Measurment Tolerances

[TrooperKbC]

I finally made my first hundred rounds today. I used my Lee Classic Turret press with Lee Deluxe 45 acp dies, an AWS digital scale, and General brand digital calipers. I noticed my primed Magtech brass varried by as much 1.35gns. The Rainier 230gn JHP varied in seating by +/-0.003 without adjusting the dies! The IMR 700-x is a PITA to use with a Lee Pro Auto-disk Powder measure, so I charged with it, then added or removed on the digital scale to charge more accurately. The scale I bought measures in 0.02gn increments; but, I can place the charged case on the scale to get a reading, then remove the case and put it back and I commonly get a slightly different reading(+/-0.04gn)!

Are these amounts of variation in seating depths and powder weights(on the scale, not the auto-disk) normal? What is the maximum safe standard deviation?

 

[Texasjack]

You're overthinking the issue. For one thing, brass is dense, so very slight differences will show up as different weights. It should not affect the velocity or accuracy.

I long ago gave up weighing all my pistol ammo loads. I use a powder measure or a Lee scoop (depending on what I'm loading) and I check the powder measure every 10 or 20 rounds to make sure it hasn't drifted off.

What you can do, if you're really trying to make sure your bullet velocities are the same in a batch is to weigh and sort the bullets. Depending on brand, there can be some differences. Keep the same weights in a single batch. Even that is probably unnecessary, given all the other variables in shooting a .45.

For what it's worth, 700-x wouldn't be my first choice for the .45.

 

[ROGER4314]

The overall length is a maximum dimension. Experts like to seat bullets just to the start of the rifling but I never worried much about that as long as the round is not over length. The .45 acp head spaces on the end of the cartridge mouth so it's important for the bullet not to engage the rifling before the cartridge mouth seats or a head space condition could result.

Regarding charge weights, the measures take a volume of powder and equates that to a weight. If you knock around on the loading bench, the powder packs down, becomes more dense for the same volume. Be very careful to use the measure the same way every time to keep your actions from packing the powder down. Another thing I do is to place tape markers at a high point and low point of the powder in the measure reservoir. Typically, I separate high and low marks by about 1 1/2" and refill it more often than is necessary if you use the entire reservoir of powder. Again, that's to keep the powder from packing down more with a full reservoir or less with a nearly empty reservoir.

Two things should be done about your powder charge variation. There are 7000 grains to the pound and +-.04 grain is a very small amount of variation! My experience says that a small power variation like that is not significant. You may need to let that ride. The docs on your scale may list it's accuracy. Some of what you see in powder charge variation may be inaccuracy of the scale. You may need to let that ride, too.

Hope that helps!

Flash

 

[Texastransplant]

I am guessing most do this but if not it's not a bad idea. After you get your charge set for your loads, go ahead and through 10 charges and weigh it. That will give a a very good idea of how close your charges are ie. 3.5gr should be 35 grains. It just gives you a better idea of how even your charges are running. Like said above make sure you are doing the same steps each time for consistancy.

 

[dalto]

When you say the weight of the primed brass differs by 1.35 grains are you measuring two different cases? Because there is substantial variance of brass case weights, even within the same batch.

If you are seeing a difference in powder of 1.35 grains then that is a problem.

As far as the length goes, are you sure it is a difference in length and not your calipers? If you measure the same round 3 times do you get the same results?

 

[Texasjack]

Put it this way: Max load on a .45 ACP with 700-x is 4.7 g, so 0.1 g is 1/47 or about 2% variation.

 

[TrooperKbC]

The 1.35gn variation is in the brass, not the charges. LOL. It was a useless point for me to mention it in this case.

IMR's web page lists the max load as 5.0gns and a couple other places I have seen say 4.9gns. I know it's not a real big deal for pistol ammunition, but I just wasn't really sure what variation is normal so when I begin to do rifle loads I will be more prepared. My scale will say something over, then something under, and if I continue measuring the same thing the values will have an average. EX: 4.58, 4.62, 4.60, 4.56, 4.64...=4.6gns. In my mind, a 2% variation in powder weight is large. I guess with a rifle, 0.1gn will be much less than 2%, though. Maybe I just need to get a more reliable scale. Time will tell.

The calipers, on the other hand, give the same number repeatably, so the variation in seating is not a measuring problem. With seating to the lands, I have heard that principle for accuracy, but never thought about the consequences in pressure. I thought the seating depths listed in load books are mandatory because:
A) Seating too shallow will create a larger case volume and reduce pressure, and B) seating too deep will create a smaller case volume and higher pressure. Certainly you don't want to stray too much from the seating depths in a manual and change the pressure and oxygen available for the charge, right?

I will take Roger's advise and let the variation in charge weights ride for now if they are less than .04gns. Seating on the other hand...I still am not sure.

 

[Texas42]

You can get real good accuracy with +/- 0.1grain. Its pistols. Most people can't shoot near the mechanical accuaracy of their gun. You should be able to get a small ragged hole at 7 yards, and a large one at 15.

 

[dalto]

The calipers, on the other hand, give the same number repeatably, so the variation in seating is not a measuring problem. With seating to the lands, I have heard that principle for accuracy, but never thought about the consequences in pressure. I thought the seating depths listed in load books are mandatory because:
A) Seating too shallow will create a larger case volume and reduce pressure, and B) seating too deep will create a smaller case volume and higher pressure. Certainly you don't want to stray too much from the seating depths in a manual and change the pressure and oxygen available for the charge, right?

Assuming your decimal point is in the right place you are talking about 3 thousandths of an inch. This is slightly less than the thickness of an average sheet of copy paper.

 

[TrooperKbC]

I see your point with the pistol ammunition. So, when we're talking about adjusting seating depth to the landings for rifles, is it only going to be a variation of a few thousandths? What is a safe amount of adjustment when measured in inches or % case capacity? At what amount of recessed seating would the powder charge need to be also adjusted?