Pressure issues with IMR4955?

[Curator]

I recently loaded some 260 Remington cases with Nosler and Hornady 123 grain match grade bullets using IMR4955. On the IMR data site, they recommended a start load of 45.3 grains and a maximum of 48.0 grains at 2.780" COAL. My brass is twice reloaded R-P, and I am using Winchester Large Rifle primers and COAL 2.85" (about .005" off the rifling leade). I selected .46 grains as a reasonable starting point feeling that this would be reasonably conservative. My first shot resulted in a severely enlarged primer pocket with the primer falling out upon ejection. I believed this to be a defective case, not an overload since I have been very careful checking the exact charge weights.My second shot seemed normal with a primer indicating high but not excessive pressures. My third shot resulted in another severely enlarged primer pocket and loose primer. I did not fire any more of these reloads, taking them home and carefully disassembling them to weigh the powder charge. Every one of them had exactly 46 grains of IMR4955 powder. These are my first loads using this "new" powder. Anyone else have experience with IMR4955 that resulted in over-pressure from published load data?

 

[Blue68f100]

Your jump to lands may be off. Jamming the bullet into the lands will take most any load over the edge. Did you have any extractor marks on the brass to go alone with overly expanded primer pockets? Have not used that powder so I have no experience with it.

 

[LoonWulf]

not directly helpful I know.....

Ive not used 4955, but imr4451 is at max, right at book max in my .243 and my buddies .270. It even hits expected velocity in both.

I've honestly never seen that before

I wonder if the Enduring data might be a bit closer to the edge than their normal data.

 

[243winxb]

Your bullets are not Sierra, looking at the Hodgdon data. Bullets make a big differrnce. Weight, not as much if close.

Look at the amount of powder used for the 120 gr and 125 gr bullets. Less powder then your choice.

 

[ArchAngelCD]

Your jump to lands may be off. Jamming the bullet into the lands will take most any load over the edge. Did you have any extractor marks on the brass to go alone with overly expanded primer pockets?

This was my first thought too.

0.005" off the lands is extremely close and anyone can easily mistakenly put the bullet into the rifling. Why so close to the lands? IMO I would set the bullet 0.030" off the lands to start, then develop and accurate load and if need be play with the jump after that.

 

[lordpaxman]

I had a similar experience with IMR4955 and .243. Excessive velocities and primers (WLR) loosened in the pocket, or falling out of the pocket when the case was ejected. This was Federal brass, but, I loaded 4350 and 4831 with the same brass and primers within published load data and they behaved as expected.

https://www.thehighroad.org/index.p...mr-4955-results-in-excessive-velocity.848156/

 

[Curator]

This was my first thought too.

0.005" off the lands is extremely close and anyone can easily mistakenly put the bullet into the rifling. Why so close to the lands? IMO I would set the bullet 0.030" off the lands to start, then develop and accurate load and if need be play with the jump after that.

Thanks to everyone who replied! Your advice and comments are appreciated.

I originally started at 0,02" off the lands with these two bullets. 0.005" turned out to be the "sweet spot" with both of them when loading Varget or IMR4064. Neither of these bullets are known to create higher than usual pressures like solid copper or Nosler "partitions" do and these were not crimped. This is my first experience loading IMR4955 which is significantly "slower" than powders I have been using with previous loads. It appears that 4955 does not behave like other IMR slow rifle powders in this particular case. Usually, reduced loads of slow powders do not produce uniform velocity or give good accuracy. I can see that I need to re-think using IMR4955 in the .260 Remington.

 

[243winxb]

4955 is a double based powder containing Nitroglycerine. But as with any powder, using to much is not good.

The older IMR powder are single base. 4350, 4895 ........

Different bullet = Different pressure. Thicker jacket, harder lead core, longer bearing surface, all increase pressure.

 

[ArchAngelCD]

@Curator,
Did you try different charge weights @ 0.02" before you shortened to 0 .005"? Even with very expensive dies errors can and will occur when reporting to seat a bullet to 0.005" off the lands. There is just too much chance of error IMO. I really don't think it's the powder.

 

[Curator]

@Curator,
Did you try different charge weights @ 0.02" before you shortened to 0 .005"? Even with very expensive dies errors can and will occur when reporting to seat a bullet to 0.005" off the lands. There is just too much chance of error IMO. I really don't think it's the powder.

Obviously, not with the IMR4955 load. I am using a Sinclair micrometer seating die and seating depth had been determined with the 2 bullets with prior loads using near Max 4064 and Varget loads. It is difficult to argue with getting 1/2 MOA groups. You may be correct that this was an error and that pressures would be normal with more jump to the leade. I am reading about several others on different forums who have run into extremely high pressures with IMR4955 similar to what I have encountered. As 243winxb commented 4955 is a double based powder and I probably should have begun to work up a load differently.

 

[lordpaxman]

Obviously, not with the IMR4955 load. I am using a Sinclair micrometer seating die and seating depth had been determined with the 2 bullets with prior loads using near Max 4064 and Varget loads. It is difficult to argue with getting 1/2 MOA groups. You may be correct that this was an error and that pressures would be normal with more jump to the leade. I am reading about several others on different forums who have run into extremely high pressures with IMR4955 similar to what I have encountered. As 243winxb commented 4955 is a double based powder and I probably should have begun to work up a load differently.

I’d be interested in how’d you do that work up. In my case I was below minimum powder charge and the velocity was already over the maximum listed, and the primer pockets were expanded. Jump was close to .060”. I’m ok going a bit below minimum but this would put me in new and exciting territory. Is this the point where QL would help?

 

[Texas10mm]

Sigh,

You used data for Sierra bullets with Hornady and Nosler bullets. You either need to buy Nosler and Hornady manuals OR use the Sierra bullets listed in the data.

EVERY bullet is different, even if the weight is the same.

This seems to be lost on those who have never read the front of a reloading manual.

 

[Richard Jay King]

The simple answer is that your load was an overload in your gun with your components no matter what the loading manual said. I have not used IMR 4955 but I suspect that if you chronographed those loads you would have seen unexpectedly high velocities due to the overload pressures. I have had overpressures with some powders at two grains under "book" max and it just is what it is! Use a chronograph, work up slowly and watch for pressure signs just like you are doing. Good luck!

 

[243winxb]

Other possibilities.
1. The powder is not IMR 4955. Hodgdon put wrong powder in container? Any recalls? OP loaded wrong powder?
2. Brass is heavier then most brass. Volume is less then normal?
3.Tumbling media was left in the case , then loaded. This reduced internal volume and raised pressure?
4. Brass trim length to long?
5. Bullet jammed into rifling. This can raise pressure 10,000 PSI. The 260 Rem runs at 60,000 PSI under normal conditions. A jam may be dangerous.

I would load 3 rounds with 42.3 grs . Test fire in new brass and look for pressure signs. Go from there.

Screen shot from the IMR powder website. See any think wrong?

 

[243winxb]

Google found this.

I sent the powder back into hogdon and it was confirmed that this powder was burning hotter than advertised. They sent me all new stuff and it performs within parameters now. The jump in pressure came from getting into a new lot of powder, i found. Was blowing primers out with the minimum charge. Hogdon took care of the issue though.

https://forum.snipershide.com/threads/imr-4955-issues.6258187/