I'm fairly new to the AR forums, but have been monitoring them for months.

I'm having a 375 Wby Magnum built and am getting conflicting advice. Some of my buddies are telling me to have it chambered with no "free-bore", i.e. able to reach the lands. My smith says he'll do that if I wish, but that it won't make that much difference in accuracy with heavily constructed hunting bullets; I will lose some velocity; and I won't be able to shoot factory ammo.

I don't plan on shooting factory ammo anyway and am a little anal about accuracy. What do y'all think? Saeed, do you chamber with free-bore when you build on a Wby cartridge?

I have a 375 Wby with the freebore and in my rifle the accuracy is better than I can hold. I really don't think it is that much of a concern in a hunting rifle, but the not being able to safely use factory ammo could be. What happens when you sell the rifle? Some poor shmuck buys it, reads "375 WBY" on the barrel and figures "oh boy, I'll just pick me up some ammo to go with my shiny new toy and go to the range". Off he goes with that hot loaded WBY ammo and boom! I think that if the chamber says 375 WBY you should chamber to spec. If you just want a 375 Improved, then chamber it that way. IMHO - Dan

For me, there are four issues when considering freebore with developed handloads: 1)parallel throat or funnel throat, 2)bullet close to the lands or not, 3)ammunition fits in the magazine or not, and 4)what to stamp on the barrel.

A-Square says that with a parallel throat, even substantial bullet jump (common with freebore) won't decrease accuracy. For me, having the bullet close to the lands (hairsbreadth off)with a caliber length of bullet bearing surface left in the neck (hairsbreadth off) makes accuracy more likely, but not certain. Every rifle is different. The trouble you can run into, is with a lot of freebore, if you go chasing the lands, your ammunition is then too long to fit in the magazine, and with less bullet in the throat,if you're not close to the rifling, it seems like sometimes the bullet isn't stabilized to go straight for the jump to the rifling.

What to stamp on the outside of the barrel is easily addressed: just put .375 Wby. ST, which could stand for "Short Throat," and anybody who should be trusted with a firearm in the first place will know the chamber is not a standard .375 Wby. chamber. However, this can introduce another problem: I keep hearing that some African countries won't let ammunition in that doesn't match the rifle, which is of course not an issue if you do not contemplate taking that rifle to Africa.

I have a .300 Win. Mag. with what I thought was a standard chamber when I got it. It had come straight from a mail-order gunsmith to me, and as far as I know he had used a standard reamer to chamber it. Accuracy was very poor with 168 grain HPBTs. I started chasing the lands, but it took 190s to get to the lands, and by then there wasn't much bullet left in the case, so I switched to 220 HPBTs to get more bullet bearing surface in the neck and got sub 1/2 MOA for all groups (6, 3 shots each, haven't had it out since), with one under 1/4 MOA. I gave up economical bullets and a flatter trajectory and feed from the magazine to get there.

I'm still chasing accuracy with my .338-.378. It shot 1.25 MOA with moderately lengthy seating, but I didn't know when I had a good thing, so I tried seating to the lands, with not much bullet left in the case. Groups opened up. So I backed them up to having a caliber length of bearing surface in the case, and it looks like it's going to shoot factory velocity (3050 fps) and 1.25 MOA, and almost fit in the magazine (hundredth of an inch too long). Shorten it up a little more and I might be in the magazine. I wanted better than MOA, though. I'm shooting the Sierra Gameking 250 gr. SBTs. It seems like the 300 gr. HPBT would be pricey (might try it anyway), and because I think my problem is excessive freebore, I wouldn't expect to see much improvement with the 250 gr. HPBT. Surely I can beat 1.25 MOA with the Gameking?

Are there .375 RUMs yet? What's the freebore like?

The reason I chose 375 Wby instead of Ackley Improved was to have the availability of factory brass, the option of shooting factory 375 H&H in an emergency (I assume I could still do this with a short throat), and correctly headstamped brass when I go to Africa. The smith would stamp the barrel "375 Weatherby Magnum - No Factory Ammo" or something like that, so I'm not worried about someone blowing themselves up.

SDS,
The rifle is being built on a Mod 70 action with a box of 3.6 and the only COL I can find for 375 Wby is in the A-square manual and it shows 3.563 (it is confusing looking at the differences in case dimension between A-square's manual and Barnes' manual - can't quite figure that out). But my smith says he won't have problems chambering to get to the lands with the 3.6 box. The 375 RUMs have been out for a while, but I don't know anyone with one and I haven't heard about their free-bore.