Dead Beat, From what I read on here they are calling the 338-300 ultramag an edge. In other words a necked up 300 ultra which gives a bit more case capacity than the 338 ultra. Kinda like the STW, somebody gave it a name and claimed it after we had been necking that 8mm remington to everything imaginable and making some great wildcats with it for 20 years. I am still old school I guess and just call everything what it is. Me and a bunch others had the 257-8 rem mag, 264-8 rem mag, 7mm-8 rem mag, 30-8 rem mag and the 338-8 rem mag all years before I ever heard of Layne Simpson or a STW. I'm not complaining though because all this standardization of wildcats helps everybody. I had a local gunsmith build me a hot 22-250 Savage or varminter wildcat one time also, but that shows my age and look how that one turned out!
With the ultamag I had the 300, 338-300, 7mm-300, and 358-300 ultramags and loads developed for all just as the first 300 ultramags were hitting the stores 10 or so years ago. It was an act of congress to get the brass to work with though because as some of you may remember the ammo started showing up in the stores a bit late after the rifles came in. Most stores had the rifles to sell but no bullets for the customer to shoot in it. We thought that was kinda funny at the time. Obviously I had a few connections to get a jump there. By the time most people ever heard of an ultramag I had already built over 20 338-300 ultra wildcats for me and my buddies. Enough of that, here is what you want to know.
Like I said in an earlier post thousands of shooters will tell you of a story where there pet rifle took an elk at a 1000 yards. I took a nice 6x6 bull at 1100 yards with my pet 270 wby back in the mid 70's but was it the best choice, NO. The 338-300 ultramag is an excellent choice. You can put it on some standard magnum actions without much modifying for feeding problems saving you a bunch. It gives adequate velocities within 200 fps of the big 378 wby case. Brass is much cheaper and readily available. And most important it is a shootin' son of a gun. I have never built one that didn't shoot lights out. It is just super inherently accurate in the 300 and 338 versions. I remember the first one I did shot a 3 7/8" 5 shot group at the 650 yard target with 5 of the first 6 shots. The first shot was at 200 after bore sighting then I adjusted to the 650 and fired the group. I had a 1000 yard range on my farm right outside the door to my shop. That was with winchester wmr powder and I bought all I could find later (about 200 pounds) after winchester discontinued it. It was good stuff.
You will not go wrong with the 338-300 ultramag.
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