Troubleshootin' Time!

[1911Tuner]

Knock at the door the other day. Guy from the range that I recognized, but didn't know very well with a NRM Colt GM in hand...and some stange lookin' brass. Brass had a distinct "U" shaped mark that looked like a cookie cutter hadn't quite made it through the dough. Slightly bulged.

Teardown revealed that the floor of the chamber had cracked through to the outside, about centered with the lower lug and about a third of the way to the chamber centerline and angled toward the front/top of the barrel. Curved part of the U is at the rear...Spread "legs" are at the front.

Barrel unlock/linkdown/drop timing is perfect. Link slidestop pin hole is very slightly egg-shaped...about .001 inch out of round. Link pin didn't fall out, but was much looser than I like. Pinhole not out of round. Locking lugs not damaged. Lower lug doesn't show signs of unusually hard contact with the vertical impact surface, and is impacting near the top third, near the lug/barrel junction.

What cracked the chamber?

Tic-Tock...Tic-Tock...

[Hunter]

O.K. I will give this a shot. Let me know how far off I am. Could it of been a defect in the barrel? How about overcharged and or incorrect handloads? I would imagine that there would be more signs of too high pressure other than a cracked chamber. I am going to guess defect in the barrel or ammunition problem.

[stans]

It would have to be waaaaaaay over powered ammo to crack the chamber and probably would have blown out the case head before rupturing the chamber. A metallurgical flaw is plausible. Could be the radius where the back of the lower lug meets the barrel is impacting the barrel bed.

[1911Tuner]

O.K. I will give this a shot. Let me know how far off I am. Could it of been a defect in the barrel? How about overcharged and or incorrect handloads? I would imagine that there would be more signs of too high pressure other than a cracked chamber. I am going to guess defect in the barrel or ammunition problem.

Nope...Overpressure usually results in a longitudinal crack. The only ammo that had been used in the gun was PMC ball and his reloads, which were below max about 5%. He didn't report any obvious overcharged rounds.

Flaw? Possible. Hard to say...but the crack would have come even if the chamber didn't have a defect.

[1911Tuner]

It would have to be waaaaaaay over powered ammo to crack the chamber and probably would have blown out the case head before rupturing the chamber. A metallurgical flaw is plausible. Could be the radius where the back of the lower lug meets the barrel is impacting the barrel bed.

Didn't I TELL ya'll that stans was good? Didn't I TELL ya?

The barrel was impacting the bed before it hit the vertical impact surface
and was being stopped by the link while it was still in transit...pulling...for lack of a better term....while the barrel was still moving backward.

Most of these failures separate the lug from the barrel, starting at the rear radius well before they can crack the chamber...but this one did. Indicates either a defect in the chamber or a helluva stout lug/barrel junction.

The slightly stretched link and the loose pinhole was the "clew" here.

For the record...in a correctly spec-ed gun, the barrel shouldn't sit completely down onto the bed, but should stand off of it about a thousandth of an inch when completely linked down. Most these days do sit on the bed, and it usually doesn't cause a problem...but this time it did. In this case, the rear edge of the barrel lug was just far enough forward of mid-spec to cause a tolerance stack and bring on the failure. If the bed had been just .002 inch lower, the chamber wouldn't have cracked.

Kudos stans! Looks like you're one of the ringers! ;)

[garrettwc]

Teacher, a follow up question to make sure I got it right.

If I got this right then the sharp (90degree bend) portion of the barrel bed was hitting the bottom of the barrel radius like a hammer and chisel?

Correct function would have been for the flat of the lugs to contact the flat of the bed first and then cam over the radius and come to rest on the barrel bed?

[1911Tuner]

Teacher, a follow up question to make sure I got it right.

If I got this right then the sharp (90degree bend) portion of the barrel bed was hitting the bottom of the barrel radius like a hammer and chisel?

Correct function would have been for the flat of the lugs to contact the flat of the bed first and then cam over the radius and come to rest on the barrel bed?


Not quite there yet, garrett.

Imagine yourself being a millimeter tall, and watching the sequence from inside the gun. The slide is moving and the barrel is unlocking and dropping at high speed. The bottom of the barrel strikes the frame bed first, stopping the barrel's downward motion while it's still moving backward. As long as it's still moving backward the link is still "pulling" it down. The link undergoes tensile stress, and so does the lower lug and barrel. If the barrel drop is correctly timed, the rear of the lower lug hits the frame's impact surface just a hair before the barrel comes to rest in the bed...and it can't hit the surface too low on the lug, or it will do pretty much the same thing...except the crack will usually start at the junction of the barrel and the lug instead of somewhere in the middle...and it usually doesn't result in the chamber breaking through...but can.

Read it about 5 times and it'll get clearer. ;)

[garrettwc]

Read it about 5 times and it'll get clearer.
OK, it took about that many times, and I had to drag out the 1911 animation on the homepage and watch for a few cycles. :D

Two things happening:

1. The barrel is coming down and is stopped short by the frame bed. Sort of like a hammer striking an anvil.

2.Everything is still moving rearward at this point so the link is pulling on the lower lugs. Sort of like yanking a chain tight when you're trying to pull a stump out of the ground.

This is caused, if I understood right, by the face of the lugs being too far towards the muzzle end and allowing too much rearward motion of the barrel?

Assuming I have it right, what would the fix be if caught before it broke? New barrel? Weld up the lugs and refit? or something else?

[1911Tuner]

Almost got it!

The problem can be caused by the vertical impact surface being too far rearward or the rear face of the lower lug too far forward...OR...the frame bridge being just a little too high. In the barrel described above, it was the third option.

On the first two, the lug usually cracks at the rear junction, and doesn't usually crack the chamber. On the third, the crack starts a little farther forward of the rear junction...and does usually crack the chamber. Stated more simply...with mislocation of the impact surface or lower lug, the barrel is stopped by the link and THEN drops the rest of the way to bed. In this case, the bed stopped the barrel's downward motion, and the link continued to pull
until the impact surface stopped the barrel's rearward motion.

The other clue that indicates that it wasn't a mislocation of vertical impact surface or barrel lug was the fact that the gun had seen several thousand rounds. If it had been either of those, it wouldn't have taken that long, and the link would have been stretched more. Rather than a few hard pulls on the lug, it was thousands of repeated, lighter pulls that cracked the chamber.

A quick-check for the slidestop pin and link in a bind with the barrel held hard to the rear confrimed it. The lug and impact surface were good.

[garrettwc]

OK, so instead of moving down and to the rear in an arc, the movement was more L-shaped, down and then rear?

The problem wasn't too much horizontal travel, but not enough vertical?

So how is the out of spec block in the frame cured? Do you go to work grinding on the frame to get it in spec? Or do you get a new oversized barrel and fit it to the frame?

[1911Tuner]

L-shaped...That pretty much describes it.

I replaced the barrel with one from an earlier 1991A1 Colt that required narrowing the rear hood, and actually wound up with a much better fit to the slide than the original. Unfortunately, the barrel still hit the bed too hard, and I had to do a no-no according to one noteworthy expert who shall remain unnamed due to hero-worship and the possibility of triggering a flame war...I ground on the frame bed with a Dremel and a large sanding drum that just happens to be a near-perfect match for the radius. Light passes with the drum angled so as not to let it grab the bed and zip across the rail.
I lowered the bed about .003 inch or a little less, which should put it into spec, or at least get it close enough that the replacement barrel won't suffer the same damage before it's shot out. A strip of carbon paper between the barrel and bed indicated that the barrel was barely making contact just as it hit the vertical impact surface. Another thou would have gotten it, but I didn't want to push my luck. A little polishing with 600-grit paper gave it another bit of clearance...very little...and the guy reported that he ran a thousand rounds through the gun this past weekend without any sign of hard contact in the bed. He's bringing the gun back to me toward the end of the week for a followup check. I should know more then. As a side-note, he said that the gun feeds smoother since the fix. Probably due to getting the barrel a little lower and letting the rounds enter the chamber at a shallower angle.

[garrettwc]

the guy reported that he ran a thousand rounds through the gun this past weekend without any sign of hard contact in the bed.
Is this a feel thing, or are there visible signs like peening marks in the metal?

Thanks for the schooling.

[1911Tuner]

Is this a feel thing, or are there visible signs like peening marks in the metal?

Thanks for the schooling.

Usually first shows up as bright marks in the bed and lightly peened or polished areas on the barrel. The bright spots can be small. It doesn't take a wide contact area to cause the problem. All it has to do is stop the barrel before it hits the vertical impact surface. Of course, the sooner it does hit the bed, the faster the damage will appear. Some guns are timed so close that the barrel doesn't fail until it's nearly worn out.

Also, if it hits on one side instead of centered, the lug will crack earlier due to the torque...and it'll usually crack on one side before the other.

[garrettwc]

That makes sense. Thanks again.