Weird .308 issue

[geetarman]

I am shooting a Remington 700P and I have a concern that you folks might be able to explain.

Most of the time, when I shoot 168 gr. bullets or 175 gr. bullets, I am using different powders and charge weights. I expect to see some change in POI.

I ran across a load using the same charge weight powder for two different bullet weights and what I am seeing makes me wonder.

I am using new brass, 175 gr. SMK and 168 gr, Nosler over 43.6 gr. of BLC(2) and Winchester LR primers. This load is right out of the Sierra manual and is not a maximum load for either weight bullet.

I have the 168s zeroed at 100 yds. and when I shoot the 175 gr. bullets, the group shifts to the left almost 1 inch. I see very little change in elevation.

The other thing is, the primers are backing out .009/.013 inch when shooting the 175 gr. SMKs while the brass from the 168 gr. Noslers is still flush.

There is no problem chambering or extracting the brass.

You folks have any hints what is going on?

Thanks!

Geetarman

[WWWJD]

I'm no expert, cause I just started reloading a couple of weeks ago, but....

I'd say you lucked out in the elevation being the same between the bullet weights; at 100 yards though, you'd not see a significant enough different to really see. I think you'd have to push out to a couple of hundred yards before you'd pick up on an elevation change... maybe.. all other components being equal.

The left to right shift could be a couple of things I think. The heavier bullet is affecting the overall rifle system vibration enough that you're revealing a bedding issue which is pulling you left... or you were at a cant with the 175s vs. a level rifle with the 168.. or you've got incidental barrel/stock contact.... the factory hogue stock on my SPS was crap; had to go.

Is this something that you just now are seeing with the last dozen rounds or so, or is this a reoccurrence from batch to batch?

What kind of dies you using? I'm assuming that since your using new brass, you're also full length sizing. I'd suggest as an experiment, taking some of these now fire formed brass and neck sizing them only; the new stuff may be coming out of the die a little wonky and pulling the necks out of whack, creating another variable in your grouping. (I experienced this on my first batch of reloads a few weeks ago, bought a collet die... now my concentricity is perfect).

The primer's backing out on the higher pressure load may just be a result of the brass being new and having a shorter than normal head to shoulder length, thus creating a headspace issue. The fire formed brass may put an end to this too. If not, then the primer pockets are loose and the brass is scrap. Any primer flow or signs of leakage around the primer (carbon tracks, etc.)? All said and done, still may have a headspace issue.

I'm the farthest from an expert here that you can imagine... but these are my amateur thoughts on it... and you've also gotten the subsequent thread *bump*

Good luck. I subscribed to this thread... cause I want to learn something.

[Kayser]

Prevailing wisdom says backed out primers are a sign of low pressure not high pressure. Specifically - pressure is not high enough to stretch the case itself, but enough to push the primer out of the pocket.

I experience backed out primers somewhat regularly with my middle-of-the-road 150 gr reloads. Nothing punchy about them. So I've always suspected that the components themselves (in my case, Remington core-lokt brass and Federal LR primers) can be a cause simply because of mechanical reasons (less tension in the pocket, etc).

As long as you're not experiencing other pressure signs (cratering especially) you're probably fine.

[WWWJD]

Low pressure... I remember reading that somewhere now. But... 43.6 grains of BLC2 doesn't really register in my head as being a potentially low pressure load on the .308; it's actually closer to the high side of the scale according to Sierra's 5th edition. It's only a few clicks slower than Varget or 4064 according to Hodgdon. Should he just try switching to a different primer?

[geetarman]

What is really weird is to see such a difference with the same lot of brass, the same lot of primers and only 7 gr. difference in bullet weight.

The powder charge is pretty close to maximum for the bullet weight but is not exceeding anything in the manual.

I will probably load up some more and select a different powder.

I agree the signs I am seeing do not point to high pressure.

I did full length size the brass and checked the trim length.

This will be a work in progress for a while. Any additional insights are always welcome.

Geetarman

[1Hobie]

Too loose of a neck tension maybe? You might check your once fired brass against the yet to be loaded brass.

I'm watching and learning even though I've loaded many rounds for my 700 PSS with very good results.

Hobie

[HiBC]

My error,dup post

[HiBC]

If you are running your brass through a fl sizing die ,and if you set up the die by bumping it on the shellholder,you are likely setting the shoulder back to approximately minimum spec for the ammo.Tolrerances and variables can add up to shortening it to less than minimum.

So,while your rifle probably was made to perfectly acceptable headspace dimensions,it is possible/likely you are creating a condition that resembles excessive headspace by your resizing process.

The primer backout is an indication of the space you have.

While there are more sophisticated and recomendable means to measure this and correct it

For an experiment,try putting a feeler gage that is .002 less than your primer protrusion between your sizing die and the shell holder .

Size a few,then see if the sized brass will easily chamber before you run a bunch.

If you get away with a .008 or .009 feeler gage,you will save that much case stretch each time.Your brass will last longer.If the do not chamber easily,try a .006 or .007.

Good luck.You may find a bushing type case gage as made by Wilson,Dillon,or Forster a good $25 investment.

Given you have .009 to .013 "mush' in your firing pin strike,your ignition may be substandard .That could effect your pressure curve and accuracy.I have very little experience with BLC-2,but,iirc,it needs good ignition.

[Bart B.]

I suspect the cause of those primers backing out is low pressure, as has been mentioned by others. And the cases may also have been excessively sized in a full length sizing die setting their shoulder back too far.

This issue came up some years ago so I ran some tests to see what happens with a .308 Win. case and reduced loads. Starting with a max load of 44 grains of IMR4064 under a Sierra 165-gr. bullet, 10 Federal brass cases that had only 2 thousandths head clearance in the SAAMI spec chamber were sequetially loaded with 1 less grain of powder; 44, 43, 42,41 grains and so on down to 35 grains. Before and after each round was fired, case headspace was measured with an RCBS Precision MIC and the results listed.

The first 4 rounds all had virtually the same before and after firing case headspace and fired primers were all flush with the case head. 5th round with 40 grains of powder had a .001" shorter case headspace after firing and its primer was backed out about .001". Evidence that the case shoulder was set back a bit from firing pin impact but there wasn't enough pressure to stretch the back of the case such that its head stopped against the bolt face.

Subsequent firing of the next 2 cases showed the shoulder was set back further and the primer backed out further as well. I didn't shoot any more test rounds. The last one fired had 38 grains of powder and had its primer backed out over .007" and shoulder set back about .005".

It's my opinion that your cases had case headspace too short for your chamber and/or the powder charge was too low for your barrel.

PS: One other reason may be the barrel's bore and groove diameters are somewhat oversized. With a max load from loading data, that'll end up with lower than normal pressure often resulting in what I described above. Happened to me with an M1909 Argentine Mauser rechambered for .30-06. The .308" diameter '06 bullets in that barrel's .311" groove diameter didn't put factory ammo pressures up where they should have been; primer's backed out a tiny bit.