Pet load showing sudden pressure?!

[drenge]

Need your help. Been doing some ladder testing with my 7 mm RM and found a load that had a low vertical dispersion at 1,000 yards. The load showed no pressure signs and was developed in temps above 90. I shot the same load yesterday in 70 degree weather and my brass, primer, bolt showed major pressure signs. I've changed nothing... Here is some basic information:

Load is 72.5 gr of Retumbo (I know it's hot but was working before)
168 grain bergers
Fed 215 GM primers
Brass is on 4th shooting
I've only bumped shoulders and bushing sized the necks (which are turned)
I've only shot about 200 rounds through the barrel with proper break-in
I clean thoroughly after 20 plus rounds

Why did this load work before and now seems way too hot? The speed jumped so much I was over my target a couple of feet. I'm fairly meticulous with my load development so I am sure I didn't change anything from before.

Does anyone know what would cause this to happen?

 

[Broz]

ammo laying in the sun?

Chamber hot form firing a string of rounds?

This is pretty much why I quit using Retumbo in one of my 30-378's. I was close to max and knew it, but the next time out it stuck the bolt. I changed to H-1000 and have been happy since.

Jeff

 

[Boss Hoss]

As stated above-----something has changed. This is a common issue when shooting marginal loads. Be smart when working up your loads.

 

[drenge]

We were shooting out of the shade (the brass was cold) and my second shot out of the barrel showed bad ejector marks and the bolt was very difficult to lift.

Obviously I'm backing off of 72.5 by at least a grain but was just curious as to what could have caused this sudden jump in pressure. Maybe the best explanation is that it was a "marginal," inconsistent load...

 

[Catfish]

I ran into this problem with a .22-6mm when I had the barrel set back and rechambered. I figured it was due to the shortening of the throught. Primer changes can have an effect also. A different lot of powder can also change things. Brass with a slightly smaller capascity will cause it. I`m sure I`m missing some, but this is a strat of where to look.

 

[drenge]

I'm using a redding body die to bump the shoulder back. That may have tightened it up a bit? And come to think of it I did just buy another can of Retumbo which was used for these loads. It went from zero pressure signs at 72.5 grains to major pressure with the same load. It scares me that a different lot of powder could produce such drastic changes in pressure.

I hadn't thought of that possibility.

 

[SBruce]

They used to say "when you find a good lot of powder or primers that work for you, buy 8 lbs of that lot and alot of primers" I know this isn't always possible to do, sometimes its impossible if that particular stuff is really popular, but I guess this is one of those situations when it would apply?

 

[highridge1]

I bought new stock of retumbo and H1000 and had trouble with both being too hot with proven loads. The H1000 I backed off 4 grains and still had pressure issues. Went back to RL25 which has never gave me problems and the problems went away and my accuracy got even better. Also a bit more velocity.

 

[RT2506]

I have not worked a whole bunch with Retumbo but it is the powder for my 264 Win mag. When working up loads I did notice that there was a very fine line between no pressure and lots of pressure signs between 1/2 grain of powder. I ran into big time ejector marks and stiff bolt 1/2 grain below the suggested max load. Backed it off 1/2 gr. and everything is perfect.

 

[BobbyL]

I assume that you are checking the OAL of the brass and its not getting to long? Were the bullets in the lands? How often do you clean the barrel? Bumping and neck sizing usually dont cause problems like this it would have to be extreme but its possible something like that could be the problem. What about the lot of primers? What about the bullets? There can be a major difference in how long the bearing surface is and the thickness of the bullets form lot to lot no matter what bullets you shoot and they will cause these kinds of problems very very quickly. What does the firing pin strikes look like? Is the chamber clean? Sometimes not enough brass grip will cause this to happen as well. I always clean up my brass with 00 steel wool pads in my cordless drill so i have the same grip ever time. When the cases get hard this can happen as well.

I have went through at least 20 different lots of retumbo and given there was sometimes a .5 grain difference in the loads but never found any problems with it. Not saying that this is not the problem because it just happens but its rare for the H extreme powders to vary from lot to lot because they mix 3 lots at the same time so this doesnt happen. I have had more problems with 25 this way then any other powder. It can be sensitive when it comes to max load having .5 grain being to much.

 

[drenge]

I do trim my brass after every firing. I also sort my bullets first by length from base to ogive then by bearing surface. I am usually very meticulous about cleaning my gun thoroughly. I did not, however, use a chamber mop last time and I will check to make sure the chamber is clean. It usually is as my brass is very polished and clean.

I wouldn't think that Retumbo would vary by so much from lot to lot but I will continue to check and double check everything that's been suggested. I appreciate the input.

 

[drenge]

My load is:

72.5 grains of Retumbo
Fed 215 gm
nosler brass (trimmed)
168 grain bergers (sorted)
bump shoulder .002
load .002 off of lands (jump)

I clean my barrel every 20 rounds or so and usually clean the chamber. The firing pin strikes were fine last several week but were cratered last time when the pressure arose.

It is possible that I need to clean my chamber better. The only real difference so far that I can find is the new lot of Retumbo.

 

[txsendero]

drenge, if you have a chrono, lower your charge of Retumbo a little bit, and work your way back up to the same velocity that you were getting BEFORE you got the new lot of Retumbo powder.
I am using about the same load with WW brass in my 7RM, 71.9 grams Retumbo, Fed 215 primers, 168VLD.

 

[BobbyL]

I do trim my brass after every firing. I also sort my bullets first by length from base to ogive then by bearing surface. I am usually very meticulous about cleaning my gun thoroughly. I did not, however, use a chamber mop last time and I will check to make sure the chamber is clean. It usually is as my brass is very polished and clean.

I wouldn't think that Retumbo would vary by so much from lot to lot but I will continue to check and double check everything that's been suggested. I appreciate the input.

Yes be careful on how polished you get the brass. Maybe try some rougher stuff like o or oo steel wool pads. They look like the scotch brite pads.

bobby