Headspace conundrum, comparators, fired brass and go gauges

[Ape_Factory]

I've been experimenting quite a bit with 300 blackout in a gas gun. I've purchased a variety of factory loaded stuff, taking dimensional measurements and seeing how each performs in my rifle. I've also tested hundreds of my own rounds as I prepare to do a big loading on a Dillon.

I read another thread about headspace and not comparing fired brass to SAAMI specifications. Made sense and never did it anyway. Out of curiosity which killed the cat, I pulled out my go/no gauges in 300 blackout which are made by PTG. They're designed so you do not have to take the extractor out. All is normal, bolt will close (even dirty) on the go gauge, won't on the no go.

So I measured with the Hornady comparator set and a flat anvil on the other end of my Hornady calipers. Just out of curiosity.

-Go gauge came in at 1.074. Interesting. I don't have a field gauge.
-No go gauge came in at 1.078. Again, the bolt will not fully close on the no-go gauge.

Here's where it turns into a conundrum for me.
-Fired brass with primer removed will typically read right around 1.082, sometimes even 1.083 but generally in that range depending on charge weight.
-Factory Hornady Sub-X and a few other cartridges, all measure 1.078 or greater pre-fired, with the Hornady rounds consistently measuring 1.080.

They all function in the rifle. I do have some factory rounds which are sized in the high 1.06 range or low 1.07 range. Again, they all function. My initial headspace setting when resizing was 1.076 and I'm getting pretty solid accuracy although I need to get consistent crimping in check.

So what gives? I sized everything to 1.079 since I shoot suppressed 100% of the time. I've shot my own cartridges down to 1.074 and even loaded a few pre-sized brass which was sized at 1.068 but according to my Sheridan, still drops right in and meets SAAMI specifications. Even at 1.080, it'll drop in to the gauge no problem and not be out of specification. I haven't tried dropping the go gauge into the Sheridan, maybe I'll try that and see where it ends up.

Thoughts? And thanks in advance.

 

[Cascade Hemi]

Thoughts? And thanks in advance.

It's fine. You're welcome.

 

[bax]

Keep in mind, Hornady says that their comparator gauges are not headspace gauges - they are for comparison. For example, measure a case, size that case, re-measure. Did I just push the shoulders back by .002?

Are your PTG headspace gauges a cylinder with a flat end or do they have shoulders?

When you measure fired brass, your Hornady comparator has a cylinder with a hole in it. The diameter of the hole is a SAAMI datum point diameter.

Your headspace gauges could be a cylinder or they could have shoulders like a case. If they are cylinders, check that the diameter of the cylinder is the same as the SAAMI datum size. If your headspace gauges have shoulders, measure you headspace gauges with your Hornady comparator.

 

[Ape_Factory]

I did have to wrap my head around the whole comparator thing and came across the measurement being the datum point which really helped me understand the process as I wasn't sure where it was taking the measurement from vs. the SAAMI diagram.

I'm adept at pushing the shoulder back and can get consistent resizing case to case. That hasn't been the issue. It's just deciding what the best measurement would be moving forward. Do I trust the fired cases?

The PTG's have a "shoulder", pic below, but will measure as it really appears to meet not at the datum but at the actual shoulder.

 

[Ape_Factory]

It's fine. You're welcome.

Ok that did give me a good laugh and I get it. Nothing's blow up. I just want to fully understand and make any necessary changes to improve my loads as 300 blackout has been the most challenging for me and has provided the most setbacks (I see what you did there.)

 

[Dobbs02si]

If the reamer used in your chamber isn't ground to the exact angle of the gauges it would easily show the different measurements. Even gauges from to different companys will be different.

I wouldn't be terribly worried about the "why", more of "my resized brass chambers without excessive headspace" and roll on. Figuring out the "why" normally involves chasing of the tale for basically usless knowledge.

If fired brass rechambers after a .002-.003 shoulder bump you should be g2g.

 

[r.tenorio671]

...if it works, no need to fix...

...Actual chamber dimensions can vary due manufacturer of reamer, age of reamer, which number in sequence YOUR barrel was for the reaming process, etc.

...a fired case from a full charge factory round or reload gives you post-firing case measurements that are the most compliant with your actual chamber. If you have the ability to disable the cycling of the rifle to fire a round, the casing from that firing will be the most compliant as it remains in the chamber for the full expansion/retraction period. If you use an AGB, just close the AGB. The manual extraction may require a little more force as you pull the charging handle.

BTW, the "extractor" isn't what would cause an issue when gauging, it's the "ejector" as it applies spring loaded pressure on the back of the gauge, just a FYI. My personal preference is to gauge the barrel BEFORE assembly... especially as I have the new bolt fully disassembled anyways for absolute cleaning. Only finger pressure required to rotate the bolt and I gain more info from the "feel".

YMMV