Question on cartridge length consistency

[cw308]

Metal god
I think we're on the same page , I use 3 different brands of brass , HSM , FC and ADI the powder I use is IMR 4064 My ogive measurement is 2.219 the OAL is 2.794 I have a Rock creek M24 5R barrel installed with hardly any free bore , big difference in the Rem. Barrel that I changed , it had so much free bore I just set the OAL to 2.800 an worked on powder charge. When thinking of jump or jam you would have to take into consideration your case headspace , Now I do feel sorry for the OP. Chris
P.S. My loads are on the low side of the scale 40.8 all my shooting is at 200 yards. Seems my other hobby is picking others brains . Thanks guys

[jamaica]

First Question: No need to trim unless the cases are so long they hit the lands. Trim to specs if necessary.

Second Question: Don't worry about it. Load them.

Third Question: If the case has been sized the neck is sized down to make the new bullet fit tightly. Again, don't worry about it, load them.

[cw308]

Jamaica
I don't think the case length could hit the lands , there is a range that the case is safe to load an a length to trim back. Jamesf553 is having an issue with loaded lengths . James hasn't chimmed in to let us know how its going . That's what we're here for .

[Unclenick]

Chris,

When I was experimenting with super exact bullet jump, my system involved this tool (sorry for the old blurry picture; I should take new ones), which is one of the first things I made after I got my lathe twenty-odd years ago (I did not make the cheap Chinese plunger indicator, obviously):



The outside threads were to allow me to mount it in a press, but in the end, I mostly used it free-hand. The knurled bottom is the end of a case adapter that screws into the body to provide the correct chamber shoulder profile for a specific cartridge. It's .223 in the photo. Inside, the floating ram depressed by the dial indicator plunger resembles a seater die ram. The two parts, including lathe-turned threads on the adapter, were made in one workpiece to maximize alignment. It was drilled and reamed to nominal .220" bore diameter and then reamed with my .223 chamber reamer to form the chamber profile. I then parted the work at the junction of the freebore and case mouth portion of the chamber profile to make wiggle room for the plunger to move the ram. This way it meets a bullet at the actual location a real chamber throat does. The adapter at the bottom stops the case at the shoulder, same as occurs in a chamber and by then the bullet lifts the dial plunger via the floating ram.

In operation I put a finished cartridge in and hold it against the shoulder recess while I turn one of the markers or the dial scale zero to match the dial indicator hand position, giving me a comparison reference for the shoulder-to-ogive throat contact diameter distance. Subsequent cartridges then show plus or minus from that position, showing how much closer to or further from the throat they will likely be.

What I found with this tool was no advantage to perfectly exact seating depth. I was using the Redding Competition Seating Die which contacts and seats the bullet from a location above the location that actually meets the throat. Using new factory brass that I'd sorted with a case comparator to have the same head-to-shoulder datum length, was that a span of about 0.002" variation was pretty normal using Sierra match bullets. In other words, bullet ogive location varied between where the seater made contact and where the throat made contact by at least that much. If I didn't match the new brass, I got brass variation added in and was seeing at least another 0.001" in the mix.

In my experiments to try to see how much absolute precision mattered, I sorted my gauge's results so I could shoot identical and mixed length rounds, based on the indicator results. My thinking was that if it turned out to matter, I could use the instrument to sort the lengths and then adjust the micrometer head on the Redding seating die to do additional seating that set all those lengths to match the shortest length. But the matching rounds did not prove to shoot any better than the ammunition just left as it came off the seating die originally, so I gave up on this idea. I have a theory to explain why it didn't help, but let's leave that for another day.

A caveat here is that I was, IIRC, about 0.025" off the lands at that time. Would it have made a difference if I'd been trying to get 0.002" off the lands? I can't say. Someone else can try that. But the second factor in all this is that a case can undergo a little bit of sizing in the chamber. Hatcher showed .30-06 shortening as much as -0.006" just from rapid bolt work with an Enfield. It will vary with brass hardness and the gun, but it's a good idea to experiment with ejecting chambered live rounds to see if the length has changed, especially in self-loaders. I don't get a consistent result this way. Sometimes I'd get no clear change and sometimes another couple thousandths in the AR, indicating yet another potential source of error if you don't chamber slowly and carefully.

It seems like there's always something. But now I really am getting way in the weeds, again only to prove there's a limit to precision in the realm of seating depth, and for various reasons, so it's best not to count on it, but rather to accept this will be a slightly noisy aspect of the process.

[cw308]

Unclenick
I want to thank you for taking the time explaining your experiments , showing your gauge , absolutely amazing . I'm making You my go to guy . So if you see a post with Unclenick I need help , its me .
Respectfully , Chris