Why didn't they go with a proven model like the P220 or the P226 ?
Because they are not modular in nature (allowing the guns to be ergonomically adapted for different shooters).
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Blimey!'"ergonomically adapted for different shooters", eh? Pretty fancy talk applied to squaddies.
When I was in the (British, so lower budget) army, it was difficult to get an issue sleeping bag ergonomically adapted to me becase the issue scales were probably still based on undernourished wee Malkies driven out of the Gorbals by 1930's unemployment. You got the one, and the gun, you were given.
Tell that to kids today, and they'll not believe you.