I recently built an lr-308 pattern AR10 in 358 Winchester. I have posted details of the build and initial testing here in another thread. The barrel is a KAK industries 16" in stainless steel. I have function tested the rifle with Horndady 200 grain soft points, and it runs just fine.
Coming to my problem...After the basic function test I loaded up some 298 grain hollowpoints I cast from NOE mold 360-310 with the hollow point pins installed. The boolits were powdercoated and then loaded crimped in the first lube groove. This maximized case volume and left rounds that would still fit in a p-mag.
Then I tried to test chamber a dummy round. No go. The bullet nose was hitting the rifling long before the action was closed. To make the issue more visible, I created a dummy round with an un-coated boolit, and cycled the bolt until it fully chambered:
As you can see, i have lots of rifling engraved on the nose of the bullet. Measures a bit over .300".
My first thought was to just shoot jacketed bullets until the throat eroded, and then clean it up with a throat reamer. But, when I tried loading 225 grain Sierra SPBT to Hodgdon spec COAL for BLC2, those rounds also won't chamber for the same reason.
This leaves me with a gun I want to shoot heavies in, but only works with 200 grain or lighter jacketed bullets. I could probably get an NOE 360-230RF to work with this barrel too, but still that is a lot less bullet than a 298.
Is it advisable to increase the freebore by .300"? That seems like a BUNCH to me. But, my vision for the gun is certainly boolits that would fill that freebore. I have done some chamber reaming before, rechambering a Para P16-40 to 10mm. I am only a bit nervous at sticking a reamer in my new $250 barrel.
Things I already get: Yes, a 298 is a big bullet for 358 Winchester and velocity will suffer, especially in a 16" barrel. I am fine with only getting 1900-2000 fps with full power loads. I might even try loading them sub-sonic if I can get the action to cycle. In fact, with that in mind I used a low mass titanium BCG, which also helped a ton (actually about 10 oz) with my weight target for the rifle.
Yes, especially for subsonic loads I could seat much deeper in the case and not have to mess reaming my new barrel. And that is probably what I will do it the general consensus is that increasing the freebore is a very, very bad idea. However, it seems my Remington 700 in 300 win mag came from the factory with about .250" of freebore, and it shoots just fine.