SH, you're right, it looks like a rife cartridge. Straight blowback is correct.

The round measures out to .224 cal...roughly the same as .22WMR. If you compare ballistic data, it is most similar in performance to the .22 WMR round.

5.7x28 40 gr. FMJ (According to manufacturer)
MV: 1950 ft/sec
ME: 340 ft/lb

The comparison data below is from the posted information published at www.winchester.com

.17 HMR in Winchester 20 gr. Super-X® JHP
MV: 1830 ft/sec
ME: 149 ft/lb

.22 Magnum in Winchester 40 gr. Super-X JHP
MV: 1910 ft/sec
ME: 324 ft/lb

.38 Special +P in Winchester 125 gr. USA JHP
MV: 945 ft/sec
ME: 248 ft/lb

.357 Magnum in Winchester 110 gr. USA JHP
MV: 1295 ft/sec
ME: 410 ft/lb

9mm in Winchester 115 gr. USA JHP
MV: 1225 ft/sec
ME: 383 ft/lb

.40 S&W in Winchester 180 gr. USA JHP
MV: 1010 ft/set
ME: 408 ft/lb

.44 Magunum in Winchester 210 gr. Super-X Silvertip Hollow Point
MV: 1250 ft/sec
ME: 729 ft/lb

.45 Auto in Winchester 230 gr. USA JHP
MV: 880 ft/sec
ME: 396 ft/lb

.223 Remington in Winchester 45 gr. USA JHP
MV: 3600 ft/sec
ME: 1295 ft/lb

Left to right for comparison: 9x19mm Parabellum, .40S&W, .45ACP, [color:"blue"] FN 5.7x28mm[/color] , 5.56x45mm, .300Winchester Magnum, 2.75 and 3.0 inch 12Gauge





5.7x28 Dimensions