View Full Version : Broken Colt bolt, and what to look for...
I have just a little over 6,000 rounds through one of my Colt 6933s. This particular carbine has been my training rig, and close to 2,000 of that has been run suppressed. This carbine since day one has been run with an H3 buffer. The last time I cleaned this carbine was at the first of June, just prior to Pat Rogers' Alliance Ohio shoothouse class. Last week I broke it down for cleaning and inspection, then noticed a small crack at the cam pin hole on my Colt bolt. I snapped some pictures of the crack, then used a dye penetrate kit from work to look the bolt over real good and took some more pictures. You can see the crack, as well as some pitting on the bolt lugs and bolt tail. The pitting shows up as the white powdery looking areas, once the bolt is wiped down after the dye penetrate process. As far as the crack at the cam pin goes, most that I have seen, seem to start at the top side of the bolt (where the cam pin slides in), and the side opposite the extractor. You can see the discoloration of this area in the photos, in a V shaped pattern. Most cracked bolts I've seen also seem to crack from the inside of the cam pin hole outwards. So pay attention to the inside of the hole more so than the outside. If you have worked around broken metal parts, you will know, usually the rough torn looking edge is the last to fail. The clean break area is the first. Most pictures of broken bolts at the cam pin hole online show this same pattern. Pay close attention to the inside of the cam pin hole opposite the extractor. If you have any doubt, there are several companies that sell dye penetrate. You local welding shop is the easiest place I've found to find it. It is a very simple process and will tell you if you have any surface cracks.
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If you can catch it when it is like this, you can replace it and prevent a bad day at the range or in a fight.
Those are very informative pictures. Thanks for posting them.
Was there any abnormal wear to the carrier?
Good find!!
Excellent and informative post, Thank you!!
:cool:
Excellent example of why regular cleaning and inspection is more than just getting carbon out of the gun. :cool:
very detailed thanks for the post
Send Pat a picture of that, apparently Colts rarely crack at that location, the lugs usually failing first.
Those are very informative pictures. Thanks for posting them.
Was there any abnormal wear to the carrier?
Nothing over anything I have seen on any other well used carrier. I have been told by some elsewhere not to replace the cam pin unless you replace the bolt (stating that mating parts should be replaced together), but the bolt carrier, a mating part, takes wear from the cam pin also. I'm not onboard with this practice.
I know a few folks in the industrial field who do forensic metallurgy. Just for shits and giggles I plan on taking the bolt carrier, cam pin, and bolt to them and have them look it over to see what they say. I will try to get some good magnified and electron microscope pictures of the crack, cam pin, bolt , and carrier wear marks. I'll post those pictures when I get them.
LukeMacGillie
08-31-10, 06:00
Great Photos
carshooter
08-31-10, 09:04
Excellent pictures, and that was a good catch during your inspection.
A solid armorer can use x-ray to check for the initial hairline cracks in bolts. Often seen between 7 & 14K rds depending on firing schedule.
BTW I have only seen Colt bolts shear lugs before. However, plenty of DPMS & BM bolt snap at cam pin. Was this in a Colt carrier?
A solid armorer can use x-ray to check for the initial hairline cracks in bolts. Often seen between 7 & 14K rds depending on firing schedule.
BTW I have only seen Colt bolts shear lugs before. However, plenty of DPMS & BM bolt snap at cam pin. Was this in a Colt carrier?
Yes Sir, Colt factory 6933. The only changes to this rifle are a Knight's Midlength URX, Colt H3 buffer from SAW and a SAW or BCM extractor spring upgrade w/o ring.
Like I said before, this rifle has never skipped a beat and was still running fine when the crack was found. How long it would have continued to run is unknown.
You should shoot it to destruction. Maybe drop it in at the range and see how many more rounds it'll take.
Something else to consider. IIRC shooting suppressed will also accelerate wear (unless I am wrong) due to the excess backpressure that is created.
As Dano noted this sort of failure is rare in Colts. Pat Rogers told us in 2005 that he has never seen a Colt fail at the cam pin hole, only pictures.
I completely agree that running a can accelorates parts wear. I think I'm gonna do what Markm suggested and put it back in the rifle and run it until failure. Take it out every few hundred rounds (if it last that long) and measure the progress of the crack. I'd like to get some good magnified photos of the crack as it is now before I start though. I will report back.
I also noticed a fair amount of throat wear using a small borescope at work. Once it breaks, I'm gonna pull the barrel and cut it in half length wise to get some good pictures of the throat and gas port wear patterns. It needs some 11.5" BCM hammerforged goodness anyways.
Aristogeiton
09-01-10, 09:01
Thanks Hootiewho for running the bolt till it fails. I'm looking forward to the report.
Man! Don't cut that barrel if it still headspaces good!! :eek:
I have a Colt 11.5 commando with nasty throat erosion. But it still head spaces good and shoots very good groups.
I mainly want to do it for another reason. I have been doing some testing on some AR rifles comparing them to the Scar using some specialized test equipment. This equipment allows me to look at every event of the firing cycle, comparing the two systems, and comparing different ARs to each other. I have a 6933 that has hardly been fired and I want to compare the firing cycle of one in new condition to another well used. I want to be able to get a good idea of the gas port errosion and throat wear.
I have a very interesting write up comparing the Scar to the AR that I will be posting in coming days. One thing I found was going from H to H2 to H3 buffer weights, on average drop the rate of fire of the 6933 about 60 rounds per weight change. I have tons of other info coming shortly.
So that is my logic for wanting to cut the barrel.
You can measure the lead in the throat without destroying it. But get pics of everything in any case. :cool:
I think the Short Barrel, Carbine Length Gas System and Shooting Suppressed probably were the reason for a crack like that showing up.
Those three together, that's just about as hard as you can run an AR... unless you add F/A into the mix.
Good to know that a Mil-Spec carrier will run to around the 7,500 round mark that they're rated for even in a SBR.
I wonder what kind of condition the carrier key is in, mind taking photos of the carrier too, I bet it has some unique wear in other places...
Were you running 5.56 mostly or a mix of .223 and 5.56?
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