Could use some help deciphering Chrono data

[Archis]

I've been reading the forum for a while now and finally came up with an issue that you guys might be able to help me with.

So I headed to the range to chrono a batch of .243 that I have been developing and has been showing good progress as an accurate load. I also wanted to see if having a Magnetospeed Sporter strapped to the barrel would really affect group size (I knew it shifted point of impact up about 2MOA).

Load Data:

• PPU Brass (twice fired, annealed, neck sized using bushing die, primer pocket squared, flash hole reamed, trimmed and chamfered).
• Sellier & Belloit LR Primers
• 42gn H1000 (96.1% filled)
• 115gn DTAC (Boron Nitride Coated)
• OAL: 2.841" (bullet seated .005" into lands)

Rifle: Ruger Precision Rifle w/ Vortex 6-24x50 HS-T.

As I have been working up this load I calibrated my QuickLoad data based on the chrono speeds I was seeing. This recipe should be yielding around 2678fps and about 42K psi.

Ok, enough setup, now for the weird chrono results. I fired a string of twenty shots (4, five shot groups, 1min. between ea shot, 5min. between each group) and I was seeing good speeds (~2660fps) but on shot #2 and #10 I got a reading of 2927fps and 2962fps! Bolt lift on each of those shots opened the same as the other normal rounds, no sticking. There was ever so slight cratering but the primers weren't flattened. Part of me wants to think that it was an issue with the chrono, but the slight cratering makes me second guess that. If I back out the two "hot" rounds, I get an average of 2663fps, ES=41 and SD=16.

What I can't wrap my head around is what could even cause this degree of pressure spike. All of these rounds were loaded at the same time with no breaks from start to finish. Powder was dispensed / trickled using a Hornady Auto Charge. Quickload suggests that I would have to throw an additional 4grs. of powder resulting in a 5% compressed load to hit those kind of velocities. I just don't see how I could have accidentally overcharged by that amount. If the Auto Charge dispenses more than .2gr over the target load it warns you and the charge goes back into the hopper.

Just curious if anyone has experienced anything similar. I didn't see any extreme high flyers from these "hot" rounds (granted I was only shooting at 100yds) and shots #10 through #14 produced the best group I've yet seen from the rifle. I'm...uh...very confused. Thanks for the help.

 

[Homerange]

From Cartridges of the World there's a footnote for 243 explaining how this caliber sometimes produces unexplained results for handloaders and in all the copies I have of it (3) the same footnote is present.
There are only a few other calibers where this footnote is added and my interpretation of it is: don't try to exceed maximum published velocities in a 243.

 

[Endyo]

No idea what happened. I highly doubt you overthrew the charge to justify that velocity difference.

That chrono is accurate.

Any possibility a lighter bullet got mixed in somehow?

 

[Archis]

Homerange: Therein lies the rub. Any of the online published data for .243 Win that I have seen stops at bullet weights around 105 to 107gr. I've had to use anecdotal load data from users on this and other forums as well as extrapolate from published load data. Best I can tell, I am at the light end of the charge weight at 42gr of H1000. I've been seeing examples of people pushing the 115 DTAC's with nearly 47gr of H1000! If anything, I may be potentially below the min. threshold and that might be giving me unreliable results.

Endyo: I doubt it. The Boron Nitride bullets have a dull brown finish and the only other .243 bullets I have on hand are 105 AMAX. Now, is there a possibility that a lighter bullet was introduced at the factory? Could be, I have not weight sorted the box of 500 that I purchased from Tubb's website. So far, I really like the Magnetospeed and I'm inclined to believe the data its telling me. Only thing I can think of is that 42gr is too low of a charge and causing some kind of weird unreliable combustion.

Thanks for the input guys.

 

[eric32]

Try keep working up on the charge weight to find the velocity flat spot. I have a feeling that you might need to go up a grain or two.

Also possible the primers are giving inconsistent ignition. But that will go away with a higher density.

But if not try to get more powder in the case. 2660fps is on the really slow side. You arent really taking advantage of the 115gr BC.

But in any case a diffrent bullet might be in order.

One last thing make sure the bearing surfaces of the bullets arent drastically different from one fround to the next.

 

[Archis]

Try keep working up on the charge weight to find the velocity flat spot. I have a feeling that you might need to go up a grain or two.

Also possible the primers are giving inconsistent ignition. But that will go away with a higher density.

But if not try to get more powder in the case. 2660fps is on the really slow side. You arent really taking advantage of the 115gr BC.

But in any case a diffrent bullet might be in order.

One last thing make sure the bearing surfaces of the bullets arent drastically different from one fround to the next.

Will do. The next accuracy node should be around 44gr (2810fps) which will put me just a tad over 100% case fill. I also just picked up a case of BR2 primers to try instead of the S&B. Hopefully, between the two changes we'll see the velocities level out. The only other bullet I have tried yet is the 105 AMAX and have had okay results, but haven't done much of a workup on them. I have been meaning to try some Berger 105 or 107's if I can find them locally. If not I'll order from Midway. Thanks!

 

[eric32]

No worries bud but dont give up on the DTACs try the new primers and more powder.

 

[Endyo]

Hopefully adjusting powder charge and new primers solves the issue.

That still seems to be an abnormally high velocity spread based on those two variables.

In my testing jamming bullets had slightly higher SD/ES.

 

[Liven The Dream]

I shoot D-Tacks for years. That Bullet works best a 2950 + .
I had a 6 mm Remington case, 44.5 grs. of H 4831SC Wolf LR Win cases.
Av 3025 FPS . One great 1000 Prone Rifle.
Bottom line short barrel life..

 

[Dusty Stevens]

Case capacity

 

[CJ6]

If anything, I may be potentially below the min. threshold and that might be giving me unreliable results.

Only thing I can think of is that 42gr is too low of a charge and causing some kind of weird unreliable combustion.

Thanks for the input guys.

Bingo....Slow powders and light charges don't go well together, 42Kps (I) would consider low.

 

[Zero333]

Don't be afraid of compressed loads.

Some of my best loads are quite compressed, yielding very low es & sd.

I noticed Quickload says 105%+ case fill before I start to compress the powder.

You probably will not get any signs of pressure till you get over 48gr of H-1000

 

[Joe R]

I have used an Oehler 35 and now a Lab Radar and I can say with complete confidence that each of those chrono at times as given me weird reading of 100-200 fps slower/faster. When the difference is that big the only possible cause is a chrono error. I sort my bullets and weigh my powder on two scales. So chrono error would be my number one suspect.

I played with a .243, H1000 and the 115 DTACs and quickly concluded that there is no way to get near 3,000 fps. Not enough case capacity even if you have long freebore. But, with IMR8208 that will do it.

 

[CJ6]

I wouldn't have thought 8208 would work well with the 115 DTACS

I played with a .243, H1000 and the 115 DTACs and quickly concluded that there is no way to get near 3,000 fps. Not enough case capacity even if you have long freebore. But, with IMR8208 that will do it.

 

[Hengehold]

No idea what happened. I highly doubt you overthrew the charge to justify that velocity difference.

That chrono is accurate.

Any possibility a lighter bullet got mixed in somehow?

+1. I have heard of people getting a different weight or caliber bullet in a green box of Sierra bullets. But that was rare, like once in a lifetime, not 2 out of 20.

I shot 39gr of H4350 behind a 115DTAC in my 6xc and had LOTS of room left in the case. You would have to run quickload to find out what the fill percent is on that. I did not have any symptoms like yours due to excessive airspace inside the case. However, the burn rate between H1000 and H4350 is drastically different so perhaps one is more sensitive to fill percentage than the other.


-Trevor

 

[Endyo]

I have used an Oehler 35 and now a Lab Radar and I can say with complete confidence that each of those chrono at times as given me weird reading of 100-200 fps slower/faster. When the difference is that big the only possible cause is a chrono error. I sort my bullets and weigh my powder on two scales. So chrono error would be my number one suspect.

I played with a .243, H1000 and the 115 DTACs and quickly concluded that there is no way to get near 3,000 fps. Not enough case capacity even if you have long freebore. But, with IMR8208 that will do it.

I've had a few funny readings on my Oehler in the past. In part believed to be in user error, they don't make it dummy proof.

I haven't had any funny readings yet on my Magnetospeed V3. I have had sensitivity be off and have had missed readings.

Hopefully it WAS the chronograph and he can lay this case to rest.

Joe, I'll have to PM you and see how you liked the Labradar.

 

[GLZ]

What app are you using for group size?

 

[Endyo]

It looks like Ballistic App.

I posted a thread wondering myself:

http://forum.accurateshooter.com/th...e-calculator-phone-app.3899667/#post-36748750

 

[Archis]

Thanks everyone for the replies. I loaded up some cartridges with 44.3gn of H1000 up to the next node (2828fps) and we'll see how consistent velocities are. I still have about 2lbs left so I'll see if I can make it work for the time being. I'm not going for ultimate speed, just trying to stay around the nodes for my barrel. I think this is the fastest node I can get with H1000 before I run out of case capacity.

I also have some 8208 that I use with good success in 6.5 Grendel that I might have to try a workup on.

I was looking at the .243 cartridge guide and it looks like people were getting good accuracy from N165. I can get that locally so that might be another powder choice to try.

The app I've been using is Ballistic AE. Pretty decent app for storing load info and drop tables as well as recording group size. It's not the cheapest ($20 I think) and iPhone only, but it has been accurate and a decent tool at the range. It is also comparable with some Kestrel models for weather data, but I've not tried that yet.