The good old 30-06 with the right bullets has planty of energy to kill deer at 1000yds. With the right bullets I mean 175gr and up , I would shoot the 190-210gr pills to get all the good out of it assuming that the gun is up to it with those pills.
Not sure of the specs on that weapon as far as barrel lebngth twist rate and others but with a 24" barrel the 30-06 and a 1-10 twist the 30-06 can launch a 175gr Berger VLD at 2700fps easly ,that bullet at that speed will cleanly kill deer at 1000yds as will the 178gr A-max. chances are that the gun will have to be fed single shot for the long bullets to be seated out far close to the lands.
You have to get the lighter bullet cranking some serious speed for them to beat the heavier VLD's at long range ballisticaly.
If the barrel is at least 24" he could have it rechambered to 30-06 Ackley or 30 Gibbs , that would give him a bit more speed and if the gun if factroy it'll give him a better chamber also , but if its shooting .5" groups then its accurate enough to shoot long range
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