I think I have a cracked chamber...

[geargnasher]

This started with a brand-new bore light and a really good cleaning job.

The gun is a late-'50s Marlin 336 micro-groove .30-30 that I've had since I was a kid. Today I took it to the range and ran several loads over the chronograph in preparation for testing the free sample of PB Blocker that the company gave me to evaluate. Shot about 50 rounds and went home for a really thorough cleaning prior to applying the PB Blocker.

I noticed what looked like a scratch in the top of the chamber from 1/4" forward of the rim recess all the way to the shoulder, where it seems to disappear. Not having a good bore scope it's hard for me to tell what it really is, but it does seem to "snag" the sharp tip of a dental pick ever-so-slightly when I drag it across the mark.

Has anyone experienced this before with Marlins? I've never seen any kind of crack in a chamber before, so I'm not sure what I'm looking at. All the brass looks normal, no marks or irregular shapes, but fortunately I haven't fired anything close to full-pressure stuff in it for quite some time and I've been able to neck-size only over and over again. I know for sure the gun won't get fired again until I get my gunsmith to check it out with a borescope, just wondering what other's experiences with chamber cracks have been.

Gear

[John Taylor]

More than likely a scratch, otherwise you should be able to see it on the outside of the barrel. The best way to check for a crack is with a magnaflux but the barrel would need to be off the action to do it. This uses an electro magnet and powdered iron. The iron will collect at the crack. Used mostly on auto cylinder heads to look for a crack. It will show a crack you can't see otherwise.

[shoot-n-lead]

More than likely a scratch, otherwise you should be able to see it on the outside of the barrel. The best way to check for a crack is with a magnaflux but the barrel would need to be off the action to do it. This uses an electro magnet and powdered iron. The iron will collect at the crack. Used mostly on auto cylinder heads to look for a crack. It will show a crack you can't see otherwise.

You can check it without removing the barrel. This link is for a spray die kit that will show flaws and it is reasonably priced.

http://www.mcmaster.com/#crack-detectors/=atkdbx

[btroj]

Load up some really heavy charges with heavy bullets and have a politician go fire em.
Either way it turns out, you are a winner.

[geargnasher]

More than likely a scratch, otherwise you should be able to see it on the outside of the barrel. The best way to check for a crack is with a magnaflux but the barrel would need to be off the action to do it. This uses an electro magnet and powdered iron. The iron will collect at the crack. Used mostly on auto cylinder heads to look for a crack. It will show a crack you can't see otherwise.

That's why I posted the question, I don't know how cracks go in rifle steel but I knew someone here might. As far as I can tell there are no visible outside cracks and the OD mic's normal, in the chamber area, no bulges.

I'm very familiar with Magnaflux, used to do it at the last shop I worked at for race engine blocks and what few iron cylinder heads we saw, I didn't know that method was employed by gunsmiths, but it makes sense. Good for finding really tiny cracks, this mark is easy to see but difficult to see see well, if it can't be identified with a borescope debarreling is probably the next step.

Sooo, if this turns into a rebarreling job, are you interested?

Gear

[geargnasher]

Load up some really heavy charges with heavy bullets and have a politician go fire em.
Either way it turns out, you are a winner.

Unfortunately, those kinds of politicians don't like guns, remember? [smilie=l:

Gear

[geargnasher]

That McMaster Carr dye kit looks interesting, but I'll see what my 'Smith says first.

Gear

[Wayne Smith]

I'm confused, but that's not uncommon. How does a die kit distinguish between a scratch and a crack?

[357 Voodoo]

Wayne the dye doesn't unless the user properly removes all excesses dye before the developer is applied.

gear I would offer to send some but all I have is the floro penn and not the dye and no developer

[hickstick_10]

I'm confused, but that's not uncommon. How does a die kit distinguish between a scratch and a crack?

The quantity of dye is absorbed into the crack, when you spray the developer (white powder contrast looking stuff) over the crack, the dye will bleed back out. As 357 voodoo says, you rinse most of the dye off before putting the contrast on, and it would wash out from a scratch but not a crack.

I would hesitate using it on a gun, only for the reason that stuff goes everywhere.

[John Taylor]

Sooo, if this turns into a rebarreling job, are you interested?

Gear[/QUOTE]

Sure, that's how I pay the bills, got to keep them coming in. My mortgage company says I have to work till I'm 90.

[Ford SD]

the easiest might be go to a gun smith for a bore scope

if you use the die penatrant method look for one that is water washable

you might get the best visibility / access if you remove the bolt/ leaver assembly

Clean Barrel with a oil remover spray (ie. brake cleaner) and let it sit in a warm place
(over a heat duct for a couple of hours

put the die in the chamber and as long as it stays wet there is no problem (re brush every 15min 3-4 times)
the small cracks take time to weap/ sink in
a big crack you will get results in 5 min small ones take longer

wash with tap water, dry lint free cloth /patch

the developer is going to be the problem as too much is no good and none will not show you results (too much will act like a cleaner and wash die away)

you could try a fine artist paint brush ( spray developer in to cap of can and before it drys) dap paint brush in and brush in chamber brushing 90 deg to possible crack
if you brush with crack you might get a false positive

and the die pen is also usable on all steels and alloys cast/iron, steel, stainless, alum

the ac yoke might not work for some thing that small

[geargnasher]

Wow, thanks for all the responses, tips, and offers! I got a call from my gunsmith this afternoon, he finally got a minute to look at it and he told me to get my stupid a$$ over there, pick up my perfectly sound rifle and get out of his sight (you have to know Jeff [smilie=1: ) He showed me that the scratches were from the flutes where the chambering reamer was pulled out when the chamber was originally cut. When I picked it up he showed me another one like mine on his "for sale" rack and it, too, had the same marks. I've examined this chamber many times before, even done sulfur castings and two impact slugs, and I never observed these marks before, nor on any other gun I own, and I would have put money on at least one of these marks being a crack. I guess the new bore light really works, I just didn't know what I was looking at.

So I spent the next 20 minutes answering automotive questions and giving some valuble free tips, and he didn't charge me anything for his expertise, either. Hard items aren't the only thing worthy of being trade goods.

Anyway, better safe than sorry, and thanks again for all the good input, there really is a wealth of knowledge on this forum.

Gear

[btroj]

Glad to hear it worked out for you.