fixed sight 2 1/2 or 3 inch stainless L frame.
Yes,3 inch. Would the L frame put it into the 5 shot category like the M69 44 mag.
They made a 6-shot L frame .40- 646. With modern materials, there is no reason why it couldn't be a 6-shot.
10 is only 7.1% more pressure than .40.
True if you go by SAAMI specs. But a lot of 10mm fans insist on ammo with 50% to 60% more power than the same bullet in a 40 can generate. You don't get that kind of power increase from an extra .15 inch of case length and 7% pressure increase.
A 10mm revolver should be strong enough to handle Underwood, Buffalo Bore and reloads from someone that thinks they bought a 41 magnum, not just "weak factory ammo". Anything less and it will be bad mouthed for not being able to handle "real" 10mm ammo.
While everyone that tests ammo on YouTube has a chronograph none of them have a strain gauge to measure pressure. If they did I think, but cannot prove, that a lot of the hottest 10mm loads fall into the +P category and in some cases +P+.
I bought a Springfield Omega back in the 80s when it was the first 10mm I could buy and get magazines for. Being younger and poorer the price of factory ammo drove me into reloading. With that gun I found I was never able to get up to the maximum loads without seeing over pressure signs, including a couple of blown primers. In particular I remember trying to work up a load with 155 grain bullets that would generate the biggest splash possible when they hit a plastic milk jug filled with water. The gun mags at the time said AA7 was THE powder to use for 10mm and recommended a max of 13.7 grains as I recall. My 1991 Hornady manual says 13.9. I could only get to around 12 before I started having over pressure signs. The bullets were seated to the recommended depth. I later traded that gun for a 610 and found I could get up to 13.7 grains without over-pressure signs but past 12 grains there was little increase in velocity and accuracy went from tight groups to shotgun patterns. Loading long in the 610 helped.You can get "real" 10mm rounds within spec. Look at old data, the modern load data is lawyered down to .40 levels.
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.I find it hard to believe that Underwood or BB could stay in business while lying about their pressure levels.