300 blk build cycling issues FTF

[Quacker]

Ok... update:

Got a little time at the range today. Colt sent me a new bolt carrier to try. I swapped the bolt out of the old carrier group and installed it on the new carrier. Same thing happened as before... binded on the feed ramps.

Then I took the bolt carrier group out of my LE6920 and install it on my 300 blk. Dumped 20 rounds out of a Pmag flawlessly. I inspected the brass and there were no issues there.

Next I use the problemed bolt carrier group in my LE6920. Dumped 20 flawless rounds through it as well. I inspected the brass and saw some light scratches in the middle of the 223 case.

In short, the crappy bolt will function in my LE6920 but not my 300 blk and my LE6920 bolt will function flawlessly in both. Troubleshooting revealed it is not the bolt carrier but the bolt. I will see if Colt will send me a new bolt as well.

Glad to know it was not my build that was causing the issues. And not magazine issues.

 

[Quacker]

If you look closely you can see the marks on the 223 case in the same location as the ones on the 300 case. This is with the same problematic bolt in two different guns.

 

[Plinker66]

Does anyone know how or why a bolt would cause that? I’ve seen some of my brass marked like that.

 

[Quacker]

Ok... I think I have finally figured out the core issue with the bolt. I got to thinking about the feeding and cycling process and compared my two bolts. See picture below... The one on the left is my well used bolt from my LE6920. The one on the right is the problematic bolt. Take a look at the bolt face... The left is well worn and smooth - the right has a rough sandpaper like finish.
Looking at the problematic bolt itself, it has a very inconsistent parkerized finish. It is somewhat splotchy in that certain areas have a very smooth, typical parked finish while other areas have a very rough, crystalized, sandpaper like finish. The bolt face has the crystalized finish. I took a samll piece of aluminum I had laying on my desk and "tested" the frictional coefficient of the two... aluminum is soft like brass and I was suprised at how much more friction was present on the problematic bolt. This friction is worse with soft metals like brass, especially when a little force is applied. This prevented the case head from slipping up the face of the bolt during feeding and thus jammed the case into the feeding ramps because the case head was too low and the case was feeding at too high of an angle.

 

[Quacker]

So I decided to polish the bolt face slightly with some 600 grit sandpaper to make it smooth. Here is the pic below of the difference.

 

[Quacker]

See images below of what case feeding should look like... Notice how the case head has to slide up the bolt face in order to chamber and go into battery. My cases were never sliding up the bolt face due to the high friction / crystallized finish on the bolt face. I think this issue pretty much explains the problems I was having.

I should have mentioned that I did a dimensional analysis between the two bolts with a set of calipers and found no noticable difference between the two. The finish on the otherhand was definitley out of spec.....

If it weren't so bad and would actually cycle my weapon, im sure it would have broken in after a couple of hundred rounds and been fine. That really speaks to breaking in your weapon IMO.

 

[Quacker]

All this is still an unconfirmed hypothesis until I go to the range but I am pretty comfortable with my theory at the moment.

 

[Cohutta68]

Ok... I think I have finally figured out the core issue with the bolt. I got to thinking about the feeding and cycling process and compared my two bolts. See picture below... The one on the left is my well used bolt from my LE6920. The one on the right is the problematic bolt. Take a look at the bolt face... The left is well worn and smooth - the right has a rough sandpaper like finish.
Looking at the problematic bolt itself, it has a very inconsistent parkerized finish. It is somewhat splotchy in that certain areas have a very smooth, typical parked finish while other areas have a very rough, crystalized, sandpaper like finish. The bolt face has the crystalized finish. I took a samll piece of aluminum I had laying on my desk and "tested" the frictional coefficient of the two... aluminum is soft like brass and I was suprised at how much more friction was present on the problematic bolt. This friction is worse with soft metals like brass, especially when a little force is applied. This prevented the case head from slipping up the face of the bolt during feeding and thus jammed the case into the feeding ramps because the case head was too low and the case was feeding at too high of an angle.

View attachment 1886683

Hey man great catch and diagnostic work on this. I may try the same on a similar issue I have. You may be onto something. I agree with your theory being plausible for sure.

It will be interesting to hear your final report after you try your newly polished bolt face.

 

[Quacker]

Hey man great catch and diagnostic work on this. I may try the same on a similar issue I have. You may be onto something. I agree with your theory being plausible for sure.

It will be interesting to hear your final report after you try your newly polished bolt face.

Thanks! I may call colt to report my findings. I actually work in a plating / metal finish lab for the Air Force so I may know a thing or two about metal finishes. The only QC they probably do on the parked finishes are salt fog/corrosion tests. It would take a scanning electron microscope, good optical microscope, or a XRD to detect the porosity/crystal structure differences. Luckily, I have access to all of those. I’m sure they don’t look at frictional coefficient which would be a better property to measure.

I just want my dam rifle to work tho.

 

[Quacker]

Pics of the inconsistent finish. Microscope pics didn’t turn out that great through my phone. Microscope is not hooked up to a computer at the moment so I can’t get digital pics... top microscope pic is the smooth finish.

It’s almost like the bolt was slightly rusted before it was parkerized.