doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com 2.75" shells in 2.5" chambers

Excellent, 2-p!! I think we can add more to this model and refine it. Instantanious acceleration will be identical to instantanious pressure. Looking at time-pressure or displacement-pressure curves may give us a clue as to felt recoil. It may happen that we have to average pressure over some short time intervals to account for human reaction. Acceleration from a peak 7000 psi load is less than from a 9000 psi load.

Jumping around a bit here, since I'm multitasking on a telecon meeting right now, but...on the topic of pressure working against the base of the payload vs. working against the base of the shell/breach: Is there any reason to believe there is any difference in how this accelerates the payload and how/if it is the element that accelerates the shell head/breach aft? I've always believed that the pressure was the force acting in equal but opposite directions, one against the payload and the other against the shell head. Where else would the opposing force act against?

Chuck, have you ever shot shells without shot in them? Is pressure less than when they do have their corresponding shot load? Does the gun recoil the same as when there is shot in the shell?

JC

Drew, as Joe Wood's example of a Parker hang tag illustrates, it was not unusual for American gunmakers to recommend the use of longer shells in shorter chambers. In fact, as reported in a two-part article in The American Rifleman in the 30's--written by a man named Curtis who had worked for most of the major gunmakers for about 40 years--some American gunmakers intentionally short-chambered their 2 3/4" marked guns, because they found that slightly longer paper shells in slightly shorter chambers improved patterns. That's because, without the advantage of a plastic wad, one of the problems with deformed shot came when the shot charge exited the shell and came in contact with the barrel. If the case mouth opened slightly into the forcing cone, that offered some protection during the period of initial contact. It's no longer a factor, with plastic wads, but Curtis experimented with a Marlin 90 (starting with a 2 1/2" chamber) and found that the pattern percentage dropped as the chamber was lengthened.

Bell's experiments indicated that lengthening just the forcing cone--not the chamber--often resulted in a pressure reduction of as much as a few hundred psi.

The Brits regularly recommend the use of 67/67.5MM hulls in 2 1/2" chambers. This usually works out fine because the shells are loaded to CIP pressure parameters. However, both Thomas and contemporary writer Charles Fergus reported increased recoil and blown ends on hulls in older guns (I think all pre-1900) with very short and sharply angled cones. Using true 2 1/2" shells solved the problem.

One can always err on the side of caution, but even the SAAMI guidance posted earlier won't keep you totally out of trouble, unless you know how to interpret foreign proofmarks. There are certainly instances in which factory 2 3/4" shells should not be fired in guns with factory 2 3/4" chambers. It's unfortunate that it's not that simple, but unless you're talking strictly US guns and US shells, it's not that simple.

I hesitate to enter into a discussion where physics and engineers tread, but, here I go anyway.

Newton's third law of motion: For every action, there is an equal and opposite reaction.

Years ago I had some contact with the 4.2" mortar and the 106mm recoiless rifle. The 106s were mounted on jeeps. When fired, the projectile left is such a hurry, a vacume was created, sometimes popping headlights and watch crystals. You could watch dust jump off the ground. Yet, the jeep barely moved. Why, as much energy went out the back of the rifle as out the front. The "four deuce", however, has a projectile you can follow with the naked eye for quite a while. I can also remember digging the base plate out of soft ground or firing a couple of rounds to "set" it in the rocky soil of Ft. Hood. That in my book is called recoil.

My point of all this is, the energy of a given projectile, depending on weight and velocity, is going to send exactly the same amount of energy the opposite direction, what I call recoil.

To me, increased pressure can only speed up the projectile, creating more energy, and the same amount of increase in recoil. I think we all pretty much agree, felt recoil can be mitigated by the weight of the gun, recoil pads, and anything else bleeding off the rearward thrust of energy. But, Newton's third law is still there.

As to Joe Wood's Parker tag, I believe all 2 3/4" shells of the period and later were paper and roll crimped. When opened they were 2 5/8".

Don,
I think the pressure/recoil relationship may be something akin to BMEP as it relates to horsepower. Yes, they are independent; you can have low horsepower and high BMEP or visa-versa, but you can't have horsepower totally without BMEP... Increase BMEP and keep all other parameters the same and horsepower increases.

"I hesitate to enter into a discussion where physics and engineers tread, but, here I go anyway."

Read my last Post in this Thread.-Dick

Chuck, I agree that pressure and recoil are tied at the hip, but pressure alone is not a cause or indicator/predictor of recoil. M&M gave a very good anecdotial explaination of recoil vs pressure; he seems perfectly capable of treading along with physics and engineers.

It could be that old, roll crimped paper shells marked 2 3/4" were less than that length when opened. However, looking in my 1940 Shooter's Bible, I find no shortage of 12ga factory loads listed as "2 5/8", which was the 12ga standard until the 1920's or so. Not sure why Parker would specify on a hang tag that 2 3/4" shells are good to go in 2 5/8" chambers when there was plenty of 2 5/8" ammo available.

After reading all the posts referring to the length topic I have concluded that it is perfectly safe to use 67mm and even 70mm shells, loaded to CIP standards in my Manufrance Ideal proved to 1100 Kilos with 65mm chambers.

I had the daring some time ago to post this in the 16ga.com forum and was chastised by a pigheaded, opinionated and extremely impolite member there. I never visited again. As someone said you should not argue with idiots.

Bottom line, as Larry says, it depends on the gun and the load, not necessarily the length of the shell. And of course, if you are not sure better to err on the safe side.

JC