neck tension

[sako2]

Would neck tension cause an extreme spread of 100 fps? I'm loading a 6.5 prc with magpro.

[Unclenick]

That seems like a lot, but poor consistency of bullet interference fit with the case mouth won't help. You can try the Lee Factory Crimp Die to see if it makes velocity more consistent. That would diagnose neck fit as the culprit, if it is.

I don't know how fully your load fills the case, but I've seen close to that much variation with 80%-85% case fill in 30-06 just from having the powder alternately back over the flash hole and forward over the bullet base. Try keeping powder position constant. If it is, try a magnum primer to see if that improves ignition consistency. If it does, your powder is probably a bit slow for the bullet weight, or is hard to ignite.

Be sure your primers are firmly seated.

[PocketCamera]

here is what you do, get two boxes of different factory ammunition and shoot them over your chronograph. write down velocity for each shot. i think youll be disgusted with what you see

[sako2]

Unclenick I'm at least 80% full and using a mag primer.

[Unclenick]

The exact experience with the .30-06 was 1964 National Match ammunition (government magnum primer) with a charge of 46 grains of IMR4895. That filled about 81% of the space below its bullet. In one gun (production military barrel), tipping the muzzle down before bringing the gun level to shoot (putting the powder forward over the bullet), the average muzzle velocity was about 2530 fps. But tipping the muzzle up before bringing it level, putting the powder over the primer, it averaged about 2610 fps and the primers went from very round to pretty flat.

In another club gun with a very shot out chamber, it the velocity shift was 2495 fps and 2572 fps. A third gun was in between them. But the roughly 4% drop in velocity was consistent. Mind you, those are average velocities, so the extreme spread was higher, though I didn't record it for some reason. I can guess, though, that it was probably around 140 fps.

My point is simply to inquire if you are handling the ammunition to keep the powder position the same at each firing? If not, with 80-85% case fill, you could expect a significant difference.

Another thing to check is that your primers are all seated consistently. Ideally, you would press them in about 0.003" past the point where you can feel the anvil feet touch the bottom of the primer pocket. You could also try a different primer if you have some available. Occasionally, a magnum primer can make things worse.

For neck tension, you can test the effect of an increase in start pressure by loading with the bullet touching the lands. You should drop the powder charge 10% and work up doing that. It will increase start pressure a good deal more than increasing neck sizing will do.

[sako2]

I'm .020 off the lands. I'm not loading it from the mag when testing it. I'm going to try seating the bullet deeper in the case.

[sako2]

Seated the bullet deeper in the case it didn't help. So I backed off the powder charge a 1/2 gn and loaded the bullet .010 closer to the lands. It brought my es down to 40 fps and my sd down to 12 fps.

[hounddawg]

do a load workup incrementing in .3 gn steps from minimum to maximum to find a node where the ES to around 20 FPS then work on seating depth. If you cannot get the ES down to 20 or lower try some different brands and maybe even some non magnum primers if possible

easier said than done sometimes, even a variance in your hold's shoulder pressure can screw up the ES more than 20. I went from a 6BR to a 30 grendel recently and it took me a fifty rounds or to get my recoil management smoothed out to where I could get decent groups

[sako2]

Hounddawg I did work up the load from min to max. It shot really well at 59.6 gn but I was getting crazy es. Going to work with it and see if I can get it down some more.

[Nathan]

What is your Sd? ES is kind of hard to work with, especially on less than 10 shots.

My thought.....let’s say your ES is on a good sample size. Then Sd ~ 100/6= 17fps

This is kind of high. Still, it would be ok, if it groups well under 500 yards.

Consider a stick powder
Charge variation
CBTO variation
Case neck thickness

IME, ball powder is often 10-15 ES.

[T. O'Heir]

"...using a mag primer..." That might do it. Magnum primers are about the powder used and are only necessary if your manual says to use 'em. Hornady, who invented the PRC, uses regular Federal 210 primers. So does Hodgdon.
Mind you, some on line manuals, like Hodgdon's, call for 'em based entirely on the cartridge name. The cartridge name has nothing whatever to do with it. Their site says use 'em for .357 Mag, but not .30 Carbine with the same powders.
"...off the lands..." You can forget about that until you have a load worked up. The whole off the lands thing is a load tweaking technique that is 100% trial and error.

[reynolds357]

In my experience, MagPro works best compressed or dang near 100% case capacity. I dont usually use magnum primer with MagPro unless I am using it in a big case. ie 7rem mag or larger.

[sako2]

I'm using Western powders data and they used a mag primer. Not switching powders as I bought 16 lb . Couldn't beat the price $115 for a 8lb container. I'm mostly shooting to 300 yds but do plan on shooting it to 1300 yds. Again T. O. I did work up the load using Hornady's c.o.l for that bullet. It shoots really well I'm just trying to get the es and sd down for when I do shoot to 1300 yds.

[sako2]

Nathan my es was 100 fps. Dropping the powder charge and lengthening the c.o.l brought it down to 40 fps and my sd to 12 fps.

[kilotanker22]

I have found Magpro to be hard to work with for good velocity spreads. Works best when compressed.

[sako2]

Kilo did you try it in your prc? My average for 10 shots is 2960. Going to start to bump the charge back up and see what happens.

[Nathan]

Handy formula....6*Sd = ES for a normal distribution. It is an approximation. So, if your Sd is 12, your ES is low due to a small sample size.

[Bart B.]

How many shots to get at least 90% confidence the load's numbers are significant?

[Nathan]

Bart....I’m gonna ask you to tell me....I know that should be easy....head hurts!

[kilotanker22]

I did try it in my PRC. I don't recall velocity, but I did not pursue that powder much in that caliber, because the velocity spread was unacceptable throughout the load range I tried. I think I was between 3000 and 3100 towards the top end with 140 grain bullets.

I did find decent results with it in 6.5 Creedmoor and 270 wsm. All loads with an acceptable ES were compressed

[sako2]

The prc is basically a short mag. I'm using adg brass. I was at westerns velocity .8 gn under max. Didn't go any higher because I was getting swipe marks on the case head. Bolt opens fine.

[Unclenick]

Nathan,

The usual formula is 3 times SD = ES, not 6. It is approximately true for a sample size of 10 rounds, where your set of data are average. For other sample sizes the multiplier is as shown for up to 50 in this table, but once you get above a sample size of about 7, the standard formula for SD is better than the multiplier just because a larger number of shots starts to give outliers too many chances to appear.

[sako2]

As i mentioned in 1 of my other threads i was getting a bolt swipe on my brass. So i never made it to the max charge. Apparently the rim on Adg brass is thicker. Yesterday i loaded some hornady brass up to the max charge and i have no bolt swipe, bolt opens easily. And my es came down to 20 fps. Kilo is right Magpro wants to be compressed. Thanks everyone for your help.

[Nathan]

The usual formula is 3 times SD = ES, not 6. It is approximately true for a sample size of 10 rounds, where your set of data are average.

Aren’t there 3 Sd’s on each side of the mean for 99.7% of the population.
Like:

[hounddawg]

Glad to hear you got it straightened out

it's interesting how most powders like a full case fill, makes sense though when you think about it