Unsupported Measuring

[gbw]

I pondered on this a bit the last couple of days, and couldn't figure an accurate way to measure.

For any given 1911, I'd like to be able to find the unsupported case length at the moment of firing -the distance from the most forward point of the barrel ramp to the breech face, at the moment after firing (barrel pressed forward, case and slide pressing rearward against barrel lugs).

Does anyone know of an idea to measure this distance?

Let's assume the barrel ramp has a fairly sharp shoulder where it breaks into the chamber. If it's rounded or polished, it's an even tougher problem.

I have Cerrosafe, maybe that's the best way?

[1911Tuner]

More to it than just the chamber ramp angle and the corner. You also have to determine how the headspace affects it.

To better understand how it works requires that you understand exactly what happens when the gun fires. The first thing that you'll have to understand is that the case does not pressure-weld itself to the chamber.
It backs out as far as the breechface will permit.

On firing...the forces generated by the burning powder and expanding gasses drive the bullet in one direction and the slide in the other. This occurs at the same instant. If there's any slack...endshake...between the slide and barrel lugs...the slide and barrel separate by that amount, and the breech essentially opens by that amount...and the case backs out of the chamber by that amount. So...If you have .015-inch of unsupported case static in the chamber, and an additional .015 inch of endshake between barrel and slide lugs...you have a total of .030 inch of unsupported case head area when the gun fires.

To complicate things further...you can have zero endshake due to a close barrel hood to breechface slide fit...and still have excessive unsupported case head area because of the depth of the chamber shoulder relative to the barrel face. You can have a good amount of endshake, and have good head support, for the same reasons.

You can have the reverse of the above because of the way the lugs fit and engage horizontally...

[John]

Can I have two Tylenols please!!!!!

[gbw]

I know, that's what I meant by just after ignition - max unsupported headspace should exist at that point. Barrel being yanked fully forward by the bullet, case back against breech, breech being force rearward by the case, and any upper lug slop removed toward the unsupported direction.

The question is, can this be measured? If so, how?

It cannot be done prefectly, since it would not be possible to apply the same levels of forces. But I think it would be very close if there is a way to take the measurement with the barrel pressed forward fairly hard in the slide - this should give a close approxmimation. Then measure from the breech face to the deepest entry of the barrel ramp (if even that can be accurately determined).

This should be a worst case measurement, which is what I'm looking for.

I keep thinking there must be a way, and it would be a very useful measurement, but I haven't come up with it yet.

[1911Tuner]

The question is, can this be measured? If so, how?

Well...It's not critical to take an exact measurement, but you can make a close determination on how safe or unsafe it is. You can find the specs on the ramp depth
by setting up a right triangle that subtends the distance between the bottom of the barrel to the top of the ramp, and using the Pythagorean Theorum to find the unknown
side of the triangle. If the angle is the C/Hypoteneuse and the distance from the bottom of the barrel to the top of the ramp is the A/Altitude...figure for the unknown B/Base with C(squared) minus A(squared) = B(Squared)

Use a fired, sized case in the chamber...barrrel engaged into the slide. Use feeler gauges between the rim and the breechface to get a close idea of how much the case will back out of the chamber on firing. Then, press the case into the chamber, and back it out by approximately the same amount. Eyeball the unsupported area and compare it to the case webbing. If any of the case that contains powder is unsupported...you've got a problem. How much of a problem depends on how much is unsupported...how far into the case the web extends...how thich the case walls are, etc. As a rule, ANY unsupported area that contains powder is a potential blowout, but
again...it depends on other factors. It may produce some degree of bulging, and it may not. I've seen two different brands of brass come from the same gun in which one looked pregnant, and the other showed no outward signs of a problem...and only by careful measuring could a bulge be detected.

[gbw]

Thanks, Tuner, that's very smart - no surprise there.

I'll give this a try, and also I'll section a case or 2 lenthwise and take a look that way. You also have given good clews toward a way to get the actual measurement, if I can get the angle measured accurately - determining pretty close the depth of ramp entry into the chamber seems to be the toughest part of this.

All of this is just curiosity, I don't have any bulging cases, nor do I suspect a problem. I'm just interested to see the relative difference in worst case unsupported webbing between guns, and compared with a new untouched barrel with no lug slack (hood really doesn't matter here), or as close as I can find it, i.e., the 'best' worst case.

[gbw]

My day for idle brain strain, I guess.

I also meant to ask, it seems that the AREA of the unsupported web should make a difference as well as the depth:

Suppose that gun A has a barrel ramp that is very narrow to some specified deth;
Suppose that gun B has a barrel ramp exactly as deep and far wider - like a wadcutter ramp.

It seems to me that past a certain chamber depth, B is more likely to bulge / blow than A. Or that B is more likely to bulge at any depth, assuming there is ANY unsupported brass. True?

I'd like to understand the relationship. I think it must have something to do with area and the properties of brass and how it flows under pressure.

Something for idle pondering, if anyone's inclined.

[TriumphGT6]

Yet another advantage to low pressure cartridges like the .45 ACP :)

[gbw]

Everyone, I know you already do, but just in case - please understand ... this is an acedemic / mechanical curiosity exercise. It is NOT to find out the absolute limit to how deep a chamber ramp might be cut and still not blow your hand off.

Tr.GT6 is correct, the .45 is low pressure and therefore is somewhat more forgiving. But it's still a whale of a lot of pressure, it has its' limits, and once exceeded there ain't no turning back.

It would be foolish indeed to try to approach these limits - there are just too many unknowns (weakened brass, for example) that can easily turn you into a blind Captain Hook.

[niemi24s]

Hi GBW: Very interesting challenge in the art of physical metrology. Your idea of making a Cerro-Safe casting sounds like a good one. But, never having used the stuff, I pondered other methods. As you stated, the measurement problem gets real complicated if there's any appreciable rounding at the junction of the chamber wall and clearance cut (ramp). The following method let me judge and measure the amount of rounding in addition to measuring the unsupported case length.

MATERIALS: 6" slide caliper with depth rod; sight blackening fluid or spray; good 4-6" long straightedge, about 3/8" wide

A. Remove barrel, clean & degrease chamber and clearance cut. Clean breech face.
B. Blacken chamber & clearance cut
C. Put straightedge in chamber, contact close to stop shoulder, lower to chamber/cut edge & scrape off sight black keeping straightedge in contact all along chamber bottom. Remove straightedge. Scraped area shown below:

http://i129.photobucket.com/albums/p232/niemi24s/P3210002c.jpg

D. Measure diagonally down bore from top center of muzzle to aft-most extent of area scraped free of sight black. This dimension is Ls. (Lots of technique in getting this measurement: hold barrel in vise; don't flex caliper depth rod; use magnification if necessary; start with caliper set short and gradually increase its setting) Note whether depth rod is in groove or on land at muzzle.
E. Re-apply sight black to chamber/clearance cut transition. Use straightedge held at angle halfway (by eye) between clearance cut top and chamber to scrape it off. This scraped area shown below:

http://i129.photobucket.com/albums/p232/niemi24s/P3210001c.jpg

F. Measure as in D, above, but to the forward edge of the scraped off area. This measurement may be more than or equal to the measurement in D, but it must not be less. If it's less, repeat D, E & F.
G. The difference between the measurements in D & F indicates the degree to which the chamber/clearance cut transition has been rounded off. (In theory a sharp transition would measure the same)
G. Measure the vertical chamber diameter just forward of the clearance cut/chamber transition. This is dimension Dc.
H. Install the barrel and bushing in the slide, wedging the barrel fully forward, locked to the slide.
I. Measure the distance from the muzzle to the breech face along the bottom of the bore. This is dimension Lb.
J. Calculate the unsupported case length, Lu, as shown below:

http://i129.photobucket.com/albums/p232/niemi24s/P3210004c.jpg

My estimate of the uncertainty in Lu is about 0.010 inch for a single measurement. This could be reduced by repeating Steps B, C & D many times, discarding the high & low measurements and using the average of the remaining measurements of Ls in the calculations. This 0.010 amounts to about 4% of the value of Lu obtained for my target gun.

There's probably easier & more accurate ways of finding the unsupported case length, but this was my stab at it. Thanks for the challenge - kinda fun. Now all we need is somebody to tell us what it is in a mid-spec USGI M1911A1, huh? Regards.

[John]

Oh boy, my head spins! That reminded me of my post-graduate year, when one of our professors was deriving for us the Einstein E=mc2 equation.

Good work Niemi.

[gbw]

That is very Cool, N. I'm going to give it a shot this weekend. I don't know the mid-spec, but I do have a couple of unmodified Colt S70 1911s, so I guess I'll have to use them as a baseline. Or, I have a couple of Kart barreled guns where the ramp has not been touched. Maybe the average of these will be useful as well.

I'm going to be VERY interested to see if any of them pass 1911Tuners test of having no overlap between the power bearing part of the case and the unsupported section.

Based on using sectioned cases to observe the relationship between the case webbing and the chamber, I'm not at all sure any of them will pass.

I sectioned the cases by taking a pie-shaped cut, about 1/4 of the case circumference, in the longitudinal axes. Have to be careful to keep the cuts perpendicular to the circumference, or the thickness of the web/wall will be misleadingly high.

Putting the sectioned cases into the chamber is interesting. I'm not too sure what it proves, but it's interesting.

[niemi24s]

The only specifications I've ever seen for the 45ACP case (other than in reloading manuals) are by SAAMI, but even these specify only the external dimensions. I've got a sneaky feeling, as 1911Tuner hinted, that other stuff like case wall thicknesses, where the increase in case wall thickness begins near the base, etc, is "whatever the manufacturer decides".

Another splendid question: How thick does the case wall have to be before it can withstand the rigors of being unsupported? Or: How far away from the bore/chamber axis can the chamber/clearance cut be before it no longer "supports" the case?

BTW, the depth rod on the calipers used to make the measurements in the chamber should be round and have a nice flat end with a well-defined (sharp) edge. A depth micrometer could also be used if your slide calipers have one of those flat depth rods with an offset point.

[gbw]

The only specifications I've ever seen for the 45ACP case (other than in reloading manuals) are by SAAMI, but even these specify only the external dimensions. I've got a sneaky feeling, as 1911Tuner hinted, that other stuff like case wall thicknesses, where the increase in case wall thickness begins near the base, etc, is "whatever the manufacturer decides".

Another splendid question: How thick does the case wall have to be before it can withstand the rigors of being unsupported? Or: How far away from the bore/chamber axis can the chamber/clearance cut be before it no longer "supports" the case?

BTW, the depth rod on the calipers used to make the measurements in the chamber should be round and have a nice flat end with a well-defined (sharp) edge. A depth micrometer could also be used if your slide calipers have one of those flat depth rods with an offset point.

That really is one of the ultimate questions...but as always, probably the answer is 'it depends'. On the total area unsupported (it seems to me this MUST matter, but I'm not positive) v. the thickness, i.e. the 'ratio', which would be nearly impossible to calculate since it varies. Also what condition it's in - work hardened by reloading?, powder speed, and so on. Still be a good thing to know for a 'standard' unfired cartridge. Maybe we can get one of the grad students in materials science or engineering mechanics to do a study!

[niemi24s]

Put a quarter under my pillow last night instead of a tooth. My fairy godmother came while I slept, took the quarter, and left this note:

"For a mid-spec USGI M1911A1, the unsupported case length (horizontal distance from the barrel's clearance cut/chamber transition to the breechface - with barrel fully forward, locked to slide) rounded to the nearest 1/1000 inch is:

A. 0.246 inch for one built to 1944-vintage Springfield Armory specs
B. 0.244 inch for one built to 1973-vintage Rock Island Armory specs"

[pa_guns]

Hi

I suspect that you can use a hydraulic ram and a threaded barrel to get some pretty good data on this. Probably accurate to 0.0005 or so.

If you start cutting up brass I'm betting you find a *lot* of variation between manufacturers and age of the brass. That's going to be way beyond a few thousandths.

The next problem you get into is the yield characteristics of the brass. it eventually splits up at the mouth with use. That's a good indication that it's changing properties as its being reloaded.

One more gotcha - not all cases are made from brass ....

Bob

[niemi24s]

Another scheme: make some sort of universal receiver capable of using a standard M1911 barrel and with adjustable headspace. With it you could fire ammo and test the effects of increasing the headspace and the effects of whittling away the clearance cut - or both! Does that sound like fun, or what?

Anybody know what kind of gizmo is used to proof-test 1911 barrels? Know where I could get one?? [Dang, this is getting good!]

[pa_guns]

Anybody know what kind of gizmo is used to proof-test 1911 barrels? Know where I could get one?? [Dang, this is getting good!]

Hi

The "traditional" way to do it is to use an overpressure load. Anybody who reloads can make them.

For stuff like rifle barrels or larger stuff hydraulic setups are sometimes used. They are custom built by the manufacturers.

Bob

[niemi24s]

"What sort of fixture (gizmo) holds the 1911 barrel when the proof load is fired?" is how I should have phrased my question.

Started thinking about this for testing ammunition and barrel (not assembled pistol) accuracy instead of a Ransom (or Caldwell) rest. The idea was to eliminate as many of the 1911-related variables as possible. Did a lot of noodling on paper, but never came up with anything that seemed worthwhile. Even pondered modifying the receiver of some bolt action rifle to hold a 1911 barrel, but gave up on this too.

Any leads, hints or suggestions will be appreciated.

[gbw]

Put a quarter under my pillow last night instead of a tooth. My fairy godmother came while I slept, took the quarter, and left this note:

"For a mid-spec USGI M1911A1, the unsupported case length (horizontal distance from the barrel's clearance cut/chamber transition to the breechface - with barrel fully forward, locked to slide) rounded to the nearest 1/1000 inch is:

A. 0.246 inch for one built to 1944-vintage Springfield Armory specs
B. 0.244 inch for one built to 1973-vintage Rock Island Armory specs"

That is one smart fairy godmother. Now I can start looking at various case webs, and pistols, to see where both fall relative to this spec. I get the feeling some of the guys who grind into the chamber to move the barrel ramp forward are gonna be way past this, which would not be too smart.

[pa_guns]

"What sort of fixture (gizmo) holds the 1911 barrel when the proof load is fired?"



Hi

Depends on what country the manufacturer is located in. In some countries the tradition is that if you put it together, you go out and fire the proof load with it in your hands. Same idea as jumping on a parachute you just packed ...

In other parts of the world you clamp in in something simple and pull the trigger with a wire.

In either case the proof load is not used for accuracy testing, it's just to demonstrate that the pistol is safe.

Bob

[niemi24s]

Have been considering gbw's idea of making a Cerro-Safe casting of the chamber, clearance cut & breech face to have something to directly measure the unsupported case length from - as opposed to the roundabout method in my earliest post.

I've never used my Cerro-Safe, however, and am a bit concerned about getting the barrel stuck in the slide with it. Is this a valid concern?

[niemi24s]

Almost hate to revive this thread, but the two dimensions given in Post #15 for unsupported case length did not include the specified break in the corner where the barrel ramp meets the chamber wall. Not that anybody really cares, but I do feel obligated to tidy up.

A. 0.246 inch was given for a mid-spec gun built according to 1944-vintage Springfield Armory blueprints. This blueprint calls for the corner to be broken, but gives no dimension for the break. As such, the unsupported case length here will be somewhat more than 0.246 inch - depending on the break.

B. 0.244 inch was given for a mid-spec gun built according to 1973-vintage Rock Island Arsenal blueprints. This blueprint calls for the corner to be broken with a 0.02 inch radius AND gives an alternate ramp geometry (not previously included).

(1) For the arched (standard?) ramp the corner break radius increases the unsupported case length from 0.244 to 0.252 inch.

(2) For the straight (alternate) ramp the corner break radius results in an unsupported case length of 0.249 inch.

Note that these dimensions are the distance from the breech face at which the chamber wall/ramp radius begins, effectively enlarging the chamber diameter. Just how much enlargement is needed for the case to be considered unsupported is another matter & beyond my reckoning. Regards.

[pa_guns]

Hi

Since the brass is smaller in diameter than the chamber technically it's "unsupported" over most of it's length.

You have two basic things to worry about with the brass. The first is how far it can deform elastically. The second thing is how many psi it takes to blow a hole in it at what cross section.

In order to fully answer the question we're going to have to cross section a lot of brass.

Bob

[gbw]

Thanks, Niemi. The unsupported length is more than I would have thought - I did section a couple of cases, and measuring .250" from the end puts the unsupported section well into the tapered section of the case wall, not under the web, which surprised me a bit. Placing the sectioned cases in a new, unmodified Kart NM barrel confirmed this, at least part of the unsupported area is well into the wall, past the web - and these were not full length cases. I think the nominal case length is .898, and these were .890 (longest I could find). But actually, on firing, case length doesn't matter too much - the unsupported area will be determined by 'dynamic' headspace and the dimensions of the barrel at the point where the chamber is shortest.

With even minimal headspace, this condition becomes even more severe on firing. So I think 1911Tuner's (and others) advice NOT to deepen the chamber for any reason, even to alleviate a 3-point, is well taken.

Pa, I don't think I'd agree that the case is unsupported over its' length. To the extent the cartridge case is surrounded by a steel chamber, within the limits of brass' elastic deformation, I'd consider it supported. By your logic, technically, there is no such thing as a supported case. I suppose that's literally true, but by the practical definitions we use the case is supported. We've all seen split cases with no adverse effects (except accuracy), rifle and pistol. So, even if the case ruptures, provided there is steel chamber wall adjacent over the entire length of the break, we're ok. If it blows out in the un-supported area, we'se got troubles.

The answers to you 2 basic considerations are the key, but there are so many variables that it may be impossible to answer them for a specific round when it's fired. e.g., brass composition, how work hardened it is, headspace, exact structure of the case (how thick the taper is, where it starts, and so on), etc., etc., etc. So maybe the best we can do is stay well within the empirically established safety margins, plus pay attention to what our guns are telling us.

Still, it's been interesting looking at the sectioned cases and then comparing them with Niemi's calculations.

He's been right on the money so far - I've gotta admit, he's good with that slide-rule.

(BTW, I'm compiling a sort of Table of all of the critical dimensions that affect builders that I've run across in this forum - mostly from 1911Tuner and Niemi.)

[pa_guns]

Hi

I agree that a split at the mouth of the brass is less dramatic than a split down at the base. I would still argue thought that is splits so some degree because it's unsupported by the steel to enough of a degree that it can stretch past it's elastic limit.

Bob

[David Rose]

But "elastic limit" changes due to work hardening. Or perhaps the brass was too hard in that batch? Nominal brass in a nominal chamber "should" be able to stretch and not crack, shouldn't it? I'm sure there is some "give" or chamber expansion to help things along. But still, I think that "normally" it is accounted for.

Or maybe I should keep my nose out of your's and GBW's conversion... :butthead: :)

David

Hi

I agree that a split at the mouth of the brass is less dramatic than a split down at the base. I would still argue thought that is splits so some degree because it's unsupported by the steel to enough of a degree that it can stretch past it's elastic limit.

Bob

[pa_guns]

But "elastic limit" changes due to work hardening.



Hi

Exactly the point. The objective here is to come up with a number for the chamber dimensions that guarantees you don't exceed that limit at the web end of the case. Since it changes with time and with brass design it's not an easy thing.

Bob

[gbw]

Anyone's welcome to chime in...we'se just talkin'. To me, "unsupported" means, in practice, that if a case ruptures there will be nothing to safely contain the expanding gas. In a 1911, of course this is the area under the rear of the case where the barrel chamber is shortest to create the barrel ramp.

What really has surprised me is the amount of unsupported case in an in-spec pistol / cartridge combo, and that blow-outs are not more common. I've fired many tens of thousands rds. of .45, some of it pretty shaky stuff in the early days (haven't we all?). I've never seen a blown case, not even a badly bulged case. Many many splits, of course. Clearly, brass is stronger than I realized.

Pa-Guns is exactly right - the question is: can we find the combination of

brass thickness / brass chemistry / workhardening value / pressure, AND unsupported area v. all of these

that makes a particular pistol / cartridge dangerous?

Not possible, I'm thinking, without massive testing. That is beyond us.

We do know in-spec guns and ammo don't blow. Since we cannot determine in advance where things become dicey as specs are exceeded, the answer for me has to be to stay within spec.

That's not a very satisfactory answer, since I'm pretty sure there is a usable safety margin. But, lacking any reliable way to determine where safety ends, I'm stuck with it.

[Lubaloy]

Here is a pic of an odd one!
The bulge is right in the thickest part of the web. I suspect a reloading mistake.

http://img.photobucket.com/albums/v294/utahmom/Shane/P1060614.jpg

[gbw]

THAT was close, I've never seen one like it! Love to know how it was loaded.

[pa_guns]

Here is a pic of an odd one!
The bulge is right in the thickest part of the web. I suspect a reloading mistake.


Hi

I have a nice big bag here of 9mm's that have the same thing going on. None are as extreme as the one in the picture. I don't see this sort of thing as often in 45's. All of what I am looking at is random range brass so I have no idea at all how it was loaded or what it was shot out of.

Bob

[niemi24s]

Could this case bulge result from the cartridge being fired when the gun was out of battery, but not far enough out of battery to cause case rupture?

If it hasn't been deprimed, does the primer show evidence of excessive pressure?

[Ten_Ring]

I'm by far not an expert, but looking at the case mouth it looks as if it was over crimped or something along that line.