Carbon covered cases, but getting high velocity

[MikeMcCasland]

Hey Guys,

Wanted to sanity check a theory.

I was playing around with some 107 TMKs in a 27" 6.5 CM this past weekend, and I was getting carbon all over my cases, which would typically indicate a pretty significant undercharge.

That said, I was getting relatively high velocity with the 107s (from 3010FPS all the way to 3150FPS). SDs were also pretty high, in the ~20FPS range, which is uncommon for this powder in most applications I've tried.

I was using Shooters World Precision which doesn't have data for a 107 and a 6.5, so I was subbing Varget data, which is usually a pretty safe bet. i.e. in most every case it's almost grain for grain.

The only change/deviation I made was seating depth. I put the bullet at .005" off the lands, which meant I had very little bullet seated into the case neck. Probably right around .150"

My theory is that because I wasn't seating it way down in the case like Hodgdon did with their Varget data, that I've actually got quite a bit of headroom to keep going. That said, it feels weird looking at the prospect of running a bullet up into the 3200-3000 FPS out of a creedmoor.

Gun is a custom action with a mechanical ejector so I was really left to draw these conclusions based on the carbon on the cases and the outer edge of the primers; no obvious pressure signs.

What do ya'll think?

Carboned up cases on the right; clean ones on the left for reference.

 

[Longtrain]

Just a guess, did the shallow seating allow the bullet to move out of the case on firing before the the pressure peaked sticking the case to the chamber walls, allowing carbon between the case and chamber?
If this is feasible, try seating one deeper and see it this repeats itself.

 

[Bc'z]

What's your neck tension?
I'm playing with some 123's seated long, and I found better accuracy with .002 increase in neck tension.

 

[MikeMcCasland]

What's your neck tension?

~.002ish.

New Alpha brass, taken out of the box and mandrel expanded, then turned the necks down to .013". Done on a 21st century lathe with their mandrels.

Box stock the necks were ~.014"

 

[Bc'z]

 

[dskogman]

The shoulder angles look a lot different? Is it an AI? I’ve had carbon if the shoulders are bumped too far back.

 

[MikeMcCasland]

The shoulder angles look a lot different? Is it an AI? I’ve had carbon if the shoulders are bumped too far back.

New brass. Pulled it out of the box, turned it .001, and loaded it.

I didn't take a reading on the shoulder, but I loaded up some 130 Hybrids using RL16 with this same brass and it shot as you'd expect.

 

[47WillysGuy]

Mike wrote, "The only change/deviation I made was seating depth. I put the bullet at .005" off the lands, which meant I had very little bullet seated into the case neck. Probably right around .150"

Tighter neck tension and seating the loads to hard jam plus 0.003". This should give enough resistance to cause the case to expand sufficiently. Unless free bore makes this impossible...

 

[Lone Hunter]

On a 6XC I have I did the Berger seating depth test. I used .001 neck tension . They carboned up some but not as bad. I increased the neck tension to .003 and that stopped plus the groupings were a lot better. Loaded round was .272 used a .269 bushing. First I used the expander mandrel then the .269 bushing to set my .003 tension. Try a little more neck tension and see if it stops.

 

[Chuckhunter]

I agree with 47 WG, stick the bullets about .010 into the rifeling and fire them again, your shoulder should sharpen up. Check the base to shoulder length of a fired case that you say works as it should then adj. your bump to .002 shorter. I think it will disappear.

 

[gman47564]

Just a guess but im guessing that no more bullet than you have in the neck its not giving the neck time to seal itself in the chamber before the bullet is out of the neck and gone.. i bet if you seat the bullet to where you have at least the diameter of the bullet down in the neck your problem goes away..

 

[Ggmac]

Are the ones on the left new or fired in this chamber and cleaned ? The ring on the shoulder looks odd , as in having a chamber issue , 2 different angles on the shoulder . Ive seen this a few times when someone wants an ackley but doesnt set the barrel back . They just run in the reamer till the headspace ( original cartridge) is hit . What do your primers look like on the carbon cases ?

 

[MikeMcCasland]

Are the ones on the left new or fired in this chamber and cleaned ? The ring on the shoulder looks odd , as in having a chamber issue , 2 different angles on the shoulder . Ive seen this a few times when someone wants an ackley but doesnt set the barrel back . They just run in the reamer till the headspace ( original cartridge) is hit . What do your primers look like on the carbon cases ?

Ones on the left are once fired out of this gun. They were cleaned, AMPed, resized (bumped .002), and tossed in a vibratory tumbler. The ones on the right are straight out of the gun.

There are no issues with this rifle's chamber. It's the ~3rd-4th barrel I've had cut off this reamer, and the first 50 cases I shot came out the same fired dimension as brass out of my last chamber (I haven't measured the sooted up cases admittedly). Still, chamber is good. This batch of brass is good.

No pressure signs on the primers or cases. They were fired out of a custom action with a mechanical ejector, so I really don't have much to go off of other than FPS, primer condition, and the obvious carbon/soot. The SDs were terrible, but that really only reiterates that I wasn't getting good ignition (i.e. something I already knew).

Reamer is a .199fb SAAMI spec 6.5CM chamber. PTG reamer, cut a few barrels for me, and all have been good shooters.

I have a couple of theories:

1.) Folks might be correct about not having enough bullet in the case neck/maybe not enough tension, although new brass with just .001 turned on it usually still has a decent bit of tension. That said, I've also run some 80gr bullets out of my .169fb .223 that had like ~.060 worth of bullet in the neck, and I never saw this.

2.) With the 107s seated as long as they are, maybe I do have quite a bit more headroom with powder capacity. It's basically the same theory as our FTR guns no? No many manuals call for a 200.20X to be seated way the heck out there, and running 2650.

I think I'm going to test both the theories.

1.) I'll put some crazy tension on the bullet...like .004 and seat it .040 deeper, then see what happens.

2.) I'll probably just keep going up the ladder seated .005" off until I get obvious pressure signs.

 

[Chuckhunter]

Wouldn't be surprised if #2 does the trick.

 

[243winxb]

The Collapsed Shoulder is in an old Speer manual. Light loads & slow powder are the cause. Use a faster burn rate of powder. Pressure is low & neck not expanding to seal. Make sure the bullets have enough neck tension. Magnum primers must be used. A to long OAL, will not let powder burn correctly.

 

[Hohn]

The speeds you are running are slow for a little 107. Lots of folks are running 120s nearly as fast.

I would personally seat deeper first before increasing powder charge. That will improve burn and raise pressure closer to ideal. I worry that if you go hard jam, you’ll have almost no bullet in the case.

 

[fyrewall]

Hodgdon gives pressures up to 60,000 psi with equivalent velocity loads of Varget with the 107 Si MK. Possibly the Shooters World powder charges were lower than the OP thought or Shooter World changed power lots. Did I miss seeing measurements of case mouth, an indication of obduration? My thinking is that playing around with seating depth will have little effect on pressures except for hard jamming of bullet into lands. No views of case end & primer, a crude indication of pressure. Selection of standard primers should work as this is a 6.5 CM with powder charges in the 41 gr. plus range. I would be tempted to try some other powder.

 

[MikeMcCasland]

Hodgdon gives pressures up to 60,000 psi with equivalent velocity loads of Varget with the 107 Si MK. Possibly the Shooters World powder charges were lower than the OP thought or Shooter World changed power lots. Did I miss seeing measurements of case mouth, an indication of obduration? My thinking is that playing around with seating depth will have little effect on pressures except for hard jamming of bullet into lands. No views of case end & primer, a crude indication of pressure. Selection of standard primers should work as this is a 6.5 CM with powder charges in the 41 gr. plus range. I would be tempted to try some other powder.

Charges weighed on an FX120i. They're on point. The same scale weighed charges for 120gr bullets, using the same powder/same time, and there was no 'smoking' of the cases.

Powder lot has been tested in multiple other applications (.223/.308/6GT). It performs in line with Hodgdon Varget data in all other calibers. It performed as Varget would with the 120gr bullets.

Primers looked fine; no flattening. This was a custom action, so they're not going to crater.

Back of brass looks normal; mechanical ejector, so you'll never get ejector swipes.

Primers were CCI No. 200 LR.

 

[fyrewall]

Charges weighed on an FX120i. They're on point. The same scale weighed charges for 120gr bullets, using the same powder/same time, and there was no 'smoking' of the cases.

Powder lot has been tested in multiple other applications (.223/.308/6GT). It performs in line with Hodgdon Varget data in all other calibers. It performed as Varget would with the 120gr bullets.

Primers looked fine; no flattening. This was a custom action, so they're not going to crater.

Back of brass looks normal; mechanical ejector, so you'll never get ejector swipes.

Primers were CCI No. 200 LR.

Looks like the Shooter's World powder passed. Any unfired ammo of same load available to pull bullets & weigh charges? Did all same load ammo have black stuff deposits?

By mechanical ejector - is this a spring loaded, blade like ejector that pops up into a slot in the bolt when the bolt is retracted to hit the back of the case causing ejection as opposed to a plunger type ejector located in the bolt face? I would not expect to see plunger ejector swipes or ejector slot swipes at 60K and lower pressures. I don't know how brass is ejected from your rifle.

If the pressures generated were in the 60K range, as per Hodgdon data, there would be little or no primer cratering unless the fit between firing pin & hole in bolt face was sloppy. Was the diameter of the hole in bolt face & diameter of firing pin measured? Little primer flattening would be expected with lower pressure loads but at 60K there should be a degree of noticeable primer flattening .

Getting into the obduration stuff - any comparisons of neck diameters between cases having black stuff and clean cases assuming equal neck wall thickness?

Hard jamming bullets into rifling would jump pressures but minor differences in seating depths should result in minor variations in pressure.

It kind of looks like the pressures shown in the Hodgdon data using Varget & 107 Si MK bullets up to 60K psi were not reached for any number of reasons.

 

[243winxb]

SO62 seems to be the powder your usung? https://www.lhs-germany.de/en/powder/smokeless-powder/lovex/s062/