Velocity increase on 6 month old ammo

[The Oregonian]

I had maybe 30 extra rounds from hunting season last year....i hadn't shot that gun since then until a week ago, and had put it away clean. To my surprise, velocity jumped from 3010 avg to about 3070 avg. i cleaned the rifle again and used simichrome to check for carbon ring (there was none). I loaded down a grain with the same jar of powder (N560), same primers (F210), same brass, etc. temp wasn't really any different from my sessions last fall - all in the 60's and 70's at the same location.

Today I worked back up and got 3020 with the same powder charge on ammo that was loaded yesterday...I also shot 2 more of the old ammo and got 3070 again. No pressure signs the other day or today so I felt comfortable with those old rounds.

My guess is that brass springback is what led to the velocity increase...I go .003 under with a FL bushing and then .002 with a neck turning mandrel, so it could come back tighter over time. I anneal with AMP after each firing so the brass isn't worked hardened.

Nothing else changed...could it be springback? Or could it be bullet and neck getting sticky? I tumble with STM and then coat with graphite powder prior to neck expanding and thought that would cover any metal stickiness issue.

Gun is a 280AI with 168 hunting VLD's. Ammo has been kept inside where temp range is normal with heat and AC.

 

[Deputy819]

Tagging in on this. Had the same thing happen this year with my .243 (87gr vld's) except I lost 40 fps since I worked the load up 8 months ago. Same powder and primer and I also use a .003 under bushing, .002 under mandrel and Imperial Dry Neck Lube prior to expanding.

 

[snox801]

I'm watching this also. I had Kirby do 100 rounds of .375 ammo with my new rifle and it will take me years to shoot all that. It's been 1.5 years and it's still running same FPS but. This has me worried a bit.

 

[ShtrRdy]

If you have some of the 6 month old ammo available would you consider trying something for us? Set up the seating die in the press and place one of the loaded cartridges into the shell holder. Back the seating stem off a couple turns and then raise the cartridge into the seating die. Next turn the seating stem down until it contacts the bullet. Drop the cartridge out of the seating die and then turn the seating stem down a little. While listening carefully raise the cartridge into to seating die and feel/listen for a sticky bullet breaking free from the brass.

 

[gohring3006]

If you have some of the 6 month old ammo available would you consider trying something for us? Set up the seating die in the press and place one of the loaded cartridges into the shell holder. Back the seating stem off a couple turns and then raise the cartridge into the seating die. Next turn the seating stem down until it contacts the bullet. Drop the cartridge out of the seating die and then turn the seating stem down a little. While listening carefully raise the cartridge into to seating die and feel/listen for a sticky bullet breaking free from the brass.

My suspicions as well..

 

[Gord0]

My suspicions as well..

Same here. Cold welding of the brass to the bullet.

 

[kiwikid]

Norma has shown that this can be caused by the ammo being stored at a lower RH which would reduce the moisture content of the powder in the loaded ammo. They loaded 30-06 ammo with Norma 203B powder and stored half at 20% RH and half at 86% RH and then tested the velocity at various intervals over 663 days. After 200 days the ammo stored at 20% RH had increased in speed by 18 m/s (59 fps), after 360 days it had increased by 28 m/s (92 fps). The ammo stored at 86% RH lost velocity.
The Oregonian you mentioned that the ammo is stored inside with heat and AC which may have altered the moisture content of the powder in loaded ammo.

 

[codyadams]

Cold welding. I almost gaurantee it. If you try to seat your bullets deeper or pull them with a hand puller, you will feel them "pop" when they break free. This extra pressure that it takes to break them free increases chamber pressure, thus increasing velocity.

If you want to get rid of this issue, the best option is to HBN coat your bullets. You can clean your necks, use different lubes, it wont really change it. Using HBN coating on your bullets will get your ammo consistent shelf life and also likely lower your e.s., that has been my experience.

 

[The Oregonian]

If you have some of the 6 month old ammo available would you consider trying something for us? Set up the seating die in the press and place one of the loaded cartridges into the shell holder. Back the seating stem off a couple turns and then raise the cartridge into the seating die. Next turn the seating stem down until it contacts the bullet. Drop the cartridge out of the seating die and then turn the seating stem down a little. While listening carefully raise the cartridge into to seating die and feel/listen for a sticky bullet breaking free from the brass.

Wouldn't just seating the bullet deeper by turning the stem down do the same thing?maybe turning the stem down by .020 or so?

 

[Greyfox]

Cold welding. I almost gaurantee it. If you try to seat your bullets deeper or pull them with a hand puller, you will feel them "pop" when they break free. This extra pressure that it takes to break them free increases chamber pressure, thus increasing velocity.

If you want to get rid of this issue, the best option is to HBN coat your bullets. You can clean your necks, use different lubes, it wont really change it. Using HBN coating on your bullets will get your ammo consistent shelf life and also likely lower your e.s., that has been my experience.

I have also experienced this cold welding effect when when pulling bullets after being stored for extended periods. Another remedy is to not clean the brass to the point that the "inside of the case is brought bare metal........leave a carbon film on the inside of the case. When cleaning my brass, I use corn cob with no additives, and tumble/vibrate just long enough to completely clean the outside of the case, but leaving a carbon film on the inside. This also has eliminated the long term cold welding issue for me. This carbon film seems to also aid in consistent seating pressure, velocity, and low ES.

 

[ShtrRdy]

Wouldn't just seating the bullet deeper by turning the stem down do the same thing?maybe turning the stem down by .020 or so?

Yes, this is what I'm basically asking. What we're looking for is some additional force needed to break the bullet free. This would indicate some sticking between the bullet and case.

 

[The Oregonian]

I'll seat some further before I pull them all. I'll also measure the brass before i run it through the sizer again. Thanks for the help.

 

[Gord0]

I'll seat some further before I pull them all. I'll also measure the brass before i run it through the sizer again. Thanks for the help.

You might be able to pop them out just a little with a puller, then reseat to the proper depth and be good to go.

 

[cbobclark]

Well, I'm not a leader so I can't do a technical answer, but six months ago it was colder than it is now, 90° here. Was your recent target fired with the ammo in the sunshine? Just a low tech guess.