To get DG ballistics at 32,000 PSI or less, I'd be going .50 3 1/4". That will give you a 500gr. to 520gr. at 2,100fps, maybe 2,150fps. With a premium bullet, it would work just fine on about anything.

I would not go with the .45 3 1/4", unless there are some powders or loads I'm not familiar with or can't remember the loads. I am seeing loads in the 1,850fps range at 30,000psi with 500gr. lead bullets.

If allowed 40,000 - then OK for .45 cal. If not, it'd be .50 for me. Too - the 3 1/4" case is better ballanced for .50, than .45.

I am thinking in terms of ease of finding accurate loads and consistancy of those loads, shot to shot uniformity - important in a DR, I'd think.

I've never had a ctg. DR - but have worked with both 3 1/4" cases, .45 and .50 in single shot rifles. The .50 was much easier to find accurate, consistant loads with both smokeless powder and black.