I have had my go round with a couple 10" .223 Contender Barrels over the years, and always went back, RUNNING BACK to Super 14" .223's.
The very first shot I fired out of a 10" .223 Contender was with Black Hills Shooters Supply Commercial Reloads. Man I was not ready for that much muzzle blast. I was also shocked at the amount of recoil. That was the end of that, I decided I needed to find a good handload. I ended up shooting 50gr. Winchester PSP Bullets with AA 2015BR Powder. This combination produce really good accuracy, but I never was overly impressed with the velocity or range.
About 10 years later I just had to have another 10" .223 Contender. This time I concentrated on 40gr. Nosler Ballistic Tips and used H-4198, IMR-4198, H-322 & W-748 Powders. The absolute best combination I came up with was with IMR-4198 and boy did it shoot. Velocity out of the 10" Barrel with the 40gr. Nosler Ballistic Tips with IMR-4198 was right around 2900 FPS (I'd have to look it up to be sure). Accuracy was quite good, and this load was much more pleasant to shoot, both in terms of recoil but more importantly to me muzzle blast also.
Well that Barrel didn't stick around long either before I found myself wanting a Super 14 .223, and the Stainless-Steel verison I currently have is a winner. I have only tried one load in it to date, and the way 55gr. Nosler Ballistic Tips loaded with H-4895 in Military LC Brass with Remington 7 1/2 Primers shoots I really don't see the need to look any further. Average muzzle velocity is 2820 FPS with low extreme spread and standard deviation.
I have zero experience with 12" Contender Barrels so can not comment on them. In fact I don't think I have ever handled a 12" Contender Barrel to know how they even feel.
As per shooting Whitetails on a Crop Damage Permit with a .223 Handgun, many, many shooters and hunters will tell you that a .223 out of a Rifle is not a Deer Cartridge. For the most part I agree, but under the utmost of controlled circumstances it will do the job. I generally shoot several Whitetail Deer per year while at work at the sites of vehicle / deer accidents, and will state that given the right circumstances and putting the right bullet in the right place, yes it will kill Whitetail Deer.
In regards to the .30-30 Winchester in a Contender. I have chronographed my buddies Commercial Reload (125gr. Nosler Ballistic Tip with AA 2230 Powder) in several Super 14 .30-30's and 1 10" .30-30 Contender. In the 10" .30-30 (with this particular ammo) velocity loss from the 14" to 10" Barrel was 160 FPS. I too have heard guys shooting 110gr. Bullets out of their 14" .30-30 Contenders but have no first hand experience with that bullet weight.
Good luck
Larry