I know this has been discussed recently, but a minor resizing error led me to examine this idea more closely. I was reshaping Lee C309-1800R's in a Hanned tapering die (It -> a .302" nose and a .3095" base with a perfectly flat gc base.) for my .30-06 the other day when I either forgot to add a gc or it didn't make it to the bottom punch (is identical to the ones that come with Lee sizer dies). Btw, the Hanned die completely eliminates the gc shank. It occurred to me that if such CB's shoot accurately, I could save $$ on gc's: This sounded better by the minute. I then proceeded to load 30 of them v. 30 gas checked ones at the same OAL, but with different powder charges: 15gr. WC 820 for the unchecked ones v. 18.5gr. for those with "copper" bottoms. The lube (Liquid Alox) and primers were identical (Fed. 210's). The gas checked CB's are a known commodity and didn't disappoint, but I can't say the same for their plain bottomed brethren: 5 moa with the added pleasure of bore leading was the best they could do. I'm thinking of repeating the experiment, but with 14gr. WC 820 and a greater OAL (I'll go to 3.30" next time instead of 3.20".)
As for using the Hanned die, (a) they're no longer available and (b) a PITA to use, but they turn undersized CB designs, e.g., the Lee, but also Lyman's #311291 (The nose is undersized, not the body.) into 1 moa shooters. However, if you yearn for a tapered CB that shoots well in many .30cal. rifles, including the very CB-friendly K-31, look for Lyman #311644 and be happy.