Bullet depth and pressure

[MightyMO1911]

So I was working up my hunting load in my 30-06. Data calls for a COAL of 2.250 inches. Long story short I got confused reading the dial caliper and seated to a depth of 2.220 inches.

Now, my plan is to pull them and start over so I will not be shooting these. But I am curious. How will this more deeply seated bullet affect pressure in the 30-06?

[SSA]

Those numbers aren't right.
What bullet, powder, charge weight, oal?

[RC20]

Edit: SSA is right as 30-06 is all over 3 inches, rest is general background but need the load data.

Small case like the 9mm its big.

Large case not nearly so much, maybe not at all to a point, and the following is my experience and opinion not hard bullet or powder mfg fact.

I run mid velocity loads so I am not on the upper end.

I run a lot more variation than that for testing (in 30-06 as well), I had the bullet profile that mostly wants a .010 clearance but the recommendation was to try as far out as .040 which I did.

If you are really pushing it to the limit velocity wise and or compressed powder loads its possible its trouble.

Powder choice of course makes a difference so it would help to know what powder and bullet you are doing to know if its a risky area

[MightyMO1911]

Sorry. Typo. COAL is 3.250". Fat fingered the keyboard. Sorry about that.

150 grain Nosler BT and Varget. The range I am loading is 49.5-51 grains.

[SSA]

If this is the first time shooting this load in this rifle, you're starting out a lot closer to max than I would.
0.03" difference in seating depth seating shouldn't make any difference in pressure in an '06.

[Unclenick]

That COL is within the normal range for a 150. The Hornady 150 FMJ's normally seat 3.185". A 152 grain M2 Ball bullet seats longer, to 3.330" or so, but has a flat base and longer tangent ogive.

Change of pressure with seating depth is dominated by loss of gas bypass when you get close to the throat, raising pressure, but is dominated by loss of powder space when you seat deeply enough, also raising pressure. In between is a minimum pressure point. I can't say how close to that minimum you are. This is why we work up loads. But if you have a pointed Spitzer nose, and are more than about 0.050" off the lands, you are probably down in the trough. How deep you will have to go before pressure starts rising again, I can't say.

The example below, from Dr. Lloyd Brownells study in the mid 1960's is for a round nose bullet. Their ogives taper more gradually than a pointed bullet, so the minimum is further off the lands than with a Spitzer nose.

[FlyFish]

I agree that the small difference in seating depth is unlikely to be of any concern regarding pressure, but if you wanted to make them longer you don't need to go to all the trouble of pulling the loads and starting over. Just give each a couple taps in an inertial bullet puller, which you probably already have (or should have, anyway) - just enough to move the bullet out a tiny bit beyond your preferred 3.250 OAL, then reseat to the "correct" depth.

[Unclenick]

MightMO1911,

If you want to adjust it, FlyFish's method is the best way I can see to go about it.

QuickLOAD makes a good worst case, as the pressure reduction from backing off the lands isn't considered in its model, and it sees only the reduction in powder space factor, which means pressure goes up with deeper seating, even though it might actually be neutral or go down in your gun. With the powder file I have for faster Varget (it has varied a bit over time) which is worst case, but very close to what Hodgdon's data currently shows in .30-06, I get a 1.9% increase in pressure and 12-13 fps velocity increase for the low and high ends of your load range, respectively. If you have more velocity extreme spread than that now, you know you are actually seeing bigger variations in pressure than that with normal seating.

[green_MTman]

what does the manual,i never seat below manual C.O.L ,I usually seat a bit above it or quite a bit above it.

the problem here and why this is a good post is there is so much contradictory info on this matter,you even see load data manuals with some really deep seated bullets.

my advice
at minimum load seat the bullet .005" deeper than data source COL and then another .005" etc........ untill you get what you want and then slowly work back up in powder charge.

honestly i dont see a lower COL of any real benifit,you want to have your bullet anywhere from .050" to .010 of the rifling for best results.

third option get your chamber free bore increased,should only cost $75,problem solved there

[Unclenick]

He's using Hodgdon's load data for the bullet which uses 3.250" COL in a SAAMI test barrel. How his throat and freebore compare to the SAAMI minimum chamber used in test barrels, I have no idea. How that COL compares to what others use with that bullet, I have no idea, as Nosler doesn't list a recommendation in their databook (at least, not in my copy, which is #6). I can tell you that the seating depth of the bearing surface (so, not counting the boattail) in a case trimmed to 2.484", is 0.411". That's a little more than the usual 1 caliber rule of thumb, but the usual 1 caliber rule of thumb goes over the SAAMI and military maximum of 3.340". At 3.340" the bullet is seated 0.322", and, frankly, that should be enough case grip on the bullet. I just don't know where the throat is with respect to the lands at that point. But I can figure it out.

From Bryan Litz's measurements of this bullet, it is a true tangent ogive bullet with an Rt/R of 1.00. By cheating and using my CAD software to find the contact point on the throat, the bullet will be in contact with the land of a SAAMI minimum chamber when it is seated to 3.2674" COL. So, by seating to 3.250" COL, it is 0.0174" off the lands of Hodgdon's test barrel. That's tight.

Originally Posted by green_MTman

you want to have your bullet anywhere from .050" to .010 of the rifling for best results

I think it is fair to say a lot of tangent ogive bullets fall into that range, but it's not necessarily so with all bulelts, and certainly not so for secant ogive bullets. From Berger's experiments with secant ogive VLD's:

Originally Posted by Berger

What has been discovered is that VLD bullets shoot best when loaded to a COAL that puts the bullet in a “sweet spot”. This sweet spot is a band .030 to .040 wide and is located anywhere between jamming the bullets into the lands and .150 jump off the lands.

So, it depends heavily on the gun and bullet. But with the Nosler Ballistic tip, specifically, we have at least one case were it wanted 0.050" off the lands with a 0.224 bullet (for a .308 that is proportional to 0.067" off the lands):

In the Precision Shooting Reloading Guide (Precision Shooting Publications, 1995) Dan Hackett, relates how, in changing bullets for his 40X 220 Swift, he turned his competition seating die's micrometer adjustment the wrong way, and wound up with 20 rounds loaded 0.050" off the lands instead of 0.020" off the lands, as he'd intended. The dogma he had subscribed to at the time said nothing over 0.030" off the lands would shoot best. Faced with pulling down the loads or just shooting them in practice, he opted for the latter. To his amazement, this gun, which had never previously printed a 5-shot group smaller than 3/8” at 100 yds, gave him two ¼” groups and two bugholes in the low ones with these “incorrectly” seated bullets.


MightyMO1911,

In light of the above, you might want to leave your loads where they are. You should be almost 0.050" off the lands with your mistake. It might prove beneficial. The 2% extra pressure doesn't mean much. What everyone calls the SAAMI maximum isn't really a maximum, it's a 10 shot average that allows individual rounds to go as much as 18% over the average without disqualification, so you are going to be will within that in the worst case. I think I would just see what that load does.

[MightyMO1911]

Well I appreciate everyone's input. I always learn a lot from y'all.

As an update. I did pull them all. I then removed the decapping pin from my die and resized the neck then seated to the "proper" depth. I had one charge that proved to be quite consistent when shot over my chronograph, giving a standard deviation of only 12fps. I was really hoping for about 100fps more than the 2838 five shot average I got, and I may conduct the same test with H414. But then again I may load up 10 more and go to the range for more velocity and accuracy testing.

[reynolds357]

Pressure usually goes down when you seat slightly deeper. The freebore it creates USUALLY more than offsets the loss of case capacity.