Headspace problem?

[Essanddubyafan]

I have a Navy Arms Rolling Block pistol in .357 Mag with a problem. When I fire it, the case splits crosswise in the middle, as if it were cut with a tubing cutter. I does the same with .38 sp. target loads. I intend to remove the barrel, and using a lathe, relieve the shoulder by one thread width, shorten the breech by the same amount and cut a new cartridge rim recess. That should get it shooting .38s. If so, I can then purchase a .357 reamer and re chamber. I measured the current rim recess in the breech and it measures .066 in. One of my reloading manuals shows a rim thickness of .058 in. but I don't know if that is a cartridge spec. or a chamber spec. My question is how deep should I cut the rim recess?

 

[Jim Watson]

SAAMI headspace is .060" to .074" whether a recess or just the space between revolver cylinder and breechface. Figured for a maximum rim thickness of .059".

I am not a gunsmith, but I don't know if that will fix your problem. What is the actual headspace? Is there a gap between the chambered round and the breechblock and is there any play between the breechblock and hammer?

 

[Oldnamvet]

Measuring headspace

This is a method for measuring headspace in TC pistols but you should be able to use the same principles for yours. Sorry if it is a long post since I just copied the info from a newsletter put out by Mike Bellm, a gunsmith who specializes in TC single shot pistols.
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(If you are a bit foggy about it and still don't have a handle on headspace clearly in mind, you might want to save this, especially the latter part after my brief rant.)



Excess headspace causes misfires, eratic ignition, reduced accuracy, and shortened case life. This is the problem, a chronic problem.



If I weren't getting so many emails about excess headspace and the way TC deals with it, I would not be sending this particular Newsletter. So even though it seems I beat the headspace issue to death, it does not change the fact that 1) excess headspace is a chronic problem and 2) TC does not have an adequate means of helping you with the problem.



I have mentioned before that IF you have a headspace problem, and IF you take the measurements and lay the hard numbers at TC's feet, they cannot brush it off. The one and only good thing I can say about SAAMI is that they do lay the legal groundwork and set the legal atmosphere for you to hold TC's feet to the fire if you do in fact have an excess headspace situation.



The industry standard is a maximum headspace of .006." Anything over that is "excess." I believe what is referred to as a "Field" guage is supposed to represent .010" headspace and signifies a gun/barrel is due for repair or replacement if headspace exceeds that. "Field" applies in military applications, but honestly, I do not recall how it applies to silly villian (civilian) use.



Fact of the matter is that stretching cases .006" or more or having the case rattling fore & aft in the chamber .006" or more is 1) problematic and 2) NOT acceptable, no matter what some tech that answered a help wanted ad in a Rochester, NH paper says. And even though a fired case may "look normal" to HIM on the outside, it does not mean he is taking into account how much the case has stretched inside, nor how much the case life is diminished by stretching on the first firing.



You deserve better, and I don't give a rip what TC says or does or what their "policy" is. You deserve better and should NOT be forced to accept less.



Lately I have been getting reports of folks coming up with some misguided, twisted distortions of how to measure headspace. Giving it attention rather than ignoring it is a plus, and if they stumble into something that works, right or wrong, so long as they are happy with the way their TC shoots, I guess it is some improvement. These are fantastic guns, and we want to see only "happy campers." That is what counts in the end, but, please, why reinvent the wheel? I'm giving you what you need. Don't fold, bend, spindle, and mutilate it. Use it like it is. It works.



Point is, measuring headspace is crucial to making a chamber that fits the ammo.... my job. YOUR job is to make ammo that fits the chamber. Measurements are the same, folks. And even though you may not reload at all, it is JUST as important that you be able to measure headspace with factory ammo as it is for the guy who reloads to know what he has produced.



Headspace:

Steel Headspace Guages do not fire. Only ammo will fire. Guages do not equal ammo. Headspace with AMMO is all that matters, no matter what a guage indicates. Just because someone drops a headspace guage in a barrel and closes it does not mean you do not have excess headspace with the AMMO you are firing. Ammo manufacturers have a tolerance range also. Chamber on the deep side of tolerances and ammo on the short side of tolerances "stacks" tolerances and can end up as excess, inspite of whether or not a barrel closes on a particular steel headspace guage.



Take the headspace measurements with any given barrel, frame, and ammo combination. If the actual headspace is over .006" you need to either make corrections yourself or lay the hard numbers at TC's feet for them to correct the situation. Indications are that this is working. One customer with a headspace problem was asked by TC where he got his figures. When you KNOW what you are talking about, it is hard for them to blow smoke your way. You can call them on it. It works. So get a handle on headspace!



Short course:



Use a feeler guage set to measure the distance from the end of the barrel to the breechface.



Take the barrel off the frame.



Measure how far the head of a loaded round is above or below the end of the barrel.



If the case head is below the end of the barrel, ADD this distance to the barrel-to-frame gap measurement to arrive at total headspace.



If the case head protrudes any from the end of the barrel, SUBTRACT this distance from the barrel-to-frame gap.



If the case head is dead flush with the end of the barrel, then the barrel-to-frame distance equals the headspace.



BTW.... I still hear about guys looking at the barrel-to-frame gap and calling the gap they see "headsapce." Wrong. The barrel-to-frame gap ONLY equals the headspace WHEN the case head is dead even with the end of the barrel. And "dead even" is not by eyeball. It must be measured at least to the thousandth of an inch, and you cannot see that amount.



All the best,



Mike Bellm

 

[Chawbaccer]

I would have a chamber cast made to check for an over sized chamber before I did any metal work.

 

[Jim K]

Since every rolling block gun I know of has a perfectly flat breech face, headspace checking should be easy to do as described. Remember that for a rimmed cartridge, headspace is only the space required for the case rim. The dimensions of the chamber are irrelevant (within reason). Of course, any measurement by feeler gauge must be taken with the breech locked and the upper block held back as hard as possible against the lower block.

What appears to be happening is a classic case of excess headspace. The firing pin blow drives the case forward as far as the rim will let it go. Then the pressure from the burning powder presses the thin walls of the case outward against the chamber walls, and holds them there. Meantime, the case head, under pressure, tries to back up. Since the headspace allows it to do so, the pressure stretches the case to the point that the head separates from the rest of the case, almost as if (as you say) like it was cut with a tubing cutter.

The depth of the rim cutout is one factor, but it is only part of the picture. If either or both of the blocks is peened or battered, or the lower block pin is battered or the hole worn oblong, the necessary case support will not be there and the breechblock will not support the case properly.

I will also note that battering of the locking areas or the pin/frame can also be caused by hot loads. Since straight cases like the .357 and .38 Special will normally back up before separating, I wonder if your loads might not be a lot hotter than those recommended for revolvers in those calibers.

Jim

 

[Essanddubyafan]

Thanx for the numbers. I can measure the recess in the barrel as .066-.067. Inserting a feeler guage between the breech block and the barrel ( thanks for that idea, I didn't think of it ) adds another .006 to .007 in ( my feeler guages are incremented in even thousands, .006 goes, .008 doesn't ). That puts the headspace at about .074-.075, right on the margin of being too much.

Oldnamvets article says that regardless of actual measurement, a gap of over .006 between the breech block and the cartridge case is too much. The cases I am using ( a mix of Rem and Win ) measure .057-.058 rim thickness. This makes the headspace greatly out of tolerance.

Someone on another board suggested putting a shim behind the cartridge and test fireing to verify if that is the problem before doing any cutting. I will make a washer to allow for the fireing pin and try it. I will be back in a couple of weeks.