How critical is identical case length when loading for a double rifle? I have 200 Horneber cases and they are all shorter than 3". Will the cases grow with initial fireforming in the double being this is a low pressure round? The shortest case is 2.855, Quite a bit shorter than specification.

For a double I would rather have them shorter than longer. I always trim the brass for my doubles back to the trim to length every time I reload. If the brass is too long, it will not chamber properly. I would not hesitate to use the shorter brass. If you are still leery, make me an offer and I will be happy to take the brass off your hands. .450/.400 brass is a bitch to come by these days.

You are right. A few on the Double rifle forum spouted off about how hard it was to get 375 flanged magnum brass and I was able to get 500 rounds of Horneber from Huntingtons. I have had 500 rounds of the Horneber 450-400,3" on order for three years and I have finally taken posession of 160 of the 500. The rest are back ordered. So the 375 was easier to acquire. I also have 200 of the Hornady stuff on order but don't look for it until after the first of the year.