10mm ammo in 40mm barrell

My favorite CCW is my 10mm Glock-29 loaded with Underwood 115 gr. 1700 fps Xtreme Defender ammo. These rounds cost me almost $2 per round when you consider shipping. I put in lone-wolf a 10mm to .40 caliber conversion barrel and normally practice with 180 gr .40 cal. ammo that I buy in bulk...