Ron, I think you will be limited by the number of barrel makers out there that will do a 16 twist in a 45 caliber. Green Mountain makes that particular barrel in several different weights. I have used them and found them to be a very good barrel. BRC barrels generally come unprofiled, but you can check with them and they may be able to do that profile for you but I don't know if they do a 16 twist. I would limit the caliber to what  brass is available right now. Bob

Many of the current barrel makers can not make a 32-34" 1-16" profiled blank. I did get a large round 35" McGowen blank and had C. Sharps profile the octagon to my dimensions, but it was 1-18 twist. The Bore quality is superb, no tooling marks like my last 2 Green Mountain "covid" barrels.
kw

I'm partial to Krieger, and they do a 14 and 16 in a .450/.458.

I don't think they do octagon, but you could spec the appropriate dia's and lengths to save a bunch of time.

I have one .45 x 16 twist Green Mountain, half octagon/half round barrel left from my old barrel business. where I sold barrels specifically contoured and configured for single shots.   

It has a slight defect in the rifling at the chamber end, that necessitates a .45-90 to clean it up.  I had 30 of these barrels at one time, barreled several myself and sold many; the ones I did and tested were excellent shooters.

$325 plus shipping if you want it.  I can verify dimensions and post a picture if you're interested.  Come to think of it, I'll do that now and post it in the classifieds this evening.

It's 30" long, and 18 twist.  I mis-spoke when I said 16 twist.

I bought a lot of odd lot stuff from Green Mountain, McGowen, and Montana Rifleman when I was doing these barrels.  The entire lot of these barrels had the same reaming defect, in the same location.  The length might be from shortening them to eliminate defect, or they may have made a run that length for somebody that didn't pass QC because of the defect.  I do know that chambering for .45-90 cleans it up entirely, and they're good barrels.   

I put it in the classifieds, with dimensions.  I believe it to be a Green Mountain, but it could be a Montana Rifleman also.  Usually Montana only sold me modern calibers, and the large calibers came from Green Mountain.  So many barrel lots i bought, hard to remember for sure and I didn't mark it at the time.

Ron Smith of RKS Enterprises can do straight or gain twist barrels with any twist you desire in chrome moly or stainless steel. 
You can request either a 6 groove or 8 groove barrel or even fewer grooves if you desire.
I have used the 8 groove barrel since the bullet aligns better and is more concentric in my opinion.

His barrels are cut point precision cut where he makes 40 cuts for every groove.
The barrels have a continuous taper of .0004" between the chamber and muzzle.
The choke provides the best accuracy for shooting cast bullets (reduces ability of gas cutting around side of bullet).
The gain twist reduces barrel wear by 10% to 15% for high pressure high performance use of his barrels.

Ron Smith
P.O. Box 635
Wimborne, Alberta 
T0M 2G0
403-631-2405

Again I post the accuracy from my 5 shot cast bullet groups shot @ 100 yards with a .32 RKS 8 groove gain twist stainless steel barrel ending in a 11.5 twist. 

I have made one or more rifles with RKS barrels in the following calibers: .22 LR, .218 Bee, .22-250, .223, .25 RKS, 6mm Remington, 6.5x55 Swede, .270, .280, .308, .30-06, .300 Win. Mag., .32RKS,  .32-40,  .35 Whelan, .38-55, .40-65, .45-70, .50-70.

They have all had EXCELLENT performance in Schuetzen smokeless powder cast bullet rifles, Black Powder rifles and hunting rifles. 

Ron always says to select the weight and length of bullet you intend to shoot; in order to provide the best twist for gyroscopic stabilization before you order the barrel.
Also he can consider how best to throat the chamber.

Works better being able to start with the correct bullet instead of trying to test numerous bullets to work out your new barrel.

Quigley, there's no barrel or equipment preference, it's not a precision match.  Shooter and more particularly spotter skill are a big factor in placing well; and luck of the draw on when you shoot on a target really comes into play.  The random elements mask any barrel advantage.

The targets are big and easy to hit.  The wind is unstable and hard to call, and can get downright brutal at times.  Any 2 moa gun can win at Quigley without difficulty with good shooter/spotter and a lucky shooting slot on each target.  Conversely, a 1/2 moa gun would definitely help, but would not "buy" a win.