Inconsistent Fire-Formed brass -- Charge Issue?

[1951]

G'day, I'n new to reloading and this forum.
While measuring some once fired 6.5 Creedmoor brass I notice quite a variation from the cartridge base to the shoulder datum. The shortest was 1.5315" and the longest 1.5380"
These cases were from factory loaded cartridges, all the same lot number. I took 60 cartridges from the same box of ammo (500 rounds, every box of 20 had the same lot number stamp) and measured the cartridge base to shoulder datum, they were all between 1.5315 and 1.5325. Looks like some of the rounds didn't fire form at all and some fire formed 6+ thou.

I don't know why they would have had such a variation in the amount they fire formed. I was thinking maybe the load charge was inconsistent from some rounds to the next. A few of the 143 rounds had the primer flattened right out, no roundness on the outer edge of the primer when I inspected them, I've been told this could be from over pressure. Also had some drag on the bolt extracting some rounds.

I've also had some bad groups right after shooting some good ones. I'm not the world's best shooter, but after putting 300 rounds down range at 100 yards, all the same ammo and fairly close to the same conditions every time out, I would expect better. I will put a five round group in a one inch circle that can be covered with a dime and the next five round group could be 1-1/4 MOA. The first 150 rounds I thought it's me, not the rifle or ammo, but after seeing the difference in shoulder bump of the fire firmed brass I'm wondering if the ammo could have charge weight discrepancies. Maybe I'm just looking for excuses for being a lousy shot. But if some rounds are loaded hotter than others I can see that sending a bullet higher than ones loaded lower.

Sorry for the long winded question, should have just asked if anyone thought ammo could be loaded with a difference of charge weight enough to cause the fired brass to form at such extreme sizes.

Thanks,
Al

 

[Straightshooter1]

Yes, there can be a substantial difference in factory ammo and how well they might work also depends a lot on how they fit your particular rifle's chamber.

If you haven't see the recent articles on the Precision Rifle Blog, it might help you with your understanding of the issue(s) here:

6.5 Creedmoor Ammo Test Part 4: Live-Fire Muzzle Velocity Details

This is the second article in this series covering the muzzle velocity data I collected. The last a...

precisionrifleblog.com

 

[47WillysGuy]

If you have factory loads that are not fired, take five loads and measure the case head space, the base to ogives, disassemble and weigh the powder charges of the five factory loads. And also the primers, and the cases and the projectiles. See if anything pops out that might answer your question. Be sure to keep each load components separate from the other four. Good luck!


And welcome aboard!

 

[243winxb]

measuring some once fired

It may take 3 or more firings, to expand brass to the chamber.

had some drag on the bolt extracting some rounds.

Oil/solvent in chamber or on brass will increase bolt thrust/breech pressure. The head to datum measurement may increase .002", longer then any others. The fired shell when placed back in to the chamber will cause a crush fit when closing the bolt.

 

[Rook 6]

Could be and probably is any or all of mentioned. Measuring new unfired for a baseline might provide insight. I’d freshly anneal, FLS and try again. I’m guessing tolerances will improve and allow you to move on. Good luck!

 

[1951]

Thanks guys/gals.

That is a very interesting article from the Precision Rifle Blog. Also supports what I've read so many times since starting this precision shooting sport, that every rifle shoots a particular round or bullet differently, some shoot bullet A well others don't.

"measure case head space"? Meaning base to shoulder, shoulder bump?
I did measure the cartridge base to shoulder of 60 factory unfired rounds, they were all within a thou of each other. +/- 0.0005" Also measured the CBTOG of all sixty, the diameter under the head, diameter of the body just before the shoulder, diameter of the neck. diameter of the bullets, length of case and COAL. They were all consistent with very little variation. I also weighed 50 of the fired cases with the primers decapped and the cases cleaned. They were all within 0.9 grains. I then weighed a new box of 50 unprimed Peterson brass cases, the results were the same as the once fired factory cases. Good idea about taking a few apart and weighing the components, I wish I thought of it. If there is a variation in charge weight that may answer my question. I may have to do more than five though. Of the 143 rounds I measured for base to shoulder length 58 were between 1.5315" and 1.5345" the rest were between 1.535" and 1.538". I'll maybe try 20, two from 10 different boxes.
I use 1.535 as the reference length as I full length sized 7 cases between 1.535 and 1.538", trimmed them to the length of the unfired rounds, seated bullets to the the same cartridge length of the unfired rounds and tested them in my chamber with the ejector removed from the bolt. The longest ones had bolt drag which lessened with each smaller length, I had very light drag at 1.5355" and drag free at 1.535". I'm now using that measurement for bumping the shoulder when I size the cases. I'll just size the shorter ones with no bump.

I use a chamber guide when cleaning and mop the chamber clean after. But I could have missed some cleaner in there. I've talked to a few guys who say some cases may vary in base to shoulder length after firing and may take a couple firings to form to the chamber, but they all say the maximum they encounter is 2 thou, not the spread I've found.

I'm not at the annealing stage yet, Two weeks ago I'd never hand loaded anything, but it will most likely come to that sometime down the line in my learning adventure.

Once again, thank you for the replies.
Al

 

[1951]

I think I saved myself the trouble of taking a bunch of the ammo apart to weigh powder.
Went to the range Monday and Tuesday with my hand loaded ammo, first ammo I've ever done myself. Shot 20 five round groups same distance same rifle same bench etc, as with the factory loads. Had an average spread of 0,31717" center to center. Largest spread was 0.683". With the factory ammo I was averaging about an inch, some 3/8" some close to 2". I'm not a great shot but I was happy with the difference between the frustration of firing the factory stuff and my hand loads.

 

[searcher]

Most factory ammo is designed to shoot in any chamber - tight or loose - and that ensures the shoulders will be back quite a ways to ensure they will shoot in any chamber. It usually takes a number of firings to move the shoulder to the fullest forward position allowed in the chamber. That is because the brass is partially "unsupported" from front to rear due to the excess headspace. If one seated the bullets "long" so as to jam the bullet into the lands, that will blow the shoulders forward fully much faster, though factory ammo will never come with bullets seated out that far. So - having different base to shoulder lengths of a few thousandths on brass fired as factory ammo for the first time or two (or three) is perfectly normal in my view. If you jam the bullets - that will expedite the process.

 

[1951]

Thanks, I reloaded them with bullets approx 12 thou off the lands, they all measured within a thou base to shoulder after firing. Non would fit in my magazines so I fired them single shot.

Also found when I tried to prime the 1Xfired brass the primer pockets in most of them was too shallow, I looked at a bunch of the unfired and most of the primers were proud of the base, that can't be too good for the bolt face when the round is fired.

 

[1951]

That S&B brass must be pretty stiff stuff, I've shot some of it three times and some cases are still not fire formed to the chamber. I shot a few rounds loaded with Peterson brass and the cases all formed 1-1/2 to 2 thou over what I bump the shoulders back to. They all formed to the chamber on the first time fired. I guess I know what brass I'll be using from now on.

 

[RegionRat]

Good observations @1951
Some factory ammo is produced with very low quality in terms of the brass, others are state of the art.

I suspect your observations above included some of the effects of really low quality controls on the brass. It would also have been good to have the velocities, but your hand loads are pointed in the right direction so you may end up bypassing the chronographing.

Sometimes it can still pay to start with factory ammo, and sometimes it is a waste of your time and bbl. It takes experience and study to know the differences.

 

[1951]

I did start with factory ammo as I new nothing about hand loading when I started this. I read and watched as many articles and videos as I could on loading for precision and have learned I've made more than a few mistakes, but at least I learned.

I did get a chronograph and did a somewhat practiced load development, I had to start over just about the time I was almost done with the development when I ran out of the Berger 140 grain Hybrid Target bullets and none were to be had anywhere in the country. I found a 1,000 round box of Lapua 139 grain Scenars and took a chance on them. Did another load development using both muzzle velocity and group sizes to narrow down the charge weight and then seating depth to find the best CBTOG and finally have my load. Now the weather has the roads closed and the virus has the range closed, but I'm loading up a bunch for when things open up.

Thanks for all the advice.

 

[piie]

That S&B brass must be pretty stiff stuff, I've shot some of it three times and some cases are still not fire formed to the chamber. I shot a few rounds loaded with Peterson brass and the cases all formed 1-1/2 to 2 thou over what I bump the shoulders back to. They all formed to the chamber on the first time fired. I guess I know what brass I'll be using from now on.

Should have mentioned S&B in your first post... I know there are alot of guys who will claim S&B to be "good", but boy, I've never had anything S&B - brass or primers in rifle or pistol - that I would consider to be "good" - call me a snob...

 

[358WCF]

Also found when I tried to prime the 1Xfired brass the primer pockets in most of them was too shallow,

That's the S&B SNAFU. Absolutely normal. Otherwise the brass seems decent enough quality, but I haven't found it to be worth the effort to make it usable.

If wanting maximum brass life, you may want to partial FL (not touching shoulder) size your good brass until it starts getting snug on bolt closure, or fully fireformed to your chamber. It often takes a few firings. Only then bump shoulders until the bolt closes easily. Lock the ring there & you're good to go.