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

Hello David,

The owner of the blown barrel I posted was really lucky in that he "only" has an ugly scar on his arm to show for it.



You in the US are somewhat cursed with SAAMI standards, but modern loads do include CIP standard loaded shells that have reasonable pressures of 2 3/4" shells you can use in 2,5" chambers.

The fact that plastic shells are used should also be brought into the equation: the difference in thickness of a paper hull going into the forcing cone is not the same as a plastic one doing the same. JMTC

JC

Larry;
I think we are actually making progress. I think you have made an accurate assesment of the situation Fergus encountered. As I recall his acount the gun was a very early one & the cone was so short as to be almost a step.
It is also noted that 65mm & 2½" are not mathamaticaly identical. 2½ = 63.5mm. I have not had opportunity to measure a large number of British guns but from what I read I believe that many, if not most nominal 2½" British guns did have the 65mm chambers which are about 2 9/16". Now if this early chamber were a true 2½" (It is my understanding some were) & if that cone was on the order of just a 30°-45° chamfer at the end, then even a 67.5mm hull could have entered the bore proper, not just a portion of the cone. I highly suspect this was somewhat the condition Fergus encountered. This could indeed create a check to the early movement of the shot causing a rapid build of pressure. I can also see a sudden release taking place which would cause this high pressure to give an enhanced velocity to the charge & in fact the cutting off of the portion of the hull in the bore may have provided this release. This enhanced velocity would then create the extra recoil. It is though quite possible that with a different set of conditions A pressure spike can occur but this sudden release not happen & there be no increase in either velocity or recoil. So yes Larry, any abnormal increase in recoil should most definitely be immediately & thoroughly investigated for cause, But the point I have been trying to instil is "If" it does not occur, one cannot automatically determine no abnormal pressure has occured. To load a longer shell in a shorter chamber established guidelines must be adhered to & Recoil is not a reliable indicator of Maximum Peak Pressure.

Good points, Miller--and from what Fergus wrote, I think the situation he encountered was as you describe it.

I don't have a recent Brit 2 1/2" gun to measure. Most, of course, are now chambered 2 3/4"/70mm. The older 2 1/2" guns reproofed recently are remarked, I think, 65mm--even though I do not believe any actual modification in length has been made. So even though 2 1/2" does not equal 65mm, I think the Brits treat the two as being the same. I found it interesting that Fergus' old gun passed reproof some time after 1954. Apparently the proof loads didn't produce anything unusual, even though the 67mm shells did.

I think a reasonable conclusion is this: mouth of the unfired hull extending into the bore itself, or into a very short and sharp forcing cone, may very well produce dangerous results in terms of a pressure spike, visible damage to the hull, and perhaps increased recoil.

David;
As I recall this occurance the owner of the Remington dbl mistakenly used smokeless IMR PB powder & loaded by black powder volume. I do not now recall the exact load intended, but let's assume 3 drams. In black powder this would weigh 82 grains. Most smokeless shotgun powders weigh considerably less for an equal volume of black & while I have not actually performed a check weighing, just using figures from a Lee Dipper chart PB would have a wt of about 57% that of black. For a BP 3 dram volume it should thus have weighed about 47 grains. I don't have an IMR loadbook in front of me but this was likely approaching a double charge. Double charging with smokeless powders is not such a good idea. It is imperative to keep ones B's & P's in the right order.

It's like getting a head-start, reading from members who've read the great books. Thank you.

Miller, I also remember as you do. His conclusion was he'd never again have two different powders on the bench at the same time while reloading.

JC

Jim Legg & Jack +100..........

Well Doug . . . all of this remains in the "unknown" category, until someone does something like firing a 3" shell in a 2 1/2" chamber and seeing if the velocity increases vs a 3" shell fired in a 3" chamber. Both Sherman Bell and Arthur Curtis (the latter reporting in an American Rifleman article from 1936) did indeed fire 3" shells in 2 1/2" chambers--Bell in a 12ga pressure barrel, as part of his "Finding Out for Myself" tests, and Curtis in standard .410's with 2 1/2" chambers. Both reported blown ends on the shells. Bell reported the nearly 1500 psi increase in pressure. Neither, unfortunately, provided comparative velocity readings. Bell would not have noted increased recoil, since it was a pressure barrel rather than a shoulder-fired gun. Curtis reported no "trouble or great discomfort other than would be expected from the use of high-speed loads", and he apparently didn't bother to shoot the same 3" shells in a .410 of the same model, only with the appropriate 3" chambers, so that he could make an on the spot comparison of recoil. But we do have other reports of increased recoil from long shells being fired in short chambers, and while one would tend to say--given the major factors in how recoil is generated--that it must be due to increased velocity, we don't know that for sure without velocity measurements having been taken. On the other hand . . . with a blown end from the shell also going down the barrel, we have now increased ejecta WEIGHT vs the same shell without the end blown off. So that might be a possible explanation for a noted increase in recoil. Although not a lot of additional ejecta weight . . . who knows? Certainly easy enough to test for a velocity increase, using a .410 with 2 1/2" chambers, if anyone is willing to do as Curtis did and fire 3" shells in one of those.

Food for further discussion and experimentation . . . and truly unfortunate Jim Legg is no longer here to join in on the discussion.

"On the other hand . . . with a blown end from the shell also going down the barrel, we have now increased ejecta WEIGHT vs the same shell without the end blown off. So that might be a possible explanation for a noted increase in recoil. Although not a lot of additional ejecta weight . . . who knows?"



OK guys no sense guessing about the theoretical. To make life easy I have included a link to an automated recoil calculator. Plug in you numbers and find out for yourself.

http://www.10xshooters.com/calculators/Shotgun_Recoil_Calculator.htm

I think any increase in recoil that could possibly be attributed to a little bit of dislodged plastic will be negligible at best.

That's handy, thanks TwiceBarrel and "fly safe"......

Best Regards,