...which I would avoid. To be blunt, I wouldn't fire the Westley ammo that you've already obtained.

Their ammo line is loaded for them by Wolfgang Romey in Germany. Word in the trade is that Westley requested their ammo be "hotted up". I've heard of a number of incidents of severe overloads with it in doubles. My own experience with it was nothing if not over the top. I'll put it this way, I won't stay on the line if someone is firing it in a double.

In the event that you simply must fire it, like Bonanza said, if you have to force the gun open like that on the first rounds, STOP and don't shoot any more of it. What you described is never normal and may well be suggestive of ammunition that is unsafe.

In .375 Flanged Magnum, your best bet is Kynoch. I've never used it in that caliber, but it's been good in the calibers I've tried. Connecticut Shotgun (Galazan's) is, or was, their US distributor, not M. W. Reynolds. If they don't have it in stock, call or email David Little at Kynoch in the UK. Contact data is on their web site, and David is always extremely helpful.