doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Pressure Question for Engineers....

I apologize for being a nerd on this, but I still do not see the physics on why S. Bell found that the 12/28 system had greater pressure (measured in the "same" place) than the 12/20 system. I do not believe that the minimal shot size difference would account for this.

I will not bother this topic again, since it can only reflect more clearly my stupidity--but I would greatly appreciate insight into the Sherman Bell measurement of the two 12ga systems.

Berrien

As 2-Piper pointed out, without initial pressure measurements of each load fired in a matching-gauge barrel, we do not know the pressure developed by each load under normal use. However, even though we can't really know, a likely explanation for your question could be that the 28 gauge load develops higher pressure than the 20 gauge load when each is fired in a proper gauge barrel. If you start with a higher pressure load to begin with, it's reasonable to find that it is still higher than the 20 when both are fired in a larger bore. Most handloaders know that it is hard to load a low-pressure 20 ga. load, and harder still to do it in 28. Note that there is no conclusion stated in the report from Bell about pressure whatsoever, and certainly not that the 12/28 combo is INHERENTLY higher pressure than the 12/20. It's just raw data that shows what happened with these particular two loads.

Rocketman, O.K., I'll give to you the possibility that the wad skirt may still seal the bore in the small jump from 12 to 10 ga. or 16 to 12 ga. How ever I'm pretty certain that gas leakage would occur in the jump from 20 to 12 or 28 to 12 ga. But even where blow by does not take place, it seems to be claiming a physical free lunch to assume that increased force on the wad base would magically occur just by the area of the wad base instantly increasing in size during the transition from Gaugemate to nominal bore size. In the F=PxA calculation, if the area increases and pressure is constant, force would somehow increase just from the wad entering the larger bore. If that were the case, why isn't every shotgun maker taking advantage of this?

Berrien, there is to much information missing from the sales promotional data to allow hard and fast scientific conclusions. An Indication of a possible cause lies in the SAAMI maximum average pressures for each gauge, listed below:
10 ga 11000psi
12 ga 11500 ( except 3&1/2" shell =13500)
16 ga 11500
20 ga 12000
28 ga 12500
Even with a constant wall thickness the higher pressure would largely be accomodated by the increasing ratio of ID vs OD in the hoop stress.
Regarding the apparent gain in speed of @ 100 fps This may be a function of decreased friction, however before you could draw any firm conclusions on this I suspect you would need to conduct extensive testing on this aspect alone.
Unfortunately until I win Powerball I cant afford the time and expense to conduct such research.

Boyles/Charles law [discovered 1662] can be used to "ESTIMATE" the pressure drop as the charge leaves the 12 G. gaugemate and enters the forcing cone and 10G barrel.Boyles law states tha:P1xV1=P2xV2. This formulae can be transposed to:
P2 = [P1xV1]divided by V2


P1 = Breech pressure - 6000 psi V1= 12 G [.729]volume
P2 = Pressure as charge enters 10g barrel- UNKNOWN!
V2 = 10G [.775] volume.
Based on the following assumptions and applying Boyles/Charles Law I was able to "ESTIMATE" [or should I say guesstimate], that there would be a drop in pressure of 362 psi or 6% as the 12G charge enters the 10 G barrel.
Assumptions;
1/The 12 G shell generates 6000 psi on ignition.
2/The 12 G gas gas leaving the gaugemate has an average diameter of .752 as it passes through the forcing cone.
3/The forcing cone is 1.0 long.
4/The temperature of combustion was constant.

The reduction in pressure calculated would reduce charge velocity and thus recoil. But by far the greatest reduction in recoil is due to the difference in weight between the 12G gun and the 10G gun including the gaugemates.[ See my earlier post]

1st; Bore volume is based on the Square of the dia, not the dia. Thus the volume of the .775" bore is 13% more than the .729" bore, not the 6% stated.
2nd; Assuming the wad obturates & seals the bore, even though the pressure dropped by 13% it would be acting upon a 13% larger wad base so velocity should be unchanged from this cause only.
3rd; Recoil is based upon wt & velocity of the ejecta, not pressure. As the wt of the charge is unchanged the only thing to affect the recoil is developed velocity.
4th; While on average larger gauges weigh more this is not an automatic given, there are some 10ga guns which weigh less than some 12ga guns. If the two guns weighed the same & velocity was the same the recoil would be the same.
"IF" the velocity is actually increased as claimed by the makers, even though pressure may have been reduced, recoil will be increased in proportion to the velocity increase.

I did use the square of the diameters in my Estimate. I also took into consideration the effect of the transition of the charge from 12G to 10G as it past through void created by the 10G forcing cone. After making a scale drawing of the barrel with the gaugemate in position it appeared logical to use the square of .752 diameter in my pressure calculations[ie;.752 diameter being the mean diameter between 10G and 12G].Experience has shown that when faced with this type of problem it is adviseable to go back to first principals, in this case Boyles law.I would agree the proof of the pudding will be apparent once controlled tests results have been evaluated.
By the way as a boy, I used a Webley and Scott .410 adaptor in a 12G; it was effective on bolting rabbits so balistics was not an issue!

My understanding of the gaugemate was that it is approximately shell length & has no forcing cone. You will thus on exit have whatever the dia of the internal hull measures, which will be slightly larger than the .729" 12ga bore. The charge will not of course exit directly into a .775" bore, but rather into the forcing cone of the 10ga which will be somewhat larger. I do not see any point where you would have a comparsion of a .752" dia to the .775" bore.

Gagemates disclaim any use in damascus bbls,so why not have Briley make short tubes for your gun?More money yes,and a little more weight.Briley said they will make me companion tubes to what ever length I want,so I would opt for 10" tubes to have the actual muzzle ahead of the forearm t protect the hand.The pressure are way low at this point,and the set will be about 10oz added.