Effects of long seating on pressure?

[BigUglyMan]

I know that someone here will know the answer to this but I couldn't find anything with the search function.

I have a 300WM on a M70 action. I've gone to a full-length mag box and follower to take advantage of the length of the 210 Berger and 208 AMax and to seat them out to the lands. What effect, if any, will the long seating length have on chamber pressure? I would think that with the bullet protruding less into the case (OAL 3.664" with the 208 AMax) that there would be more room in the case and, subsequently, a little less pressure. Can anyone confirm this or set me straight? Average velocity with the 208 AMax and 72.4gr of RL22 was 2902 (though just for 3 shots) which is 50 FPS faster than Hornady says it should be from a 25" test barrel, but my rifle wears a 29" barrel. I'm wondering if there's a big more room to go up. The load looks like it wants to perform better as the charge goes up (1.2" at 71.4gr, .97" at 71.9gr but, like a dummy, I shot this one first and forgot that I was dealing with a cold clean barrel and the first shot was 1.4" from #2 and #3 which were overlapping - another reason to shoot more than 3 shots in a group!).

Thoughts? Advice?

Thanks in advance.

 

[MontanaRifleman]

The longer out you seat your bullets, the more case capacity you will have and the less pressure you will get with the same charge of powder, everything else being equal. But the difference, in my experience, is very slight. However, once the bullet engages the lands, it may increase pressure. I would treat seating bullets to or into the lands as a new load to work up.

In my Sako 85 300 WSM, when I seat the 210 Berger to the lands, the bearing surface of the bullet only catches about 2/3rds of the neck. With RL17, my velocity out of a 24 3/8" barrel is about 2935 fps. I would describe that as a "warm" load in my rifle. Definitely max.

-Mark

 

[royinidaho]

What MontanaRifleman said!

 

[Robbin]

I seat all the bullets for my rifles into the lands where possible. But touching the lands (so the bullet doesn't get a running start before it encounters the lands) raises pressures. Quick Load suggests adding 5k to the starting pressure of a round. I agree with Montana, treat touching the lands as a new load, back down to a mild load, change your seating depth and work up from there.

I have yet to shoot a VLD that didn't shoot better touching the lands.

Good luck,
Robbin

 

[Robbin]

To answer a couple of your other questions. All barrels shoot differently, just because a test barrel did X doesn't mean yours will. I've got two 6.5X47 Lapuas. Both chambers mad with the same reamer. Both 28 inches, Identical rounds shoot 60fps slower in one, with higher pressure. I can't reach the MV of the other gun and stay within safe pressures. The books MV are for reference only. Your gun will shoot what your gun shoots. Starting with a hot load can get you killed. Start moderate and work up.

Allways shoot two or more foulers before you shoot for group. 3 round groups are 69% as big as 5 shots groups. But usually tell you what you need to Know. Always hunt with a fouled barrel, and always make sure your first shot, is what your scope is sighted for. A hot barrel may move the POI and when you are hunting, only the first, cold bore shot counts.

The serious BR guys shoot 10 shot groups. But they wear a lot of barrels out... A gun that shoots MOA with 3 shots, won't shoot MOA with 5, and MOA gun with 5 shots, won't be MOA with 10. To shoot real, live, no kidding around "what is can me and my rifle really do", you've got to shoot 10 shoot groups.

But remember, when you hunt, your first, cold bore shot is the only one that counts.