doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com British 20 Ga. Pressure Question

I have a dainty little 20 Ga 2 /12 chambered single shot Army & Navy Hammer Gun. The gun is in excellent condition.

The Proof markings are:

A 20 in a Diamond
Nitro Proof
Shot 7/8 OZ MAXm

What pressure shells should I fire in it ?

I have some RST 20 2 1/1 shells on hand.

Also would it be OK to fire SAAMI pressure 2 1/2 .410s in it with a chamber adapter ?

Rockie, looks like you have a gun made between 1896-1904 by the proofmarks you listed, not reproofed. RST 2 1/2's would be fine. Pressure standard for the 2 1/2" .410 per SAAMI specs is 12,500 psi. Too hot for that old girl.

Larry thank you !

Larry, it has been my understanding that when using a chamber adapter such as Chambermates, Gauge Mates, etc. there is a significant reduction in pressure. I do not have any specific data, but I'd have to believe that a .410 loaded to SAAMI specs is yielding a whole lot less pressure when shooting that shell with a chamber adapter out of a 20-ga. gun. Am I wrong?

It would be the Standing Breech of the old piece that would take the Pressure/Back Thrust, as the adapter is quite thick, of course.

If the 20ga gun accepts the full length 20-to-410 chambermate inserts then the higher pressure of the 410 cartridge should not be an issue as the Chambermate will contain the peak pressure before it enters the bore of the 20 ga. I believe the answer is true for the old savage FourTenners. This is not true for short cartridge size inserts.

I have noticed on 16 ga guns not all bores and chambers are equal and some, in particularly german guns, are narrower than others. This leads to problems using chambermates in the gun. Of four 16 SxS I have tried to use chambermates with two would not fit all the way in. If you do not already have the chambermates on hand if you can borrow a pair for a test fit before you lay out cash for a pair might be a good idea.

Larry, you are on the right track, but I'd like to add a bit of information. There are two issues of backthrust. Should the "face" be a bit thin, esp. around the firing pin, it could deform under the higher local pressure of the .410. The other issue is total backthrust. That needs to be calculated using the .410's pressure times its base area and comparing to the same calculation for the 20 bore.


DDA

Single shots do tend to have fairly impressive thickness through the breech, which would also help in that area. But at 12,500 psi, you're getting pretty close to what the proof pressure would have been on that old Brit single.

I would presume that for an accurate calculation of this base area for pressure one would need the inside diameter of the case at its base. Not wanting to try & determine that I simply made two quick calculations, one based on the two bore sizes, the other on the OD of the shell just ahead of the rim. In both cases area of the .410 is approximately 45% that of the 20ga.
45% of 12,500 = 5,625. I would not foresee any problem with total back thrust using the .410 in a 20 ga. That max pressure is going to be realized in the chamber itself where it has the additional strength of the inset, some at least which are quite capable of containing the pressure by them selves. Pressure should drop dramatically as you exit the insert into the bore of the 20ga as its area is 225% that of a .410 bore.
It would appear to me the only area of real concern would be that possibility of a Thin area in the breech face R'Man mentioned, but thats not real likely.

Thank you all for your advice on this. This is an example of the excellence and extremely well informed advice that this forum is famous for.


The gun is a Back Actioned Hammer Gun so it may have the required strength at the breech. I believe I could estimate it, by removing the Firing Pin and measuring the length of the opening.