I've been following along with your problem here and trying to come up with a decent solution for you...Well, seeings how you live in Canada, and its Winter, its not like those dead cows are going anywhere soon....
I would ask, why do you think your cows were shot to begin with? And by who? Do you got a neighbor whom you suspect? If so, Why would they shoot your cows in the first place? if so, with what caliber? These are viable thoughts that may assist...If you do have a bad neighbor who hates your cows or you for some reason, lets walk this trail..Its logical....
A mean stupid kid would shoot a cow with a .22 rimfire right in the guts and let them wander around and suffer for days before they die...A guy that knows a bit would step up and shoot them right in the head with a .22 and drop them on the spot at the fenceline! So the location of the dead cow and other evidence may help you figure this out..tracks in the snow...other sidebar evidence...
Now, a guy shooting a heavier caliber, who hates your cows or you, would put a pill right through the boiler room from a concealed location and from a safe distance well off your property...30 cal and up I would suspect are quite common in your area...Up there, 7mm mags are considered bunny guns...
Dead frozen cows are quite a situation to deal with....I know, I once had to kill and skin a Belgian Horse during a Northern Minnesota Winter Blizzard....Lots of frozen dead cattle as well for a guy to deal with....
If I were you, I would drag in one of my unfortunate ungulates with the tractor and perform my own autopsy by skinning it...It would take a few hours tops...less with a chainsaw, very easy to see if any external ballistical malfeasance leading to the innocent bovines demise would then be quite obvious.....
Then I'd go from there...If I found evidence and suspected a certain neighbor of this aggregious impropriaty, I would confront them with the autopsy evidence and tell them I want cash money equaling twice the street value of the victim! Or, I'm calling the Mounties!
Just a suggestion.In lieu of all this, and to answer your question, I'd get a Tek T2 or an AtPro! Both would be capable of finding a bullet and hold a decent Resale value!
FWIW, if you want to get even with your neighbor, as you are driving by, simply toss a Brillo pad soaked in bacon grease into the guys yard...his dog will eat it...it will take days of painful suffering.....Its pretty much untraceable back to you as the dog killing culprit!...Everybody knows dogs will eat anything!..Word of warning..dont kill some innocent dog unless you are absolutely sure some evil peckerwood would miss him and that he was the one who shot your cows in the first place!!. Theres more to this story than you are letting on here!