Seating bullets straight, in my opinion, is much misunderstood process by most reloaders. I've used straight-line seaters such as Wilson's hand operated one, RCBS Comp. Seater, Vickerman, Bonanza and a few others borrowed from folks just to check 'em out. They didn't seat bullets any straighter than conventional seating dies. All of 'em have quite a bit of clearance between their neck area and the case neck that's inside them when the bullet's seated.
Each and every one of them seated bullets very well aligned with case necks. As long as the case neck was well aligned with the case body, one ended up with a a round that well centered its bullet with the bore when the primer got struck by the firing pin. Cases with necks a bit angled (bent?) from the case body axis tended to keep their necks bent the same amount regardless of the seater used.
So, I've come to the conclusion that if your case sizing die makes their necks straight and not have a lot of grip on the bullet, you could probably use a ball peen hammer to seat bullets very straight indeed. Most conventional dies and their expander balls leave case necks way too tight. Grinding/polishing a .311" expander ball down to .3085" is much better for 30 caliber cases that the standard one that's about .307", but it tends to bend the neck a bit more.
One can straighten the neck of a loaded round by using a bullet pulling collet sized to fit the loaded round case neck. I've taken factory/arsenal 30 caliber match ammo as well as my hand loads, measured their bullet runout, marked the case at the high point, put the loaded round's neck up into a .338 caliber bullet puller, gently closing the collet then pushing on the marked high point to slightly bend the neck. Remeasuring the rounds and learning how much bending pressure needed for a given runout number let me get ammo that varied up to 6 thousandths run out all down to under 2. Shot much more accurate after doing so.
There's more things important to accuracy than very straight bullets and case necks. A batch of .308 Win. ammo (200+ thousand rounds) a few of us developed tested some years ago had bullet runout up to a bit over 3 thousandths and charge weight spread around 3/10ths grain. In a standard SAAMI chamber, it shot 20-round groups at 600 yards under 3 inches.