Overpressure, overnight...

[OFIS]

I'll start with what I've got:
308 WIN
175 SMK
RP Brass 5x firings trimmed, chamfered
CCI large rifle
44.4g Varget
2.80 oal
R700 95 palma chamber 25"

Now the problem:
This is a load I worked up with ladder testing to be the most accurate and under max with no pressure signs.
Loaded 600+ with great success all chrono'd and tracking with dope @ 2725 avg.
Only now this last batch is @ 2771 avg with noticeable primer cratering.

I had loaded all 100 too long (2.820) since the 7 year old LEE die must have gradually backed out and I neglected to double check. A few @ 2800 with primer flow and stiff bolt notified me to the problem. Pulled a few rounds apart at random through the batch to check powder weight; all 44.4

Corrected the OAL and went out again: 2771 avg out of 10

Established load and checked the past few days all around the same temp 40-60f, same powder lot as previously 2725 load.

Help?

 

[melloyello]

Let's verify a few things here because I don't think loading .020" longer is going to cause a pressure issue.
These rounds are being used in the same chamber as they were worked up for?
These have been shot 5 times with or without bumping the shoulder back?
Has the barrel been cleaned recently? If so, was oil used?
Is the bolt hard to close on these rounds?

 

[OFIS]

MtnCreek:
Yes, despite a .003 +/- from uneven tips.
I don't have a check weight but zero out the scale before I start using it.
Haven't bought 168s in about 3 years, I keep one around in a spare drawer and checked, they aren't 168s.

Melloyello:
Same chamber
Full length sized every time, never used a bump die
I did give it the first thorough cleaning in about 300 rounds (hoppes 9) but had shot 25 rounds afterward to no ill effect.
Bolt closes fine once they were seated back to 2.80, noticeably snug on the 2.82

The scale might be the weak link since it is the Lee bargain basic I started with. Check weights are on the wish list.
I'm going to pull and re-load down to 44g to see what difference that makes.

 

[bcrich]

Do you know where your lands start? Because I have the same Palma chamber and since its throated for 155's the 175's are going to stick further out (ogive) I had to seat the 175's at 2.785 and not jam into the lands. Now that I've gotten 1,100 rnds down the pipe the throat has eroded about .020" and now can load to 2.810+- anything past 43.9 g of varget in win brass gives me sticky bolt....

 

[Dan Newberry]

44.4 grains of Varget in RP brass is pedal to the metal... it'll work, just like 45 grains of Varget will work in Winchester brass (with 175's), but Varget at the top end can do some screwy things when all is not totally well.

Probably something closer to 43.1 grains will give you a steady, day-in-day-out performer, and your brass life will be much longer. With Winchester brass, load closer to 43.5 grains...

Dan

 

[OFIS]

My round count on this chamber is exactly 2,068 but I know the chamber is a factor in necessitating the 2.8 max.

This brass has only ever fired in this gun but it is not my usual (WIN) which were starting to show loose pockets at 7-8 firings when I retired them. The RP brass has been my backup stock since it was slim pickings this winter. Never had an issue with using them interchangeably, but chalking this up as a lesson learned.

The 308 only gets used for matches so I know the 44.4 load is about as Formula 1 as it is going to get.

Thanks for the replies, I've got some homework for sure.

 

[insight3b]

Do you know where your lands start? Because I have the same Palma chamber and since its throated for 155's the 175's are going to stick further out (ogive) I had to seat the 175's at 2.785 and not jam into the lands. Now that I've gotten 1,100 rnds down the pipe the throat has eroded about .020" and now can load to 2.810+- anything past 43.9 g of varget in win brass gives me sticky bolt....

I always thought Palma chambers were cut for shorter 155 grain bullets too.

 

[KYpatriot]

Interesting, that doesn't seem like too much varget to me, I load 45grains even behind 175smks, no pressure signs at all. Every rifle is different but your complaint isn't that the load has always been hit, but that it was suddenly so.

Different lot of powder?
Different lot of bullets? Is the diameter on them correct?

If you figure it out let us know.

 

[plh]

.....Different lot of bullets?.......

Anecdotal to be sure, but my accomplished 1k benchrest go-to-guy related an episode of a sticky bolt followed by subsequent case failure with powder burns he experienced with same everything (almost) while tuning for a match that was the result of a different lot # of bullet. That and/or an occult carbon ring might be at work.

 

[xdeano]

When was the last time you cleaned your rifle? Copper fowling and carbon rings will increase pressures if you have enought build up.

xdeano

 

[eldiablo]

When was the last time you cleaned your rifle? Copper fowling and carbon rings will increase pressures if you have enought build up.

xdeano

This!!! I have had the same thing happen. Not sure why, but If I didn't clean the barrel every 35-45 round it would do the same thing. Granted it was not a 308 (22-6mm Ackley), but everytime I cleaned it everything went back to normal.

 

[capnswervon]

I just had a similar velocity jump....without the over pressure signs myself. Only difference.... New 8lb jug of Varget. I was loading a pretty big batch of 308 and I had to open the next jug part way thru the run. I took the batch out to chrono it and I always test 10 off of each end of the run. The first 10 were right at 2670 the last we're at 2730-2740. All loaded with 43.5gr of Varget in Lapua brass. The only difference.... Lot# of powder. I have since done the same testing and the velocitys are all consistently at the higher value.

I would also second....or third the recommendation that you work on the carbon ring in your chamber. With the new "shoot it until it no longer shoots" attitude that prevails today, I imagine we will see more of these issues arising. Not an issue with factory guns so much, but If you have a tight match chamber it can become more of a problem.

See you at the next match,

The Capn'

 

[Creature]

I use 44.5 behind a 178gr a max at 2.845 and haven't had any problems. 2600fps out of a 20" barrel and 2714fps out of a 22".