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Discussion Starter #1
I took my 460 Rowland to the range today and on the 3rd round I had a case head seperation. When I examined the other 2 cases, there was a ring which I hadn't previously noticed. This was with factory "self-defense" (light) loads. Obviously the slide is unlocking with high pressure still in the chamber. This is scary stuff and I need to know what's going on. This is with the boss filed down as discussed in the other post. Somebody help me out here.





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I'll assume that you've got the correct recoil spring.

But I'm wondering about the lockup. I'm assuming that .460 barrel is new with new link. I'm not sure how to measure this, but perhaps an old technique of using prussian blue? Coat the lugs on the barrel, reasseble, cycle once, dissassemble and measure area wiped free of blue.

The barrel slide obviously needs to stay together for awhile. When they part company too early, the slide will be extracting a case under a lot of pressure. You might want to check those cases for gouges on the front of the rim - indicating a serious tug-of-war between the extractor and a case under pressure.

Very good photography.

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Yes, check the extractor groove. If you are extracting early enough to yank the case apart, there will be one heck of a serious gouge in the rim.

Early unlocking will show in the primer, too. If the barrel isn't fitting properly, you should see some peening of the locking lugs. Check locking lug engagement (modeling clay in the barrel lugs to record engagement depth, and a deprimed case and no firing pin to check f-p alignment).

If the rim and primer looks OK, and the barrel fits properly, I'd call the ammo maker. Soft cases could do this, or other brass problems.
 

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Good point Patrick about the primer/brass. If the barrel/slide is moving apart during initial firing, i.e. the case head is not squarely against the breachface, then the primer will wipe the firing pin channel. This tiny movement should still show on the extractor groove. I'm also thinking that this rearward movement basically allows the case to separate from the rearward thrust.

Now I believe that Starline's .460 Rowland brass is a lengthened .45Super. But I believe this brass is supposed to under go a heat treatment. Standard 45acp pressures probably won't identify a brass process problem, but .460 Roland could. Starline makes a 45acp +P case which has a thicker web but doesn't undergo the heat treating.

I wish you well in this endevor and I look forward to reading the analysis/solution.

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Discussion Starter #5
Well it turns out the barrel has 0.019" headspace. I'm not sure what the max is for the 460, but that seems excessive for a 40kpsi cartridge. I wish Clark would respond to email, but I have sent 2 emails about different subjects in the past and neither got a reply. (He should remove the email link from his webpage if he isn't going to answer it.) I will call him and tell him what is happening and that I suspect excessive headspace.

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Well I talked to Clark Custom this morning and they said they had some kits with short leade that can cause excessive pressure. They said to send the barrel back and they will fix it. I will post the results here.

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AKM: What is a leade? I'm currious as to how they modify the barrel to achieve correct headspace.

I'm interested in the resolution of this problem as I just might learn something


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[This message has been edited by dla (edited 10-09-2001).]
 

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Discussion Starter #8
Leade (pronounced leed) is the portion of the barrel past the chamber that is not rifled. Also called freebore. It gives the bullet a chance to accelerate a bit before it encounters the resistance of the rifling. Of course you can't fix a barrel that had excessive headspace. If the chamber has to little headspace, it can be cut a little deeper. I intent to ask Clark to check both the leade and headspace of the barrel.

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AKM: Forgive my ignorance, but I thought a .45acp headspaced off the extractor. In other words, I thought the headspace is really determined by the grip the extractor has on the case as it holds it against the breachface.

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Discussion Starter #10
45ACP (and other straight-walled rimless cartridges) headspace on the case mouth. Bottleneck rimless cartridges headspace on the shoulder. Rimmed cartridges headspace on the rim.

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The .45ACP headspaces on the case mouth, except when excessive headspace calls on the extractor to keep things under control. In regular trim, (17,000psi max) excess headspace is no big deal.

Most chambers are long, most brass is short, the average ends up with most guns having .015" to .020" excess. I've seen loose military 1911's with .030", and still working fine. In standard loads, no problem. In the 40K loads of the 460, problem.

The leade is the short rifling-less section and the taper of the beginning of the rifling. If it is short, or the taper to the rifling is steep, pressures will spike. Again, in standard .45ACP load at 17K, no problem. In the 40k 460 Rowland, big problem.

All calibers have the dimensions specified, and manufacturers strive to stay within the specs. Some specs vary, as in the differences between civilian .223 leade and military 5.56. The military leade is longer, more tapered and keeps the pressure spike down. If you shoot normal-pressure military ammo in a civilian .223 chamber, you can have over-pressure problems, even though the ammo is OK in military chambers.

Tell us what happens, and how the new barrel looks.
 

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Discussion Starter #12
OK, got my new barrel from Clark's Custom. The headspace is around .003" and it has a nice long leade. I'm taking it to the range tomorrow AM. If you don't hear from me, check the hospitals.


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Discussion Starter #13
Back from the range. The pistol is much better behaved with the new barrel. The blast and recoil are noticeably less and there is no sign of case stretching. Still have the flattened primers and breach-marks on the brass, but this is a high-pressure cartridge.

Also, this barrel seems more accurate than the other one. I shot a group around 4" at 50ft offhand. The indoor range doesn't have any accomodation for shooting from a rest, so I'll have to wait to do real accuracy evaluation.

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[This message has been edited by AKM (edited 10-21-2001).]
 
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