Hornady oal gauge question

[cajun]

So using the hornady modified case to get oal with the hornady tool. If I use my headspace comparator on the modified case the shoulder measurement is about .010 less than my brass. So I would need to add .010 to the measurement for my brass?? It sounds simple but my old fart brain is having a hard time getting my head wrapped around this. I know ideally you should use your own brass but it has been real close in other calibers.

 

[Eric H]

No, that will have no effect whether you are seating to OAL or Base to bullet OGIVE.

 

[cajun]

No, that will have no effect whether you are seating to OAL or Base to bullet OGIVE.

So what I was thinking since the headspace measurement is shorter on the modified case it's going farther into the chamber before the shoulder makes contact.

 

[QuietTexan]

ON PAPER you're 100% correct. The difference between fired cases and the comparator at the SHOULDERS drives total possible headspace gap that would have to be controlled for to keep jump perfectly consistent.

But in the real world if you set BTO on resized fired cases and don't adjust the die between firings on the brass, you have a zero net difference on subsequent firings. At least zero-enough to matter IMO.

 

[cajun]

ON PAPER you're 100% correct. The difference between fired cases and the comparator at the SHOULDERS drives total possible headspace gap that would have to be controlled for to keep jump perfectly consistent.

But in the real world if you set BTO on resized fired cases and don't adjust the die between firings on the brass, you have a zero net difference on subsequent firings. At least zero-enough to matter IMO.

Just to be clear. The .010 difference is between the modified case and a sized case with .002 shoulder bump.

 

[cajun]

So when you use the tool you push the tool forward until the tool stops moving ( case has hit the shoulder in the chamber ) then you push the rod until the bullet hits the lands. I cant see how the different headspace measurement doesn't affect the overall measurement. It would be like moving the tool body .010. Maybe I'm just confused lol.

 

[QuietTexan]

Yeah, and in theory/ on paper that messes up your BTO because the case can float somewhere in that range - constrained by the extractor at least. 100% agree there. There are variances in the shoulder length that could screw stuff up.

But once you seat to a particular BTO on a fired case, use that measurement, AND it works... no change to the relative measurements from that point forward resulting from using the standard modified case.

This is why I "rough in" jump at first, usually 0.020" off/ mag length/ arbitrary starting point for the first loads. Then fire all the cases. THEN do fine seating depth tuning - because at that point the relativity of measurements has leveled out.

Basically is doesn't matter if 2.640" BTO is actually 0.005" off, 0.010" off, or 0.030" off - all that matters is all your future rounds load to whatever length worked, meaning whatever BTO your comparator shows you.

 

[cajun]

Yeah I get it if your not in the lands then once you load and move backwards it doesnt matter if your 20 of 30 off you go by what shoots best. In this case it's a friends creedmoor. I'm getting 2.820 which is only .020 off saami coal. Pretty short. For reference my creedmoor is 2.878 same bullet. 140 eldm.

 

[Eric H]

So what I was thinking since the headspace measurement is shorter on the modified case it's going farther into the chamber before the shoulder makes contact.

If you're using the purchased "Modified Case" check it by chambering it and see if it ejects with no problem. If it does, roll with it. It's within SAAMI specs. One way to be more precise is to make your own modified case from a fired piece of brass from that gun.