will say that I am scrupulous in trimming to minimum length - every case is trimmed every time. And you are right on about the generous chambers of the GI bbls, but I have a commercial bbl on a Winnie that has had the same diet. I allow that I may be on thin ice, but generally the only variable working with mine is std base dies. I am pretty conscientious about loading, and any gun work that relates to function and safety is done by full-up smiths - Clint worked on the M14; Accuracy Gun Shop, Columbus, GA (former USAMU smith), and Tony Pucci the Garands. But it might time to pick up those SB dies, anyway (and stick with the hard primers).
Go ask those guys about the number of ruined rifles they have seen, and how many they get back each year. I would be curious to know.
I was unfortunate to have two slamfires in Garands, so since then, I have been very cautious. There are people who don't wear safety belts, smoke two packs a day, drive drunk (I have met functional drunks, they drink to a certain level of intoxication and stay there till they go home), and some of these folks live a long time and don't have accidents.
I have a friend who never does any maintenance till something falls off. He drives his vehicles 200 K miles. I take care of my vehicles, 100% proper maintenance, and yet I get blown head gaskets and warped engine blocks at 125,000 miles. Sometimes life is unfair.
A primer initiated slamfire event is very rare. And yet, when you get enough people together, the chances are one will happen. I can recall one Camp Perry Garand match where the load command was given during standing, and everyone heard the shot. It was in an battery slamfire. That year's CMP ammo was made by Federal.
George Frost's book has statistics on primer testing. In his book “Ammunition Making”, provides a good description and procedure for calculating the sensitivity numbers in a primer test. His book shows that lots are accepted if a small number of primers are allowed to fire at the lowest energy impact, the "none fire" level.
Your chances of having an overly sensitive primer in your gun may be 1:50,000 or 1:1,000,000. You could go your entire life without having an in battery slamfire or worse, an out of battery slamfire. But people win the lottery every month with 1:100,000,000 chances. Why risk it? Receivers are not getting cheaper. Certain body parts don't grow back.
(If I was not wearing safety glasses I would have been blinded in the right eye. I had pits directly in line with my pupil.)
Proper sizing reduces the risk of an out of battery slamfire, proper primers reduce the risk of a slamfire, in battery or otherwise.
(speaking of Alabammy Slammy, it's 24-6 right now and the slammin' continues )
Roll Tide, Alabama Won!