I'm having a hard time deciding on my build. At first I had wanted a .280 AI, but then after comparing brass $, availability, and the fact that the 7RM had a bit more fps, I decided on the 7RM. My dad also has a 7RM which I reload for, so I already have the dies and lots of brass available. So he through out the idea, why don't I just go with a .300 Win Mag. The more I think about it, the more I think he may be on to something. Now what? Is there any reason I should go with the .300 over the 7mm? If I load for the .300, I probably won't be loading anything over 180 grains anyway. The 7mm can be loaded up to that. I thought I had it all figured out, but now my mind's a mess!
I think it would be a great idea if you were to inform those who read this forum that can really help you about what you intend to shoot, shoot at, and at what distances. How do you feel about muzzle blast and recoil?
If you do this you will probably get some great advice.
What do you want to do with your rifle? Paper punch? Hunt? Hunt what? Where? How far? Cartridge selection (and bullet selection too) depends alot on what you're hunting - though there are other implrtant factors to consider as well. Were it me, i'd go with the .300 Win Mag.
Incidently, i also currently have a 7mm RM (for like 26 years now). However, when i built my own tactical shooter 18 years ago i went with the .300 Win. The reasons were;
1) prior experience with the cartridge
2) more projectile/bullet choices in .308 (i think.....)
3) sheer power increase (this i know!!!!)
I like/liked the 7mm RM and killed deer with it, but i'll be canabalizing mine for the action one day to build something with more LR everything or will most likely mod it significantly for my son. On second thought, i will definitely re-barrel it in something different - and probably a belt-less 'magnum' velocity mid-bore.
For me, i've no problem bringing extra power to the hunt. One day i'll hunt (and hopefully find!!!) elk. When that day arrives you can bet i'll have either a .300 RUM or a .338 Edge. Would i use either of these on deer-sized game? Yup, and will tailor my bullet choice for the target at hand.
I currently own a 300WM, but I'm having a 7RM built right now by Kirby. I've killed antelope, deer, elk and buffalo with my 300WM shooting a 180 grain bullet and it has performed perfectly. I'm building the 7RM to shoot the Berger 180 grain bullet. I won't be giving up much velocity from the 300 and the 7mm bullet will have a much higher BC. I just ran it through Exbal real quick. If I start the 7mm bullet out 250 fps slower than the 300 Accubond, the 7mm will need about 2 moa more come-ups at 1000 yds. But, the 7mm will have about 100 lbs more energy and will drift almost 10" less in a 10mph wind. Sorry, you didn't ask about all this, kinda did it for myself. I'm not sure what you can do with a 300 WM that you can't do with a 7RM. I love my 300 and I'll not sell it, but I'm hoping to grow to love the 7RM when it's done. All this from a guy who swore he'd never own a 7RM. LOL!!
I will be hunting mainly whitetails here in West Central Wisconsin out to 500 to 600 yards. I will also be hunting elk, black bear, and hopefully in a couple of years, moose in northern Alberta.
If I am considering the .300, should I just step up to the .300 RUM? I assume I will have to find a different action since my donor rifle is now chambered in .30-06?
I have hunted Alaska, Alberta, Montana, Idaho, Washington etc. all with a 7mm Rem Mag. It has been my choice over all of the options I have owned. Today if I were hunting moose or brown bear I would probably use my .338 Win Mag, but I would feel very confident with a 7 Mag if that is all I had. I find it easy to shoot, fast, flat, minimal recoil for a magnum cartridge and deadly. While I don't recommend it a friend of mine shot his Kodiak bear with his 7MM Rem Mag.
While I have considered a 300 WM a few times having owned the 300 Wby Mag, I find if I want that much soup my go to is going to be the .338 every time. Same recoil but with meat and potatoes in the soup. You might consider the .338. You can load down to 180 gr bullets with the .338 for chipmunks and go all the way to truck size bullets for your moose.
Given you have a donor action you can not go wrong with any of these cartridges that fit that action. You will have to probably have some gunsmithing done to the action to feed the larger diameter case.
If you want to go with a long action you might consider the 7mm STW or 7mm Ultra Mag.