As Martin stated,I have two custom 300ultramags.Mine are built on stainless remington actions,but I use fluted aftermarket medium varmint barrels,and pillar bedding into mcmillan stocks.The total weight of my rifles with the 3x10x42 swarovski scopes mounted runs right around 10 lbs so they aren't lightweights,but the added weight makes them quite managable to shoot.My 180tsx load averages 3380fps and both rifles average 5/8" groups at 100 yards with this load.To date I have taken 9 elk and 2 moose with the 300ultramags,at ranges from 80 yards to 377 yards, and I am very pleased with the performance.
However I do hesitate to recommend this cartridge to most hunters.The primary reason is that the 300ultramag only offers an advantage over the 300win mag when the ranges exceed 300 yards.And then in order to utilize the advantage offered by the 300 ultramag,the shooter must be willing to practise at longer distances in order to learn the cartridges trajectory,and most importantly to develop and maintain the shooting skills necessary to accurately place his bullets at distances exceeding 300 yards.Most shooters aren't willing to spend the time or money,and deal with the substancial recoil in order to practise regularly with this round.Even if you are willing,you must have access to a location where you can shoot long distances,and at those distances ,a laser rangefinder becomes a necessity,as any error in range estimation can cause a miss or worse yet a wounded animal.
Another factor to consider,is that the factory loads for the 300ultramag are pathetically mild with some 180gr loads barely reaching 3200fps in most rifles.This isn't enough gain over the 300winmag to justify the extra recoil and cost or ammunition.Therefore,I do not recommend this cartridge unless you handload.
Lastly,the 300ultramag does not have a long barrel life.If you are very careful not to overheat the barrel,the life expectancy will be in the 1000 to 1500 round area.In other words,if you shoot this cartridge enough to remain proficient with it,you will be shopping for a barrel at least every 4 or 5 years.
I have several identical rifles with nearly identical trajectories so,I am able to practise a bit with each of them to prolong the life of the barrels.
However,if you have access to a 500 yard range,own a laser rangefinder,are prepared to spend the time and money to practise at longer distances,are prepared to put up with the recoil,and are willing to rebarrel when required,the 300 ultramag is a great long range cartridge.