I've used sand-filled shot bags, all sorts of high-dollar benchrest rigs and even tight rolled strips of cloth under a rifle's fore end atop a bench. Nothing's shot as accurate as using the sand-filled shot bags under the fore end when I'm slung up in prone. Sometimes, a second bag under the toe of the stock will enable a smaller holding area.
One thing I've learned is the more someone holds onto a rifle sitting at a bench shooting it as it's held against ones shoulder, the worse accuracy gets. Us humans just don't reposition the rifle repeatably enough from bench positions. Most folks will shoot their rifles more accurate slung up in prone with a bag or bipod under the fore end. F-class competition has pretty much proved this.
Best accuracy tests of complete rifles are done with machine rests having a 3-point support that lets them recoil back and up like the do when fired off ones shoulder. These will produce much, much smaller test groups than any hand-holding and aiming us humans will ever make. Benchrest rifles stocked for free recoil firing are the next best system. Rail guns used in benchrest unlimited competition and some bullet makers are best for testing barrels and ammo components for accuracy.