Stale Powder? Larger Charge Is Slower (H4350)

[Das Capitolin]

I am using H4350 powder to load 140 GR Hornady BTHP Match bullets for a 26" heavy barrel in 260 Rem, and some recent results have me stumped. Here are the details:

Over a year ago I purchased an 8 LB keg of Hodgdon H4350, which has been opened six times to pour and return powder. The powder is stored in a cool, dry, cabinet inside an insulated garage. The space is climate controlled, and all cabinets have large silica gel packs to remove moisture.

2012 was spent developing loads for a rifle that was replaced at the end of the season. A month ago I began building loads for a new rifle, using 39.0 GR as a starting point to collect data before increasing the charge. 39.0 GR consistently produced 2661 FPS for me, after analyzing more than 40 rounds fired on two separate days.

So then I recently moved to 40.0 GR, and built 20 rounds for early data along with 20 more rounds at 39.0 GR. Yesterday was a warm day (finally reached 70F after months of 20-40F days), and I fired all 20 rounds behind the chronograph. I wanted a true comparison, so a string of five rounds at 39.0 GR charge were followed by five rounds of 40.0 GR... until all ammo was tested. To my surprise, every 40.0 GR round I fired delivered almost 80 FPS less than the rounds with 39.0 GR charge.

The powder still looks the same: dark, and smells of acetone. I've been careful to keep everything sealed tight and avoid moisture. What could cause this?

 

[waste_knot]

The powder is probably the last thing that I would suspect. I clocked several IMR stuff 20 years apart and I just don't see degradation. Just my opinion based on a few test. Try 40.2 see if there is a jump.

 

[Das Capitolin]

Bullets not passing squarely over sensors?

Mixing up 39gr / 40gr loads?

I doubt the powder is stale. It should last many, many years.

They weren't mixed, since they were kept in separate boxes with labels on each. I also write on the first bullet in each row with a fine-point marker, out of habit.

The chronograph was definately square, although even if it wasn't there would be an issue with four strings of five shots at 39GR being faster than four strings of five shots at 40GR.

 

[Das Capitolin]

4 strings of 5 ea, I agree chrony angle is not it. BTW, refreshing to hear 20 rd samples.

That's odd and not something I've seen. I've had loads level out, meaning I could up the powder, but velocities stayed close to the same.

Grasping; additional powder made the charge more dense and slower to ignite. If that were the case, the SD / ES would probably be larger on the 40gr load. That's probably WAY out in left field. :-/

Thank you for the fast responses, and your feedback. I'm heading back to the workbench to load ten rounds each at 39.0, 40.0, and 41.0 GR. I'll re-test tomorrow AM and report back with my findings.

 

[Aimsmall55]

I've always heard that if you start getting no increase in velocity with higher charge weights then your rifle is at its limit. But the ops charge weights are far below max as I'm loading 42.4 with a creedmor. Just a thought ....

 

[Das Capitolin]

Yesterday I loaded another forty rounds for comparison. Each round was loaded with the charge weight double and triple checked, and bullets were seated using a Forster Benchrest Die. Ten rounds were made in each charge: 39, 40, 41, and 42.0 GR of Hodgdon H4350.

Today I returned to the range and fired five rounds from each charge weight, with several minutes between strings. First five rounds of 39 GR, then five of 40 GR. Then I walked to the 300 to paint steel (done for cool down), and returned to fire five rounds of 41 GR then five rounds of 42 GR. I walked out and painted, then returned. The barrel was warm at the chamber. I ran through this process again, repeating each string with another break for cooling. The barrel was warm at the tip, and reaching hot at the chamber. Here are my results:

String 1 (39 GR): 2592 AVG________String 5 (39 GR): 2545 AVG
String 2 (40 GR): 2640 AVG________String 6 (40 GR): 2594 AVG
10-minute break_________________10-minute break
String 3 (41 GR): 2691 AVG________String 7 (41 GR): 2656 AVG
String 4 (42 GR): 2754 AVG________String 8 (42 GR): 2732 AVG
10-minute break_________________Firing complete

I did see a few odd shots in last set of strings, with at least one round in each string deviating 40-60 FPS from the rest. I'm not sure why I'm losing so much velocity across the board as the barrel heats, since I'm allowing cool-down breaks every two strings. It's a fluted heavy (nearly bull) barrel that measures 1.05" at the end, and didn't feel hot until I finished firing. I suspect that the bullet is seating too close to the lands, but everything was built .02" back. Thoughts or ideas?

 

[CapDog]

In my experience chronographs can be pretty finicky, with readings that are different on different days. It reads one thing all day, go home and come back to the range and re-set up and it will read different. Sometimes slightly, sometimes much greater. This was with a batch of rounds all made up at the same time. Basically set up/lighting conditions effected the chrono readings I got. Typically what I will do now is use the chrono as a guide and then shoot the load out to distance, preferably 800 yards. Then using ballistic software back check it to come up with the velocity. I've found this to be much more accurate then my chrono.

 

[former naval person]

CapDog has it. Don't completely trust the chrono. Just like computers...there is a bit of fairy dust in there. A grain of dust keeping the screens a tiny bit farther apart could cause that. Shoot and enjoy. Build your own drop tests.

 

[1lnbrdg]

As above, the chrono is probably sensitive to light direction. I see the same regularly.

The stray-velocity shots though, they could be a problem, not the chrono necessarily. Just use it for the raw data it will give you (extreme spread and average velocity) and it's going to show you how well your loads are doing.

 

[Das Capitolin]

Thank you guys! I'm going to create a cover to go over the chrono, and see if that helps.

 

[BuzzBoss915]

I don't think you have a problem. It is not stale powder, in any case. If you are shooting 20 round strings for checking velocity, that's unnecessary. 5 rounds is sufficient. This is a gas gun, right? That could influence the different readings on different days, different conditions and maybe different distances from barrel to the chrono screens. BB

edit: and, your scale could be out of calibration.

 

[Das Capitolin]

I don't think you have a problem. It is not stale powder, in any case. If you are shooting 20 round strings for checking velocity, that's unnecessary. 5 rounds is sufficient. This is a gas gun, right? That could influence the different readings on different days, different conditions and maybe different distances from barrel to the chrono screens. BB

edit: and, your scale could be out of calibration.

I'm using a Hornaday digital powder dispenser, which I calibrate before loading. I also checked zero repeatedly before and after each charge, and then removed the pan and replaced it to confirm weight. I'm confident that the charges were dead-on correct and consistent.

The gun is a Savage 12 LRP in 260 Remington, bolt action, with 26" heavy barrel.

 

[BuzzBoss915]

Okay, scale's fine, powder isn't stale, and you have confidence in the accuracy of your chronograph. What would be your next guess? Assuming, for the sake of argument, that you actually have a problem. BB

 

[Rthur]

I may have missed it but what kind of chrono is this?

R

 

[Das Capitolin]

Okay, scale's fine, powder isn't stale, and you have confidence in the accuracy of your chronograph. What would be your next guess? Assuming, for the sake of argument, that you actually have a problem. BB

I am not firmly confident of the chronograph, as the sun moved overhead throughout the hour of shooting I did and dust might have accumulated in the window pockets.
It's a Competition Electronics Pro Chrono Digital: https://www.competitionelectronics....ypage=flypage.tpl&product_id=20&category_id=7.

 

[waste_knot]

I've got the same unit and it pretty solid. I have noticed that the instructions say to put the screens on when it sunny and off when it cloudy and this works well. It seem like the opposite.