Jim,
I've posted several things regarding the CVA Apex on this site...look around. While I do not own the black powder barrel, I do own two frames and three centerfire barrels. I have had difficulty with one of the barrels, which I sent back to CVA a week ago, and am awaiting their response. The .45-70 and 300 Win Mag barrels print well, and the barrels interchange with the frames tightly and solidly, without issues so far. One of the frames aligns the barrels slightly off center (left to right), so that the firing pin does not strike the exact center of the primers on any barrel I mount, but that is apparently only a cosmetic issue, as it has not affected accuracy of either barrel to this point. POI is not affected on remount. During range tests I have shot to original POI out to 300 yards after remounting the 300 Win Mag barrel and scope, and out to 200 for the .45-70. I will qualify that by saying that I have fired less than 200 rounds using that frame however, so I can't speak to how loose it may become after several years of use, especially with heavy recoilers like the 300 and 45-70. One frame has a very crisp, repeatable trigger break that I have set at 3#, the other has some trigger slack before it breaks, that I do not like at all. I had backed the trigger pull weight off to 3# on that frame as well, so am going to tighten it back up to 3.5 to see if that takes the slack out. If it doesn't, the frame is going back to CVA for correction.
On thing I might add...With CVA's reduced price on aftermarket barrels (marked down in January to $219.95 on their CVA webstore), you may may want to consider building your own rifle by buying a frame and forearm from an Internet dealer, and the barrel from the CVA web store, and come out for less than you can buy a complete firearm from one of the large bricks and mortar stores like Cabela's...say less than $550 for the black composite model, to assemble it yourself...$220 barrel, $240 frame, $32 forearm...then add shipping and the FFL transfer fee for the frame (I have a dealer that only charges me $25).
Good shooting! ...CC...