Rifle Cartridge OAL Question

[10isnotenough]

I have reloaded my first 7mm mag loads and have a question about my COL.

I used fired cases from my gun and neck sized only. Using a Stoney Point headspace gage, the various cartridges are a dimension from case head to shoulder at the point defined by the headspace gage of 2.123 inches. The dimension from the case head to the ogive of the bullet is 2.749 inches (measured with the stoney point bullet comparator bushing).

When I used the stoney point OAL gauge and used a test case from stoney point I get an OAL from case head to bullet ogive of 2.747 when the bullet just contacts the rifling (of course bullet is the same type used to loaded cases measured above).

From the above measurements, it would appear that my loads are too long (2.749 vs 2.747).

However, when I measure the headspace dimension of the test case I find that it is 2.108 (as opposed to the 2.123 of the fired cases).

So, if I want to determine the distance between the shoulder and the bullet ogive, I can do some simple math to get: 2.747-2.108=.639 to the lands from the OAL gage using the test case.

My loaded rounds give 2.749-2.123=.626 inches.

Therefore, this appears to mean that I am averaging about .013 off the lands.

However, I wonder if this is a valid conclusion since I am using two different case geometries to do the calculations. In other words, If the chamber's shoulder dimension is not identical to the headspace gauge, I wonder if this conclusion is valid?

Are my loads safe as far as length goes? I don't want the bullet jammed into the rifling. BTW, I can chamber the rounds without difficulty.

Thanks

 

[dakotasin]

wow... entirely too much math for me... handloading's s'posed to be fun!

seriously, it sounds to me like: 1- your c.o.l. is ok - if the bullets were seated long you would have a harder time chambering, and you'd see rifling marks on the bullet when you pulled the cartridge back out of the gun after chambering. unless the bullets were seated really long, they won't get stuck in the bore, anyway. 2- your data is probably invalid because of the two different case dimensions.

might also want to test for feeding reliability by loading the magazine, then chamber and extract each cartridge (i do this w/ 10 random cartridges). inspect each bullet as it comes out to see if there are l&g marks on it - which are different than the extraction mark that gets left many times as the bullet scrapes the side of the chamber on extraction). if you have no problems, i'd say you are just fine.

 

[Jaywalker]

Interesting question. I do the same thing... Some issues lead me to believe you might be comparing "apples" and "oranges," though.

I get differences of as much as 0.0035 headspace from the same lot of brass when measuring fired brass.

The Stoney Point case isn't "fired," though you can send them a case and have them drill it. If you're measuring headspace against the standard case they deliver, it isn't representing anything.

I get some really significant differences in ogive length in one box of bullets using the Stoney Point Comparator. I've posted somewhere the results of measuring a box of 100 Speer 140g 0.264 bullets. IIRC, the ogive length had a max variance of 0.729 to 0.743 inches. Most fell within a range of 0.006, but there were significant numbers outside that range. A similar box of Hornady fell within an max range of 0.006 inches.

Worst case, these errors could be additive, causing an individual case/bullet to be outside the average and come closer to the lands. I do believe I've come to close to the lands occasionally, though I neither felt it nor saw land marks.

I get a real eye-opener when I test the OAL, then do the same with another bullet from the same box; I sometimes get wildly-divergent readings. I even get divergent reading from the same bullet on subsequent trials. Technique, undoubtedly, but I think there's something else at work, also. Have you ever felt the little "snick," like two metallic parts clicking together smoothly? There shouldn't be one; it should be gradually-increasing pressure, not a sudden give. I think once you've tested a bullet against the lands, you've scored its surface, invalidating subsequent tests on the same bullet.

Personally, I've taken a couple of steps. First, I don't use Speer bullets anymore. Second, I don't try to get closer than 0.010 anymore. Third, I've stopped neck-sizing in favor of FL-sizing that pushes the shoulder back a consistent 0.003 inche amount each time. Fourth, and not specific to your question, I've changed to less-temperature-sensitive powder, such as the Hodgdon Extreme line, so that I can be reasonably certain that pressure isn't powder related. Finally, I've isolated the previously-tested bullets so that I don't reuse them in the OAL Gauge, theough they're perfectly safe to shoot.

Jaywalker

 

[Sunray]

Cases don't have head space. Rifles do. Cases have Maximum OAL and Minimum OAL. My old Lyman manual shows the max OAL with bullet to be 3.290". Max. case length of 2.5". Trim cases to 2.49".
"...I can chamber the rounds without difficulty..." This is all you need worry about. Once you have a load that shoots well in your rifle, you can tweak it by lengthening or decreasing the OAL. Meantime, you're just over complicating a simple process.