I was in this same delima about 8 months ago, however, I wanted to shoot light bullets out of the WSM Cartridge. I wanted to shot bullets from 100 to 130grn bullets for varminting out to around 800 yards.
I started doing reserch on using light bullets with one of the WSM cartridges. My goals were; to shoot the 110grn V-Max or 125grn Ballistic Tip type bullets, shoot these bullets at high velocity to help out lower BC numbers with these types of bullets, shoot them accurately and be able to use Norma Brass. After doing research, I found a gunsmith who had actually been testing such a rifle with the combination of a WSM case and shooting light bullets.
Richard Franklin discovered the 300 WSM cartridge was capable of producing awesome performances with the lighter bullets as long as the right components were used. Richard designed what is called the 300 Varminter. The 300 Varminter will shoot a 110grn V-Max at 4200fps and the 125grn BT at 4065fps.
Richard, was able to make the 300 Varminter work by using a Bartlien 30", 1-15 twist barrel, Match Tightneck .337 Chamber for Norma 300 Norma Brass, and VV-N550 powder. It is an amazing rifle for varminting out to 800 yards. The 300 Varminter, pushing the 125grn BT at 4000fps, shoots flatter than a 300RUM pushing a 200grn Accubound. However, for shots past 1000 yards, the 300RUM with 200grn Accubond is superior compared to the 125grn BT. But the 300 Varminter is built for varminting out to 800 yards, so it's perfect for it's intended purposes.
After talking to Richard, and owners of the 300 Varminter, it was a no brainer to have one of these rifles built. All who on this rifle say it is an awesome varmint machine. It was perfect for my needs. Many of my varmint hunting buddies use 204's, 22-250's, 220 Swifts, 243's, 6mmBR's, and 6mm AI. My .224 buddies use V-Max and Ballisic Tip bullets from 32grns to 50grns and my 6mm buddies use V-Max and Ballistic Tip bullets from 55grns to 87grns. All of these bullets are shot at very high velocities so the splat factor is awesome.
All of my buddies have made many kills out to 800 yards with the above mentioned rifles, mostly the 6mm's, however, some of the .22 cals have made kills at this range too. The one problem they all have once they start shooting past 500 yards, it's hard to spot the misses. I decided I wanted a .30 cal varmint rifle designed around the lighter bullets, so I could spot the misses much easier. The 300 Varminter was the answer and will be deadly out to 1000 yards. I am looking forward to varmint hunting this year, and I am looking forward to having a rifle that is so different from the norm. A 125grn BT at 4000fps will open the eyes of my local varmint hunting buddies.
Also, the 300 WSM Norma brass is performing well. If I were you, I would go with either the 300 WSM or build a 7mm-300 WSM because of the brass. Anyhow, good luck with your choice.