the bolt should still travel in and out of battery with the same relative ease

 

[Sid T.]

Take your favorite oil with you. If you still have trouble with the 115gr not wanting to cycle, try running the bolt & carrier "wet" if you did not already.


Also, did you check using more than 1 magazine? Am I correct in presuming you are using Sten mags?


A cursory check you can try before heading to the range to see if the magazine is possibly interfering due to misalignment of the magazine feed lips or from to presenting too high:

(1) with your firearm unloaded, assemble your upper on the lower (while having the buffer and spring removed from the lower)

(2) use your index finger through the ejection port to push the bolt carrier back and forth. there is a groove cut on the side face of the carrier that makes this easy to do. confirm the bolt travels easily in and out of battery with only minimal force/pressure applied from your finger. (CAUTION: you will want to be very careful not to push the bolt too far back over the hammer or it can become a challenging dilemma to some, as they will encounter difficulty returning the bolt to forward position that allows take down)

(3) check again with your mag well block and mag installed. the bolt should still travel in and out of battery with the same relative ease

Only have two magazines since they're proprietary to Olympic and they're very proud of them, (read expensive!).
There doesn't seem to be any undue drag on the bolt as it passes over the mags, but since it's a new lower and new mags it's gonna be a little tight.
I'll try lots of lube and see if that helps.

 

[OldBesty]

Only have two magazines since they're proprietary to Olympic and they're very proud of them, (read expensive!).
There doesn't seem to be any undue drag on the bolt as it passes over the mags, but since it's a new lower and new mags it's gonna be a little tight.
I'll try lots of lube and see if that helps.

Yeah, they are not cheap.

The earlier ones utilized a mag well block with sten mags. The mag well block had an interesting paddle style release.

 

[Sid T.]

I acquired the Oly upper essentially as a freebie with a RR M16. I've no intention of using it with a RR because of potential hammer pin problems, plus I have several other F/A MG's that handle 9mm quite nicely.
However, I don't have an AR15 in 9mm and this appeared to be a way to get one at minimal cost.
That said, I don't intend to let this turn into a project and if it only digests 124 grain then so be it, but it's worth a little effort and I appreciate everyone's input.
Here's pics of the items in question: