Cogitatin’ the .45-60
Drew, that’s a knotty problem.
Agreeing with your guesses as to what “might” or even most probably will happen, let me suggest one way to get a handle on the answer. It won’t be free, but it shouldn’t be too expensive:
I wonder - if you somehow set up to measure the exact timing of firing-pin-to-primer contact* to bullet exit - along with exit velocity - , you could maybe back-calculate the difference and get a good handle on the pressure curve.
As you said, you pretty much know what “ought” to be going on most of the time in the barrel, and you know the same energy is released for both (identical) loads. You could assign any measurement differences on a number of averaged shots of each type (long case vs short case) to the configuration difference and back-calculate a probable peak pressure number. It won’t be exact, but (as they say) you can have it cheap, or thorough and elegant, or soon. Pick any two.
* I’m thinking a thin sheet (a few ten-thousandths) of double-sided metalized mylar cut to size, wired with #30 or smaller wires, and set on the ass-end of the round in the breach so that the firing pin briefly shorts the two metalized sides when it punches through when hitting the primer. Yeah - the effective headspace of the receiver will be slightly altered, but not by much - and it’ll be the same for both round types. You might have to add a tiny groove in the bolt face to accommodate the detector wire(s).
Use the “shorting” signal as the start signal of your high-speed timer or chronograph and bullet exit as the stop. Hopefully pre-bullet-exit gas emission won’t cause false-tripping if you use optical triggering. But I leave that issue for the interested experimenter to solve.
Posted by ooGcM taobmaetS
06/18/2010 at 06:31 AM
Perhaps I’m just showing my ignorance as a very non-gunnie, but…
With identical rifling, identical leades, and identical bullets, the engraving force required in both firearms should be identical as well.
The bullets are not identical. Even given a perfectly equal fit, will engraving pressure be the same for a longer slug? I’d expect it to be higher.
GOF - When I read it I’d assumed both bullets were the same size - that described as applying to the shorter round (but assumed to apply to both).
Drew - another idea that may yield data useful for determining peak pressure:
instrument circumferential strain gauges ( a shitpot full of ‘em) all along the barrel length; as often and as close as practical. I’m talking every inch - or better! I bet with the right buffer amps you can tell when the round passes under each one and the transit time between each one! A bit of math may yield the delta-V peak, giving you what you want.
Posted by ooGcM taobmaetS
06/18/2010 at 09:12 AM
Grumpy - yes, the bullets are identical. I know I have simplified reality here, as no two things are ever fully identical, but I want to eliminate those side factors when determining the theoretical. The drawing is simple and the bullets shown are a bit short in the shanks.
McGoo - Inertial sensors could do the same job as your shorting plates. Or putting the shorting plates at the back end of the firing pin or even under the hammer if the gun has one. Technology exists to measure the distance from the muzzle to the nose of the bullet as it goes down the barrel. Pretty sure some sort of mini-Doppler radar is used, or a laser rangefinder and a small mirror set at 45° to the muzzle.
Strain gauge technology for guns has been around for nearly 100 years. There is a very accurate consumer version available from RSI called Pressure Trace. $500. $760 for a system that includes an integrated chronograph. I strongly recommend this rig for anyone seriously delving into wildcat development or considering a boutique ammo business. For the rest of us who reload, we’ll just follow the manuals, try and read the signs, and use the burn rate charts. Only 1 strain gauge is really needed. The RSI system can spot back-pressure waves, which the Oehler system does not. Give their webpage a read; this is a phenomena that has not been well dealt with up to now.
These gauges are driven by rather simple circuits. Easy enough to Google up, or just click here. Since the circuit is rather simple, it appears that RSI is really charging a lot for their device, but they have done all the calibration work for you, along with all the circuit development work. Even their gauges come marked with their individual scalar offsets.
For perfect calibration in any given firearm, once again you would have to go back to the method used by the professionals. Conformal transducers are calibrated hydraulically! I am not sure exactly how that is done, but I can see two methods right off the bat. One would have a cartridge shaped plug that fits the chamber and that expanded radially under pressure. The other approach would just plug both ends of the barrel and apply pressure (I’d put an unsized fired case in the chamber, so calibration would handle brass expansion at the same time). Full chamber pressure is not needed for calibration, nor is it within the strength of the muzzle ends of many barrels to contain. But strain gauges are linear in response, so all you really need is two pressure points to find the response line and from that you can derive a scalar offset. Call it 5Kpsi and 10Kpsi, both values that are well within any modern barrel’s ability at any point along it’s length.
I lost you at cogitatin but Ill bob my head and pretend to follow you just because Im a nice guy that way.
That’s Ok Rich, the internet is a big place, and sooner or later I’ll run across somebody who is into the same sort of weird and unusual subjects as I am, and who can answer my questions. In the mean time I’ll keep tossing these posts out, like breadcrumbs in front of ducks.
But I know BMEWS has several readers who appreciate the unusual, and who like the challenge of thinking about difficult things. Difficult things other than politics and what women actually want I mean.
Actually I understand some of the concepts you are looking at as they also relate to building cylinder pressure in a I.C.E. I used to spend hours with a dyno program trying to maximize the peak torque and HP for the motors I used to build for my local nascar track customers.
I do have a related Q though. Does this mean that those lil derringers with the big bullet ( .357) dont hit hard or is that 1.5 inch barrel enough to make it work effectivly?They certainly don’t hit anywhere near as hard as a hot loaded .357 fired from a closed breech 14” Contender. Snubbies and derringers are a big trade off; you get a whole lot of bang, but not a whole lot of gun, yet they fit in your pocket with ease. Better than no gun when the SHTF.
Unless you want to flash burn your target, and flash blind yourself, for very short barrels you need to select a much faster powder than would be appropriate for a longer barrel. You are usually better off using a lighter bullet as well, because it will give you higher velocities, and E=1/2MV<sup>2</sup>. One of those bullet companies now sells a special line of “SBR” bullets for Short Barreled Revolvers and has loading data or them in their manual.
Don’t overlook solid wadcutters for use in very short barreled pistols. They will cut a nice hole and will often penetrate better than most hollowpoint bullets at lower velocities. The West Coast Bullets company makes a copper plated 148gr .357 wadcutter. It has a soft, pure lead swaged core and a galvanic copper coating that is a bit thicker than the coating on the plated bullets that Ranier or Berry’s sells. These bullets can be fired at full-on .357 Magnum velocities and pressures, unlike the Raniers. And since galvanic copper has no structural strength at all (unlike the gilding metal cup that most jacketed bullets use), it acts on impact almost like a pure lead bullet. Soft and squishy, just what you want. Seems like the best of both worlds to me. I use them in my 3” .357 and seat the bullet so they are just shy of the cylinder front, creating my own “crimp groove” in their soft surface with a roll crimp die. A goodly charge of Unique gets these 148 grainers up over 1100fps, with all powder burnt. Accuracy is far more than “minute of felon” at 25 feet. Like I said, best of both worlds, especially since defensive use of hollowpoints is illegal in NJ.
The 1968 Gun Digest describes a “truly new and reliable method of taking absolute chamber pressure” (which is) now described for the first time. Here is a low cost system, available to the individual, that lets him determine the true breech pressure in his own rifle!” The authors were Michael York and Don Cantrell, both of whom had good reputations. And most used book stores have a plethora of used Gun Digests.
Some 40 years ago, I bought an internal ballistics calculator that did a decent job of answering questions like yours. Measure the water capacity of the case in grains, choose your particulars, and it did a decent job. I parked it with all my reloading stuff - eye trouble - and I cannot remember who made it. Fred Huntington of RCBS, I think. But I am probably wrong.
However, a looksee for “internal ballistics calculator” brings up many hits. If you are not looking for the ultimate in velocity, but the equivalent of a “safe in a trapdoor Springfield” factory 45/70 load, give it a try. And as usual, start 10% below the calculated powder charge, and mike the case just ahead of the base. That’s the area where the color of a fired case changes when you fire it.
A thousandth of an inch of expansion indicates you are above maximum pressure for that case in that rifle. Personally, I like to hold expansion to half a thousandth, something that is easy to “feel” even with a cheap students micrometer. And if you load your experimental load to no expansion at all, so much the better. The cases will last longer - and your wrist will feel better later.
Chronographs are not chicken feed, but loading for the same velocity as a factory load will get the job done as well. And probably just as safely as the ballistic calculator. Of course, the same velocity in a case with more space between the bullet base and the cartridge base will take slightly more powder to achieve the same result.
Regards, Stranger
Posted by Stranger
06/19/2010 at 06:25 PM
BINGO - the file retriever in the memory banks finally dug up the name. The Powley “load computer” is the internet version of the one I have.
Regards again,
Stranger
Posted by Stranger
06/19/2010 at 06:30 PM
What Drew said about the derringers, with the added observations that full-load .357 mags in a derringer do kick a bit. Interestingly, you can tell which barrel in an over/under derringer has fired without looking just by the difference in kick: the top barrel kicks harder (actually “twists” up harder in your hand) than the lower barrel. Difference in lever-arm lengths, you know…
Posted by ooGcM taobmaetS
06/19/2010 at 09:54 PM
Drew - you seem to be interested in reloading arcana on exotic rounds and receiver/chamber geometries. Have you ever been interested in load performance differences in “apparently” identical guns?
A shooting buddy and I - several times - bought identical weapons or accessories, such as 6mm TCU and 7mm TCU barrels for our TC Contenders receivers.
While both his and my barrel were ordered from the same seller (Bullberries) and received on the same day, they did NOT perform the same with identical ammo loads.
Another example: I own a S&W 686 .357 mag: so does he. The chronograph and target-accuracy data (taken at the same time, same loads, same instruments) were markedly different for the two apparently-identical pistols.
Same with CZ 52 Tokarevs (although mfg variations might account for these beasts).
This got us interested in tailoring brass, bullets, and loads for single, specific weapons - and doing chamber casts and micro-adjusting neck dia’s, shoulder locations and angles, OAL’s, etc.
A different set of critters for problems there…
Posted by ooGcM taobmaetS
06/19/2010 at 10:06 PM
McGoo - see ”Why ballisticians go gray” in any Speer load manual. Individual differences between firearms can change things a lot. This is one more reason for getting a pressure testing strain gauge rig for your own guns. When the differences can be quite large, who cares what the factory test barrel results were?
Stranger - thanks, but ... the accuracy of the Powley Computer falls off when case volume decreases and the maximum operating pressure decreases. This is a known shortcoming. The bit of software that I use is more sophisticated than Powley, but even QuickLoad has those same shortcomings.
McGoo - the link I just put in for Stranger is worth reading ... and it once again leads back to all that learning and reading in the front half of the A-Square manual. That book is THE fount of wisdom.
