N550 maybe not THE powder for heavy 308 afterall

kombayotch

kombayotch

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Well, I got an RSI Pressure Trace system over the holidays (gift to myself since no gf at the moment) so I started checking out various loads I had developed. Checking if they were at the actual OBT nodes, verifying maximums, etc...

Long story short; when I got to testing N550 with 190 SMKs in my 308 Win. I found something unpleasant.

I was checking the lower OBT node and got this group:

P1040672.jpg


Nice as usual. N550 always give me stellar groups with the heavy bullets. However, when I checked the Pressure Trace data, I got this:

N550_hump.jpg


Note the secondary pressure spike after bullet exit. I had noticed that the muzzle blast always seemed a bit louder with N550, but didn't think anything of it. I went on to test random charges up to 60k psi in both Lapua and Hornady brass and the secondary hump was always present. This load is listed in both the Lyman and VV manuals. I'm guessing they either weren't getting it or were just using a pressure detection method that only showed peak pressure.

Probably still safe and all, but not really ideal IMHO. Think I'll be sticking to RL-17 and Vargets with the 190s.
 
Both of your pressure peaks are low,so I question the worry.I would go with the best group. The secondary peaks have blown off the ends of barrels,but at much higher pressures.
 
Not worried. As I stated, its probably safe. I'm just a bit surprised by it given the results I was getting. I consider more factors than 100 yard group size when choosing a load though. N550 was more of a novelty for me due to the extreme velocities it gives at the max load (2800 fps with 190s out of my barrel). But, because of its temperature instability, unreliable availability and high cost, it isn't a powder I would use in my main loads. Plus, its only available in 1 lb. containers. That alone is a show stopper.
 
I've never used N-550 in my 308,just IMR-4064 and 155 A-Maxs.N-550 is used in Europe as a 308 match powder. It's supposed to be more temp insensistive and give more velocity then N-150.If I ever make up a 308 hunting load,it may be worth the effort. When I bought it,it was in 2# jugs,and I still have one.In 30-06 165s and 338-06 225s,other powders have worked better.
 
kombayotch said:
Well, I got an RSI Pressure Trace system over the holidays (gift to myself since no gf at the moment) so I started checking out various loads I had developed. Checking if they were at the actual OBT nodes, verifying maximums, etc...

Long story short; when I got to testing N550 with 190 SMKs in my 308 Win. I found something unpleasant.

I was checking the lower OBT node and got this group:

P1040672.jpg


Nice as usual. N550 always give me stellar groups with the heavy bullets. However, when I checked the Pressure Trace data, I got this:

N550_hump.jpg


Note the secondary pressure spike after bullet exit. I had noticed that the muzzle blast always seemed a bit louder with N550, but didn't think anything of it. I went on to test random charges up to 60k psi in both Lapua and Hornady brass and the secondary hump was always present. This load is listed in both the Lyman and VV manuals. I'm guessing they either weren't getting it or were just using a pressure detection method that only showed peak pressure.

Probably still safe and all, but not really ideal IMHO. Think I'll be sticking to RL-17 and Vargets with the 190s.
Click to expand...


The first rise, peak, and fall are essentially identical. This looks extremely uniform. The second peak varies from 25k to 42k psi in roughly 3 equal increments. Maybe this explains the vertical in the group..if the highest pressure corresponded to the highest shot. Would a magnum primer smooth out the pressure curve?

NormB
 
Note that the secondary spikes are coming AFTER the exit on my trace, but before on the ones on that page. They have a similar example to mine on the main page. His information is a bit iffy IMO.

I probably will end up trying some magnum primers to see if it makes a difference.
 
kombayotch said:
Note that the secondary spikes are coming AFTER the exit on my trace, but before on the ones on that page. They have a similar example to mine on the main page. His information is a bit iffy IMO.

I probably will end up trying some magnum primers to see if it makes a difference.
Click to expand...

It's hard for me to believe that you can produce 25 to 42K psi from an open ended barrel with no bullet in it. 42K psi is a reasonable max psi in the chamber behind a stationary bullet. But if it is fact, and not an artifact of a pressure induced bulge/vibration bouncing back to be detected again by the transducer at the chamber, then the variation of muzzle pressure/blast behind the bullet should affect the bullets flight and could produce vertical. What were the velocities of your 3 shots related to the variation of the second spike?

You know I have noted a quite a flame from time to time with N560 from the muzzle. I contributed to some lube/solvent I had in the barrel as it occurred mainly near the start of firing, I thought anyway..

NormB
 
What's the barrel length? Looks like you hit the OBT just right if I'm reading the graph correctly (just on the upslope of the second wave)(and understand it correctly ...).
 
Its a 26" barrel.

Velocities were 2495, 2533, 2525 respectively.

The trace up top was for shots 4-6 after cleaning. Beyond that, the secondary hump became pretty consistent.

The exit times are calculated (not measured) so they may not be correct.

N550_hump2.jpg

190 SMK, 46.2 gr. N550, Hornady Brass, 2678 fps

N550_hump4.jpg

190 SMK, 45.2 gr. N550, Lapua Brass, 2635 fps

N550_hump3.jpg

190 SMK, 48.0 gr. N550, Hornady Brass, 2755 fps

Varget.jpg

190 SMK, 42.6 gr. Varget, Lapua Brass

Can't explain it. But, it seems to be an accepted phenomenon among ballistics technician. The cause is still of much debate though.
 
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Kombayotch, that secondary spike in pressure, after the bullet leaves the barrel might be comeing from unburned powder, which may or may not account for the vertical dispersion, which wasn't at all that bad by the way. The suggestion of useing a magum primer may be just what the doctor ordered, especially with N550.
I, like you, don't really like Norma powders. Their inconsistancy from lot to lot is just to extreme for me and of course is just IMHO.

The dual spikes are supposedly common, from what I've read. Not haveing a pressure trace system to relate to, means I probably shouldn't be giveing an opinion at all but, isn't that secondary pressure pop the reason for developing the BOSS system? To lower or eliminate the secondary spike and level out barrel harmonics?

bearhunter
 
N550 is actually a Vihtavuori powder. Lot-to-lot consistency is excellent in my experience. Temperature stability... not so much.

One of the theories I have read is that the incomplete burn of a slow powder creates a combustable gas that fills the bore and then explodes when oxygen is introduced. The explanation was similar to that of a fire that is burning in a room and has consumed all of the oxygen. Its just smoldering, but when someone opens the door... BOOM! If gas expansion is rapid enough in the confined space of the bore, there will definitely be some kind of pressure rise. A 20-30k rise? I don't know. To me, if a magnum primer solved the issue it would confirm unburned powder being the culprit in some form. I'll probably try them just to satisfy the curiosity.
 
kombayotch said:
N550 is actually a Vihtavuori powder. Lot-to-lot consistency is excellent in my experience. Temperature stability... not so much.

One of the theories I have read is that the incomplete burn of a slow powder creates a combustable gas that fills the bore and then explodes when oxygen is introduced. The explanation was similar to that of a fire that is burning in a room and has consumed all of the oxygen. Its just smoldering, but when someone opens the door... BOOM! If gas expansion is rapid enough in the confined space of the bore, there will definitely be some kind of pressure rise. A 20-30k rise? I don't know. To me, if a magnum primer solved the issue it would confirm unburned powder being the culprit in some form. I'll probably try them just to satisfy the curiosity.
Click to expand...


Does this 2nd spike occur with most/all slow powders, or is it more with double base powders. Maybe the nitroglycerin content amplifies the effect.

My loads with Reloder22, also double based, 70gr behind a 187gr bullet in a 300 WSM just produce a cloud of smoke, no flames like N560 though.

NormB
 
It isn't happening with RL-17, which is pretty close in burn rate to N550. Not with Varget either, which is the only other powder I've tried since I've had the Pressure Trace system.

I picked up some Rem 9 1/2 M primers (all the local guy had) to try either tomorrow or Sunday.
 
kombayotch: I recall reading some similar data ( perhaps by John Barsness/Charley Sisk ) about spikes in the 7 Rem mag. If I recall correctly the spikes disappeared when the barrel was shortened an inch or two. Have you got a 308 with shorter barrel you could compare?

Interesting piece of equipment you have there.
 
"N550 is actually a Vihtavuori powder. Lot-to-lot consistency is excellent in my experience."

X2

The single base powders are very good. The double base (although the N550 is not realy a double base like other double base powders) Vht powders are quite temperature sensitive.

I have noted that some powders make a terrible blast when fired in short barrels and others do not. I have assumed that there might be an explosion of gases mixing with oxygen. Never gave it much thought. Shooting at night might be instructive.

Norma powders, and many of the Alliant powders are made by Bofors. I used to buy it 3/4 ton at a time. Had excellent results. My favourite powders. Unfortunately some of the best are not sold as cannister powders. I know at least one well known maker of match ammo uses a non-cannister Bofors power.

Powder comes in 20 kg drums (44 pounds).

POWDERDRUM0002.jpg
 
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I'm also going to test my new lot of RL-17 tomorrow. The old lot was really good, but did not live up to the velocity being hyped or being predicted by Quickload.
 
FYI: Magnum primers made it worse. Tried some Rem 9 1/2M's (all the local guy had) and the secondary spike jumped to around 40k psi. Shot mostly singles at different grain weights just to verify. Did shoot one 5 shot group at 44.4 gr. in Hornady brass. 55 fps. faster than with 210M's. Group was still 0.495".

Well, think I'm going to save the rest of the can for testing 208 AMAX loads. Moving towards Varget for the 190's, I think.
 
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