Headspace fire form Question

[Nickntexas]

Started reloading about a month ago and still reading/learning as much as I can. Sometimes it seems there are more opinions on how to use something than facts. Now that I've worked up and tested some loads I decided to learn more about fire forming. I noticed this wording on several occasions but it didn't appear to be a necessity so I put it off. Besides I'm getting excellent groups so why screw with a good thing right? Now I got the bug and want consistent 1 MOA. I decided to reread my Wilson case gage instructions which mentions fire formed brass. Previously I was just following the Lee die instructions and everything was measuring up great with little effort reusing 300bo Barnes and R-P brass. I watched a ton of videos on how to use the Wilson gage (most of the videos are inaccurate) and started researching what the heck they were talking about with fire formed brass. I discovered my shot brass is not flush in the gage though I sized them flush. I can slide a case down the gage and I have a slight catch on the rim. Not much but it's there. I backed my die out to attempt to replicate it as much as possible. I'm doing this all by feel alone. I can't seem to get an accurate usable reading using calibers from bottom of case to bottom of case neck. Been windy so I haven't been able to test anything but am I on the right tract to loading to my fire formed brass. I'm using full sizing dies. I'm just trying to load to fire form case length from sholder to rim not diameter if that maked sence. I just cut/sized some LC 5.56 but not sure I'm going to use them. The LC brass has so many different primer hole dimension and head stamp differences and my old 20/15 eyes now need readers lol, i think I'll shoot up some Hornady Nato and use those instead. From what I've read it's good brass. Anyway, any help would be appreciative. I forgot to mention I'm not flush with the gage but I'm not exceeding it either though I've read thats okay if your gun fire forms over gage spec. Also, I plan to stick with the full length dies if that will save response advise. Thanks in advance.

[rebel]

Started reloading about a month ago and still reading/learning as much as I can. Sometimes it seems there are more opinions on how to use something than facts. Now that I've worked up and tested some loads I decided to learn more about fire forming. I noticed this wording on several occasions but it didn't appear to be a necessity so I put it off. Besides I'm getting excellent groups so why screw with a good thing right? Now I got the bug and want consistent 1 MOA. I decided to reread my Wilson case gage instructions which mentions fire formed brass. Previously I was just following the Lee die instructions and everything was measuring up great with little effort reusing 300bo Barnes and R-P brass. I watched a ton of videos on how to use the Wilson gage (most of the videos are inaccurate) and started researching what the heck they were talking about with fire formed brass. I discovered my shot brass is not flush in the gage though I sized them flush. I can slide a case down the gage and I have a slight catch on the rim. Not much but it's there. I backed my die out to attempt to replicate it as much as possible. I'm doing this all by feel alone. I can't seem to get an accurate usable reading using calibers from bottom of case to bottom of case neck. Been windy so I haven't been able to test anything but am I on the right tract to loading to my fire formed brass. I'm using full sizing dies. I'm just trying to load to fire form case length from sholder to rim not diameter if that maked sence. I just cut/sized some LC 5.56 but not sure I'm going to use them. The LC brass has so many different primer hole dimension and head stamp differences and my old 20/15 eyes now need readers lol, i think I'll shoot up some Hornady Nato and use those instead. From what I've read it's good brass. Anyway, any help would be appreciative. I forgot to mention I'm not flush with the gage but I'm not exceeding it either though I've read thats okay if your gun fire forms over gage spec. Also, I plan to stick with the full length dies if that will save response advise. Thanks in advance.

Nick,
I had to reread your post several times before figuring out ( I think ) what you are trying to achieve and measure. Depending on the cartridge, a fully formed case really doesn't occur after 2 to 3 firings, depending on how hot it's loaded.
Most guys on the IBS circuit will FL size and bump their shoulders back .002. This insures reliability and ,in my experience, increases accuracy.
In a bolt gun, if the shoulder is not bumped, once the case is fully formed the bolt will be hard, if not impossible to close.

My tool of choice to measure base to datum is the Hornady comparator. They are inexpensive and work good.

[rebel]

This picture doesn't show the inserts for checking headspace. They slip over and stop at the datum. If you don't wish to purchase a whole set, the .350 insert is what works on the 300 BLK.

You can also buy the inserts to measure base to ogive of the bullet.

[rlandry6]

Another tool that looks promising, although I havn't had a chance to try mine out yet, is the RCBS Precision Mic.
It's a caliber specific tool to measure a fire formed case to determine the headspace of your specific rifle. It has standard micrometer markings and the "0" setting is minimum SAAMI spec for head space. Once you measure a fired case and know your gun's headspace, you can adjust your sizing die for the exact amount of shoulder bump. Even some of the tech guys at Redding are using them.
Whether you use the Hornady head space gauge or the precision mic, you will get your most accurate readings by measuring cases that have been neck sized only and fired 2-3 times to allow the brass to fully expand to the chamber.
If you are loading the same caliber for multiple guns, look at the Redding Competition shell holders. They change the shoulder bump in .002" increments without readjustment of your sizing die. Once you find the shell holder that gives you enough bump for a case to chamber in a particular gun, you select that holder every time you size for that specific gun.

EDIT: After I posted this, I checked the RCBS website and found that they do not make a Precision Mic for the 300 Blackout, so you are going to probably be limited to the Hornady tool.

[dellet]

I think if you go to settings and change the “use enter key to send” setting it will allow you to create paragraphs.

That will be a big help for those of us that can’t comprehend a wall of text that rambles.

Before a case can grow the max length, it needs to grow to max diameter. It looks like you are trying to fireform with a low pressure load. This will cause the diameter to grow and the length to shorten.

If you back the die off 1/2 turn, you can do a partial resize and the next firing should give you a case that is not only fatter but longer. Closer to actual chamber size.

[Nickntexas]

Thanks for the feedback. Perhaps my terminology is still lacking. Need to figure out how to post a picture. That might help explain it a bit.

I'm shooting out of an AR platform. Think I forgot to mention that. I keep forgetting there are bolt guns in 300bo as well.

Apparently there is more to head replicating fire formed brass than I thought. For now I was just attempting to properly read the Wilson headspace gage.

The directions basically say wear your case head should be. Headspace should be. Saami being flush and anything between flush and the top of the cut out groove is said to be fine.

Then it goes on to say for better accuracy you should use a fired case from the gun you are loading for to find "your" correct headspace.

After watching a video on the gages proper use it goes on to explain that your particular rifle might actually be over the headspace gage maximum which is okay if thats what your particular gun shoots.

I did not take into effect that it couod change after multiple firings if that is the case. I assumed you used a once fired case to get your correct headspace and set your die to that.

Since I only have fl dies I don't think I have any other choice at the moment.

I'll research the other tools mentioned. Thanks again. It's a nice day here so I'm going to load some up of the LC brass up and see if just backing out the die a bit makes a diffrence at this point. Thanks again for the feedback.

[dellet]

Let’s start with some basics that might be part of the confusion.

Maximum headspace will have the case sitting at the deepest level of the gauge.

Minimum will have the case at the top.

If you set your die like most instructions say, contact at the shell holder then turn it down some more, this will generally form a case giving you maximum headspace.

When the round is shot, the case should stretch some, the distance from the base to the shoulder lengthens. When that case is in the gauge the distance from the top of the gauge the the base of the case should be less. Sometimes it’s as little as .002”. It will depend on the amount of pressure when fired.

Depending on then size of the chamber, and how the brass was initially formed the brass may need to grow .010”. SAAMI spec for chambers has a .010” window as does the base to datum length on the case shoulder. With a low powered load it could take 5 firings to reach max length.

The idea of the gauge is the measure the distance from top the of the gauge to the base of the case, before and after firing so you know how much it has grown. Then when it has stopped growing you set your die so that a sized case sits about .004” shorter for a Semi Auto.

This is why you need to back off your die some, you don’t want to keep setting the shoulder back to maximum, if you are trying to get closer to minimum.

If you are trying to get the case to stretch to near full chamber size, best is to use a load that is a near max super.

[popper]

Use a feeler gauge between the case head and step on case gauge. Set the size die to form case to this minus 2thou. Your chamber is not Saami, this corrects it. The mouth should not extend past the other end of the gauge.

[warthog97]

One of the biggest differences between loading for semi-autos and loading for bolt/single-shot guns is the inability to neck size brass, which it sound like you are asking about. Because of how semi-autos work, full length resized brass is a must. Some even argue for a small base dies for semi-autos but I've not found that to be necessary. We also don't necessarily turn the brass necks either. The last thing we look for is indications that the load is causing the action to cycle too soon (this is especially important for heavy supersonic loads designed for bolt guns (more of an issue for say heavier than 77 gr 556 loads). I like that you're asking questions and learning the terminology, keep it up!

So, basically, full length resize your brass, work with known powder, primer, and bullet combinations and play with seating depth on your more promising loads. Also use good loading techniques and keep everything consistent. Measure every powder charge and make sure you're using consistent components. I have a lot of LC brass for plinking, but for serious work, use quality brass that's headstamped 300 Blackout (or Whisper). Once you get the hang of it, swage your LC brass and dive in.

[rlandry6]

I was going to start another thread for this question, but it seems appropriate to ask it here. Is there a brand of brass that is softer than others that might yield a good fire formed case short of having to neck size, reload, and do several firings? I'd really like to get a formed case with one firing if that's possible.