Your initial comments, Ted are pretty much on track. Your knowledge of such things is obviously greater than mine. But, how would using the wrong length ammo cause this problem if the gun fires and extracts the wrong length ammo with no issues when doing more normally paced shooting. I do plan to order some of the RST ammo that Skeettx pointed out to see how that works out. This gun has the BP over oval on shield proof mark indicating "Reinforced smokeless proof for shotgun barrels, pressure of 12,090 psi" (from appendix 3 in "Spanish Best", 2nd edition by Wieland). Barrel flats are marked F1 which should be 1960 manufacture. All comments are much appreciated.
You are firing the wrong length case (long) in a short chamber gun. Successful firing and extraction is not the issue, the higher pressure, and resulting heat developed in the short chamber is. The pressure marked on the flats was the PROOF pressure the gun saw at the proof house, NOT the pressure the gun is supposed to be exposed to on a regular basis. The ammunition you run should ideally develop perhaps no more than 75% of that level. An occasional blast near proof wont be an issue, most of us wont live long enough to do any damage, but, sustained use will. Pressure developed is not only a factor of the weight of the charge, and velocity, but, the quality and type of components used, in particular, powder. Powders that consistently develop lower pressure at the same velocity cost more money. Bargain promotional loads are often some of the worst offenders.
Id be willing to bet the payload in European 2 1/2 ammunition of the same era of this gun was smaller, and the load pressure was more conservative than what you can get here, now, in the US.
Again, firing and extraction, is neither here nor there. Have you inspected your spent case, keeping an eye open for pierced primers, flattened primers, ripped case mouths, or, perhaps, extractor marks in the base?
Best,
Ted
