Question About Headspacing/OAL vs Velocity

[tdogg]

Folks,

I've been developing new loads for my new to me 6mm Dasher. I have been working with RL15 along with other powders. During my initial development on 3/22/15, I found that the gun likes to crater primers. So I embarked on getting a PTG bolt head for the bolt. This required the chamber with the new PTG bolt head to be re-headspaced. I checked the headspace with the gauges prior to replacing the bolt head and it was "within spec".

During subsequent load development today 4/19/15, I noticed that the slope of the charge vs velocity curve was significantly different. What caused the change? Was it change to the headspace? The different brass lot? The "tighter" OAL? The Temperature?



Gun Details:
Savage Target Action
28 inch MTU Contour Barrel
6mm Dasher

3/22/15 Cartridge Details (Stock Savage Bolt Head):
Temperature: 67
Powder: RK15
Primer: CCI450
Bullet: Sierra 107 MK
OAL: 1.802 (0.010 off lands)
Brass: ~6x fired Lapua (came with gun)

4/19/15 Cartridge Details (PTG Bolt Head):
Temperature: 59
Powder RL15 (Same Lot)
Primer: CCI450 (Same Lot)
Bullet: Sierra 107 MK (Same Lot)
OAL: 1.810 (0.005 off lands)
Brass: 2x fired Lapua (different lot)

Any thoughts?

Cheers,
Toby

 

[mikecr]

A bunch at once, different OAL, different brass, different brass form, didn't state which way or how much head space changed, or sizing methods for both situations.
Why should anyone guess?

 

[tdogg]

Old OAL touching lands 1.812 inches, new OAL touching lands 1.815.

I couldn't tell you exactly how the headspace changed, I just know I that the old bolt head and new bolt head were/are headspaced properly (both go and nogo gauges). If you look at the OAL the headspace grew 0.003 inches.

I full length resize with a Redding type s with a .264 bushing on a COAX press. It's a tight neck chamber (.269) and I turn the brass to a loaded neck diameter of ~.266.

If you had to guess Mike what would you put your money on? I'm just trying to learn something here.

If it was a temperature offset wouldn't the curve have the same slope and just be shifted up or down? I'm struggling to believe that a different brass lot is going to shift my 32 gr charge velocity by ~55 fps but stay the about the same at the top charge. Same goes for the change to the OAL. I guess I can run a mini experiment and run the same OAL as previous and see what the velocities do.

Is there something else I'm missing?

Cheers,
Toby

 

[fguffey]

I couldn't tell you exactly how the headspace changed, I just know I that the old bolt head and new bolt head were/are headspaced properly (both go and nogo gauges). If you look at the OAL the headspace grew 0.003 inches.

It was not necessary to install the new bolt head, it was only necessary to determine how much effect the new bolt head had over the original bolt head by comparing.

I have no fewer than 40 03 bolts, I do not find it necessary to install a bolt to determine its effect on the length of the chamber, all I am required to do is measure the effect the bolt has on the length of the chamber. Of the 40 bolts 35 have absolutely no effect on changing the length of the chamber.

I offered to help, a resource type collector, builder and reloader was building a period correct 03 Rock Island rifle built in 1911. His chamber .0025 longer than a go-gage, meaning his minimum length/full length cases were .0075 shorter than the length of his chamber. I offered to size cases to off set the length of his chamber, he contacted other resource types and found a period correct bolt, all of my 03 bolts have bent back handles, he needed a bolt with a straight handle.

F. Guffey

 

[tdogg]

I don't follow you Guffey?

I installed the new PTG bolt head to minimize the cratering primer issue. I did try to just install the bolt head without altering the headspace but the new PTG bolt head locking lugs are longer than the stock Savage locking lugs. The bolt wouldn't even start to close with the PTG bolt head installed. Once I reset the headspace, performed a light locking lug lap just to check fit, then I reset the headspace again just for good measure.

I thought of another thing I changed that may also influence the charge/velocity relationship. I was trying to reduce the bolt lift force and installed a thrust washer (and bolt cap screw spacer) in the bolt. I also re-profiled the firing pin face to flatten it out a little (to help minimize the cratering primer issue). In addition I backed off the firing pin spring retainer to reduce the amount of preload on the firing pin spring. I backed it off too much as I had two light strikes during subsequent firings. I have since turned it back a half a turn to minimize the light strikes. Does the force the firing pin imparts onto the primer effect the burn intensity of the primer? In other words does the burn hotter/faster when hit harder?

I can envision the lighter hit primers not having the same burn rate and thus the internal case pressure is reduced along with the associated velocity. But it still doesn't explain the change in slope? Why does velocity meet back up near the maximum charge (started seeing pressure sign)?

Cheers,
Toby

 

[fguffey]

This required the chamber with the new PTG bolt head to be re-headspaced. I checked the headspace with the gauges prior to replacing the bolt head and it was "within spec".

I don't follow you Guffey?

OK: that is OK, "within specs", two of us had at least 110 O3 bolts, to reduce the length of his chamber I offered to measure the effect each bolt had on offsetting the length of the chamber. He had 20 plus head space gages, he could not determine the length of the chamber from the shoulder to the bolt face in thousandths. Once I measured his bolt I could compare the dimensions of the 100 plus bolts without installing them in the receiver, I could have gone home and measured my bolts with out his receiver, I offered to modify one of his head space gages from a go-gage to a go to infinity gage. .0075" was the best combination possible, his chamber was .0025 longer than a go-gage length chamber.

I did try to just install the bolt head without altering the headspace but the new PTG bolt head locking lugs are longer than the stock Savage locking lugs. The bolt wouldn't even start to close with the PTG bolt head installed

I would have compared the two bolt heads, I would have determine the effect each bolt head had when off setting the length of the chamber.

http://www.midwayusa.com/product/165455/ptg-bolt-head-savage-10-110-series-223-bolt-face-right-hand

Something I do not do, I made an exception, I called Savage. I wanted a bolt head, I wanted to know how many different bolt heads were available. They said there was one bolt head available. They said there were after market bolt heads available.

Back to "I don't follow you Guffey". I would have removed the barrel then installed the bolt head. When I suggested fitting the bolt head to the rifle they did not want to continue, again, if there was a difference between the two bolt heads I would have measured the effect each bolt head had on the length of the chamber.

I do understand the primers were cratering, replacing the bolt head was necessary, the part I do not believe was necessary was the fitting. Unless the bolt would not close without the barrel I do not understand why fitting was necessary. Most would adjust the barrel to change the length of the chamber.

F. Guffey

 

[tdogg]

I do understand the primers were cratering, replacing the bolt head was necessary, the part I do not believe was necessary was the fitting. Unless the bolt would not close without the barrel I do not understand why fitting was necessary. Most would adjust the barrel to change the length of the chamber.

F. Guffey

That was just it, the bolt would not start to close without unscrewing the barrel enough to clear the longer locking lugs on the PTG bolt head. When I say the bolt head was fit, I was referring to adjusting the barrel/headspace for the new PTG bolt head.

I still have cratering primers but I think it is a an artifact of the high pressure case design. It's definitely better with the PTG bolt head but still present.

I think I will install the backup firing pin spring assembly (fitted to the new bolt head and adjusted to same preload as originally found) and repeat the last load development to see if the charge/velocity curve returns to the original. I think there is something to the impact force of the firing pin spring and the associated pressure/velocity at a given charge.

Cheers,
Toby

 

[fguffey]

I think there is something to the impact force of the firing pin spring and the associated pressure/velocity at a given charge.

It could be a .7854 thing. The spring can be compressed with pressure inside the primer. When that happens the firing pin backs out, it is possible to have a pierced primer when this happens.

Primer cratering happens when the primer flows back around the hole and firing pin in the bolt face.

F. Guffey