TC Contenders come in a large variety of calibers, including .300 WM, but Cooter is right. The law (as I remember it) is for straight walled cases only to avoid standard rifle calibers. Although the .444 is a straight-walled case. At one point (right before I moved away and when the came up with the pistol season), there was debate about that to avoid .444s. Any thoughts on that caliber?
Better caliber than some of the other legal ones (if you can handle it), and legal. More energy and speed. Much more than some. From last year's regs.
http://www.iowadnr.gov/Portals/idnr/uploads/Hunting/huntingregs.pdf
Handguns: Center-fire handguns .357 caliber
or larger, shooting straight wall ammunition with
an expanding-type bullet. Minimum barrel length
for all handguns is 4 inches. No shoulder stock or
long barrel modifications are allowed.
Only the following center-fire cartridges may
be used: .357 Magnum, .357 Maximum, .375
Winchester, .38 Super, .40 S&W, .401 Powermag,
10 mm Auto, .41 Remington Magnum, .41 Action
Express, .44 S&W Special, .44 Remington Magnum,
.44 Automag, .444 Marlin, .445 Super Mag,
.45 ACP, .45 Colt, .45 Super Mag, .45 Winchester
Magnum, .45 Silhouette, .451 Detonics, .454
Casull, .45-70 Govt., .460 Rowland, .460 S&W
Mag, .475 Widley Magnum, .475 Linebaugh, .480
Ruger, .50 Action Express, .50 Linebaugh, .50
Beowulf and .500 S&W Mag.