doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com 2 3/4" in a 2 1/2" Chamber

Since we're making a book out of this thread, I thought I'd ad some pictures 'cause I hate a book without pitchers.

This little unit is how far consumer products have come. Oehler was the first to introduce a consumer level of pressure measurement device in his "Ballistics Lab" quite a while back as I recall. It was expensive (still is) and didn't keep up with the times IMO. The Pressure Trace system is based on laboratory grade strain gages and a common PC with software that takes the raw strain data and crunches it instantly to useable data by factoring the strain gage calibration offset, barrel measurements, etc.. The sample rate is very high (a lot faster than the Oehler) and can provide very good resolution of peak pressure, even in pistols and high intensity rifle cartridges.




Using one of the common singleshots (H&R and similar) provides for a constant diameter (not highly tapered like a sxs) at the chamber and reduces the chance of measurement error of the wall at the precise location of the strain gage. Strain gages rely on known E-modulus (Young's modulus/modulus of elasticity) which is very consistant for steels (another reason to instrument a steel barreled gun instead of a damascus gun (maybe))




This trace is of a WallyWorld Federal 16ga shell. Note the pressure. I don't believe every SAAMI compliant shell is SAAMI max pressure, quite the contrary.

Chuck,
Can you relate time to distance from the breech?

Nah, I don't have any data on that. Jim Ristow at mfr for the PT had some data on rifle time/bullet travel down the barrel.

Oehler's Model 43 Ballistic Lab used basically the same type of strain gages as does his professional Model 83/84. When I posted about this device in the past, some were very skeptical. That's o.k., they should be. They should learn how and why this type of device collects data before they invest and trust in it. I've been around instruments nearly identical to this in years past for industrial applications and I'm comfortable with them.

Now we're getting somewhere. Be very interested to see more results Chuck and I'd be happy to send samples of some of the 2" and 2-1/2" shells I have.

Chuck,

Can that pressure transducer be used as well on old doubles? Say ones with little or no taper over chambers. If so, then the door is open to getting direct data on actual old doubles and diverse ammo fired in them.

Niklas

Nik,
The less taper in the wall thickness the better. It will work on just about any gun you can get an accurate wall thickness measurement on. However, nearly all the sxs shotguns I've seen have lots of taper on the outside of the chamber. This increases the likelyhood of a measurement error by the fact of where it's placed on the chanber and the span of the gage itself over a taper.

I looked for quite a little while to find a short chambered singleshot. But, it's the simplest way to go. I bought a 12g and a 16g as well.

I've thought of ordering a barrel blank and making my own bench pressure gun for this purpose, but I'm not trying for absolutes here nor do I test much. I worked up some loads a couple years ago and haven't taken it out since. This is the kind of thing a group of guys or a club could really benefit from. I bought it specifically to work up some loads for my damascus guns. I found my 1 oz load with components and pressures I was satisfied with and stopped. I'm thinking about working up a 1 1/8 oz low press load someday, for those phez trips to SD.

Wife says I have a point, on top of my head. Thanks for the assessment of the crimp situation.

jack

I don't think anyone is saying that all American factory shells will reach SAAMI max service pressure. That's simply a ceiling. The problem is, American ammo makers load to achieve constant velocity, using bulk powders. Therefore, while one batch of American factory ammo may be well under SAAMI max service pressure, the next batch may not be, because they've changed powders.

A careful reloader has a much more exact idea about the pressure of the loads he's working up than you do if you run out and buy a box of factory shells. All you know with the factory ammo is that it's within SAAMI parameters.

Regarding the case length of 2 1/2" shells loaded to pressures deemed safe for guns proved to a service pressure of 3 tons per square inch, all meet the pressure safety requirements of proof.

However, choose the actual shell depending on the forcing cone dimensions of a given gun. Those with steep forcing cones will be better suited to 65mm cases than 67mm or 67.5mm cases, even though the pressures are the same.

This is because the opened case will partially narrow the cone urther and prouce greater pressure. It is a matter of common sense that if your cases are ragged at the ends after firing, they are too long and you should use shorter ones.

I'm glad we all now seem to be in agreement that it is not safe to put factory loaded 70mm shells in a 2 1/2" proofed and chambered gun.

If your re-loads are opening out in the gun you are using and not showing any fraying, then they must be opening fully in the chamber and are likely not a problem. If they are fraying, then you need to use shorter cases or lengthen your chamber - which, of course requires the gun to be re-proofed.

Or, you could lengthen the forcing cone.