Chamber Pressures
Not wanting to sound like Mr Knowitall, but here are some figures that some might find interesting when it comes to shooting old shotguns whether composite or fluid steel barrels . I welcome thoughtful discussion. I refer to 12 ga because that is easiest.
1. Most of these old guns were made for a working load max chamber pressure of 10,500 psi and proofed at 13,500. Later the proof was increased to 15,000. These are average pressures .
2. Present working load pressure is 12,000 psi max , proof also increased. Suggest people review SAAMI pdf on line. The interesting point is that these are average pressures, it is allowable to exceed the average by two coefficients of variation, each coefficient being 7.5 percent. That means a SAAMI allowable load could be at 13,800psi , which exceeds what used to be proof loads for vintage guns.
3. The reason we see hot loads today is twofold, first to cycle some autos that are difficult and second to reduce leading the target for shooters who have difficulty with lead( who doesn't). Browning now is selling pheasant loads of 1 1/4 oz at over 1400 fps. These are probably at 12,000 chamber pressure average again . Other manufacturers do the same.
4. So when we speak of "modern" loads and ask if they are permissible to shoot in our vintage guns, fluid or composite steel, what is modern ? There are plenty of modern loads made by manufacturers today that fall under , some well under , the 10,500 psi chamber pressure limitation , and plenty that exceed that and are still SAAMI allowable. Couple a hot load with the allowable two coefficients of standard variation for error, and it can be a hot load indeed.
5. So I personally regularly buy Remington , federal or Winchester loads, but I make sure to stay below the old limitation, with a margin of error for coefficient variation. RST shells are fine too, and Morris seems to understand the need to stay within the older pressure limitations .
6. Some weeks ago we saw a photo of bulged barrels on a 16 ga Damascus Parker. I don't know how this happened, I haven't examined the gun, don't know the chamber wall thickness, but things like this can happen if you start with a load at or near allowable max with higher shot weight and speed and then encounter coefficient of variation increase of up to 15 percent.
Best,
Bruce Day
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