Personally, I am not fond of the COW (or any other filler) being fired down a good barrel. To me, you are trading one set of problems for a worst set.
These fillers harden when they are fired and are like sand paper shot down the barrel. I question whether you are saving any barrel life with that method. Know it has been done for years, I've done it too.
If you are trying to form a few hundred cases, then a donor barrel is the way to go. I have two donor barrels, 6 Dasher and 6PPC.
Other than that, I just have always had better luck shooting left over bullets (ones that are left over from other testing) with a mid range load for the parent case. You get a clean case that is formed correctly.
When I start out a new wildcat, I load up all the cases I want to fire form. Head to the range, shoot 20 or so of them.
Back to the loading bench to start load development. Next trip to the range, I fire a few more FF cases to both fireform and foul the barrel. Fire the test loads. Clean the gun, fire form a few more cases, then fire more test loads.
You will be surprised how fast you get all you cased FF, and you really aren't firing many extra rounds. You would be firing them anyway to foul the barrel after cleaning.
When forming new PPC cases, I always load up a new lot of cases, fire them in the donor barrel. I load them again for a second fire form, keep them in a separate box. During a match, I always fire at least three of them in each relay, then a couple of sighters, then record group. PPC cases rarely fire form completely on the first firing. Doesn't take long to have all them formed and ready for the next match.
The worst cases I ever formed were for my 338 Gibbs, whew! Expand 35 Whelen cases to 375, neck back to 35 with a false shoulder, then neck again to 338. Only way I could get a good case form was full house loads behind 210 grains slugs.
I formed 100 cases for a hunting rifle that will last the life of the gun.
Also, you get to break in the barrel while fire forming.
JMHO