Some people believe that swaging improves the accuracy of cast bullets. The theory is that cast bullets often contain internal voids that make the bullet unbalanced, and that swaging generates enough pressure to collapse those internal voids.
There may be other benefits from swaging -- squaring up the gas check, creating a taper to match the throat, or changing the shape of the bullet nose. But I can square up the gas check and create a taper using a lubrisizer nose die at modest pressures, so I don't need to swage to do those things. For the purposes of this thread, I'm only interested in whether swaging improves accuracy by collapsing internal voids.
The plan is to compare the accuracy of swaged bullets to unswaged bullets. Both the swaged and the unswaged bullets will be final sized in the same nose die so that they both have the same fit in the throat.
My entry-level reloading press is not strong enough swaging, so I decided to use a 1 ton arbor press. I made a crude 7mm swaging die for the arbor press as shown. In hindsight, I would have made the die somewhat differently, but that's par for the course when you attempt something for the first time. If it turns out that swaging really does improve accuracy then I'll eventually make a better die and get a press more suitable for swaging. Nonetheless this crude die will get the job done for this shootout.
The punch holders slip over the die in order to guide the punch in straight. In hindsight that was not necessary because the arbor press already pushes in a straight line, but nonetheless it works. The die & punches were machined from drill rod, then heat treated and tempered.
My first test of the die for basic functioning used bullets that had already been heat treated, lubed, and nose sized, just because those were the only bullets I had on hand.
It's necessary for the grooves to be filled with lube prior to swaging otherwise the grooves will collapse in the swage die and then it would not be an apples-to-apples test.
The ejector punch is 3/32" so bullets exit the swage die with a small flat point.
The bullets also ended up with a slight hiccup at the start of the ogive -- that was unintentional, due to the way I machined the die and is one of those things I would do differently next time.
Nonetheless we'll live with the hiccup for this test since it doesn't change the functional aspect of the bullet much.
I'm thinking that for the shootout I'll swage air-cooled bullets in order to more easily collapse any internal voids. That means I'll have to lube prior to swaging, then after swaging remove the lube so that the bullet can be heat treated. Yes it's a lot of extra work but the idea is to end up with a bullet that is exactly like the unswaged control bullet other than having its internal voids collapsed.
How much pressure is applied by swaging? Quick and dirty measurements suggest that my 1 ton arbor press has 30:1 leverage. I can apply 100 - 150 pounds to the handle so that works out to 3000 - 4500 pounds applied to the die. A 0.285" diameter bullet has a circular area of 0.0637939 square inches, so that works out to 47,000 - 70,000 psi. A 35 BHN bullet's yield strength is roughly 17,500 psi (500 x 35) so even a heat treated bullet should yield in the swage die. Since I'm hoping to swage air-cooled shot for the shootout, they'll yield at 6000 psi.
By contrast a RCBS lubrisizer has roughly 13.3:1 leverage when sizing in a nose die. I'm guessing I typically apply 20 pounds force to the handle (if you put your weight on the lubrisizer handle you're apt to bend the handle or even break the casting as I did one time
) so that works out to 4200 psi, not necessarily enough to make the lead flow in a hard bullet.
For some reason the Idaho weather in January is not favorable to shooting
so it may be quite a while before I can perform the shootout.